diff --git a/docs/gifs/doc_identityS.gif b/docs/gifs/doc_identityS.gif
new file mode 100644
Binary files /dev/null and b/docs/gifs/doc_identityS.gif differ
diff --git a/docs/gifs/doc_signalFlat.gif b/docs/gifs/doc_signalFlat.gif
new file mode 100644
Binary files /dev/null and b/docs/gifs/doc_signalFlat.gif differ
diff --git a/docs/gifs/doc_signalT.gif b/docs/gifs/doc_signalT.gif
new file mode 100644
Binary files /dev/null and b/docs/gifs/doc_signalT.gif differ
diff --git a/examples/demo_stars.hs b/examples/demo_stars.hs
new file mode 100644
--- /dev/null
+++ b/examples/demo_stars.hs
@@ -0,0 +1,77 @@
+#!/usr/bin/env stack
+-- stack runghc --package reanimate
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ParallelListComp #-}
+module Main
+  ( main
+  )
+where
+
+import           Reanimate
+import           Reanimate.Builtin.Documentation
+import           Reanimate.ColorComponents
+import           System.Random
+import           Data.List
+import           Codec.Picture.Types
+import qualified Data.Vector                   as V
+
+main :: IO ()
+main = reanimate $ sceneAnimation $ do
+  newSpriteSVG_ $ mkBackgroundPixel rtfdBackgroundColor
+  play $ trails 0.05 starAnimation
+
+starAnimation :: Animation
+starAnimation = mkAnimation 10 $ \t ->
+  let camZ = t * 4
+  in  withStrokeWidth 0 $ rotate (t * 360) $ mkGroup
+        [ translate (x / newZ) (y / newZ) $ dot (1 - newZ)
+        | (x, y, z) <-
+          reverse $ take nStars $ dropWhile (\(_, _, z) -> z < camZ) $ allStars
+        , let newZ = z - camZ
+        ]
+ where
+  black = PixelRGB8 0x0 0x0 0x0
+  dot o =
+    withFillColorPixel
+        ( promotePixel
+        $ interpolateRGB8 labComponents (dropTransparency rtfdBackgroundColor) black o
+        )
+      $ mkCircle 0.05
+
+{-# INLINE trails #-}
+trails :: Double -> Animation -> Animation
+trails trailDur raw = mkAnimation (duration raw) $ \t ->
+  let idx = round (t * fromIntegral nFrames)
+  in  construct $ reverse [idx - trailFrames .. idx]
+ where
+  fps = 200
+  construct []       = mkGroup []
+  construct (x : xs) = mkGroup
+    [ withGroupOpacity (fromIntegral trailFrames / fromIntegral (trailFrames + 1))
+      $ construct xs
+    , getFrame x
+    ]
+  trailFrames = round (trailDur * fps)
+  nFrames     = round (duration raw * fps)
+  getFrame idx = frames V.! (idx `mod` nFrames)
+  frames = V.fromList
+    [ frameAt (fromIntegral i / fromIntegral nFrames * duration raw) raw
+    | i <- [0 .. nFrames]
+    ]
+
+nStars :: Int
+nStars = 1000
+
+stars, allStars :: [(Double, Double, Double)]
+allStars = [ (x, y, z + n) | n <- [0 ..], (x, y, z) <- stars ]
+stars = sortOn takeZ $ take nStars
+  [ (x, y, z)
+  | x <- randomRs (-screenWidth/2, screenWidth/2) seedX
+  | y <- randomRs (-screenWidth/2, screenWidth/2) seedY
+  | z <- randomRs (0, 1) seedZ ]
+  where takeZ (_,_,z) = z
+
+seedX, seedY, seedZ :: StdGen
+seedX = mkStdGen 0xDEAFBEEF
+seedY = mkStdGen 0x12345678
+seedZ = mkStdGen 0x87654321
diff --git a/examples/doc_circlePlot.hs b/examples/doc_circlePlot.hs
--- a/examples/doc_circlePlot.hs
+++ b/examples/doc_circlePlot.hs
@@ -5,7 +5,7 @@
 import Reanimate hiding (raster, hsv)
 import Reanimate.Builtin.Documentation
 import Reanimate.Builtin.CirclePlot
-import Reanimate.Interpolate
+import Reanimate.ColorComponents
 import Data.Colour.RGBSpace.HSV
 import Data.Colour.RGBSpace
 import Data.Colour.SRGB
diff --git a/examples/doc_hsvComponents.hs b/examples/doc_hsvComponents.hs
--- a/examples/doc_hsvComponents.hs
+++ b/examples/doc_hsvComponents.hs
@@ -3,8 +3,8 @@
 module Main(main) where
 
 import Reanimate
-import Reanimate.Interpolate
 import Reanimate.Builtin.Documentation
+import Reanimate.ColorComponents
 import Codec.Picture
 
 main :: IO ()
diff --git a/examples/doc_labComponents.hs b/examples/doc_labComponents.hs
--- a/examples/doc_labComponents.hs
+++ b/examples/doc_labComponents.hs
@@ -3,8 +3,8 @@
 module Main(main) where
 
 import Reanimate
-import Reanimate.Interpolate
 import Reanimate.Builtin.Documentation
+import Reanimate.ColorComponents
 import Codec.Picture
 
 main :: IO ()
diff --git a/examples/doc_lchComponents.hs b/examples/doc_lchComponents.hs
--- a/examples/doc_lchComponents.hs
+++ b/examples/doc_lchComponents.hs
@@ -3,8 +3,8 @@
 module Main(main) where
 
 import Reanimate
-import Reanimate.Interpolate
 import Reanimate.Builtin.Documentation
+import Reanimate.ColorComponents
 import Codec.Picture
 
 main :: IO ()
diff --git a/examples/doc_rgbComponents.hs b/examples/doc_rgbComponents.hs
--- a/examples/doc_rgbComponents.hs
+++ b/examples/doc_rgbComponents.hs
@@ -3,8 +3,8 @@
 module Main(main) where
 
 import Reanimate
-import Reanimate.Interpolate
 import Reanimate.Builtin.Documentation
+import Reanimate.ColorComponents
 import Codec.Picture
 
 main :: IO ()
diff --git a/examples/doc_ternaryPlot.hs b/examples/doc_ternaryPlot.hs
--- a/examples/doc_ternaryPlot.hs
+++ b/examples/doc_ternaryPlot.hs
@@ -5,7 +5,7 @@
 import Reanimate hiding (raster)
 import Reanimate.Builtin.Documentation
 import Reanimate.Builtin.TernaryPlot
-import Reanimate.Interpolate
+import Reanimate.ColorComponents
 import Data.Colour.CIE
 import Codec.Picture.Types
 
diff --git a/examples/doc_xyzComponents.hs b/examples/doc_xyzComponents.hs
--- a/examples/doc_xyzComponents.hs
+++ b/examples/doc_xyzComponents.hs
@@ -3,8 +3,8 @@
 module Main(main) where
 
 import Reanimate
-import Reanimate.Interpolate
 import Reanimate.Builtin.Documentation
+import Reanimate.ColorComponents
 import Codec.Picture
 
 main :: IO ()
diff --git a/examples/morphology_color.hs b/examples/morphology_color.hs
--- a/examples/morphology_color.hs
+++ b/examples/morphology_color.hs
@@ -6,7 +6,7 @@
 
 import           Codec.Picture
 import           Reanimate
-import           Reanimate.Interpolate
+import           Reanimate.ColorComponents
 import           Reanimate.Morph.Common
 import           Reanimate.Morph.Linear
 
diff --git a/examples/tangent_and_normal.hs b/examples/tangent_and_normal.hs
--- a/examples/tangent_and_normal.hs
+++ b/examples/tangent_and_normal.hs
@@ -14,7 +14,7 @@
 import           Reanimate.Diagrams
 import           Reanimate.Driver              (reanimate)
 import           Reanimate.Animation
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Reanimate.Svg
 
 
diff --git a/examples/tut_glue_fourier.hs b/examples/tut_glue_fourier.hs
--- a/examples/tut_glue_fourier.hs
+++ b/examples/tut_glue_fourier.hs
@@ -7,7 +7,7 @@
 import           Graphics.SvgTree
 import           Linear.V2
 import           Reanimate
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Codec.Picture
 
 -- layer 3
diff --git a/examples/tut_glue_physics.hs b/examples/tut_glue_physics.hs
--- a/examples/tut_glue_physics.hs
+++ b/examples/tut_glue_physics.hs
@@ -11,7 +11,7 @@
 import           Reanimate
 import           Reanimate.Chiphunk
 import           Reanimate.PolyShape
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           System.IO.Unsafe    (unsafePerformIO)
 
 shatter :: Animation
diff --git a/examples/voice_advanced.hs b/examples/voice_advanced.hs
new file mode 100644
--- /dev/null
+++ b/examples/voice_advanced.hs
@@ -0,0 +1,153 @@
+#!/usr/bin/env stack
+-- stack --resolver lts-15.04 runghc --package reanimate
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ApplicativeDo #-}
+module Main where
+
+import           Control.Monad
+import qualified Data.Text                     as T
+import           Reanimate
+import           Reanimate.Ease
+import           Reanimate.Voice
+import           Reanimate.Builtin.Documentation
+import           Geom2D.CubicBezier                       ( QuadBezier(..)
+                                                          , evalBezier
+                                                          , Point(..)
+                                                          )
+import           Graphics.SvgTree                         ( ElementRef(..) )
+
+transcript :: Transcript
+transcript = loadTranscript "voice_advanced.txt"
+
+main :: IO ()
+main = reanimate $ sceneAnimation $ do
+  bg <- newSpriteSVG $ mkBackgroundPixel rtfdBackgroundColor
+  spriteZ bg (-100)
+  newSpriteSVG_ $ mkGroup
+    [withStrokeColor "black" $ mkLine (-screenWidth, 0) (screenWidth, 0)]
+
+  centerTxt <- textHandler
+
+  flashEffect
+  circleEffect
+  squareEffect
+  finalEffect
+
+  waitOn $ forM_ (transcriptWords transcript) $ \tword -> fork $ do
+    wait (wordStart tword)
+    writeVar centerTxt $ wordReference tword
+
+  wait 2
+
+wordDuration :: TWord -> Double
+wordDuration tword = wordEnd tword - wordStart tword
+
+--
+finalEffect :: Scene s ()
+finalEffect = fork $ do
+  let begin  = findWord transcript ["final"] "circles"
+      ends   = findWords transcript ["final"] "flash"
+      path   = QuadBezier (Point 6 (-radius)) (Point 0 6) (Point (-6) (-radius))
+      radius = 0.3
+  wait (wordStart begin)
+  ss <- fork $ replicateM 3 (circleSprite radius path <* wait 0.2)
+  mapM_ (flip spriteZ (-1)) ss
+  forM_ (zip ss ends) $ \(s, end) -> fork $ do
+    spriteMap s flipXAxis
+    wait (wordStart end - wordStart begin)
+    destroySprite s
+
+-- square effect
+squareEffect :: Scene s ()
+squareEffect = fork $ do
+  let
+    begin = findWord transcript [] "square"
+    end   = findWord transcript ["middle"] "square"
+    path =
+      QuadBezier (Point 6 (-size / 2)) (Point 0 6) (Point (-6) (-size / 2))
+    size = 1
+  wait (wordStart begin)
+  s <- squareSprite size path
+  spriteMap s (rotate 180)
+  spriteZ s (-1)
+  wait (wordStart end + wordDuration end / 2 - wordStart begin)
+  destroySprite s
+
+-- circle effect
+circleEffect :: Scene s ()
+circleEffect = fork $ do
+  let begin  = findWord transcript [] "circle"
+      end    = findWord transcript ["middle"] "circle"
+      path   = QuadBezier (Point 6 (-radius)) (Point 0 6) (Point (-6) (-radius))
+      radius = 0.3
+  wait (wordStart begin)
+  s <- circleSprite radius path
+  spriteZ s (-1)
+  wait (wordStart end + wordDuration end / 2 - wordStart begin)
+  destroySprite s
+
+-- flash effect
+flashEffect :: Scene s ()
+flashEffect = forM_ (findWords transcript [] "flash") $ \flashWord -> fork $ do
+  wait (wordStart flashWord)
+  flash <- newSpriteSVG $ mkBackground "black"
+  spriteTween flash (wordDuration flashWord)
+    $ \t -> withGroupOpacity (fromToS 0 0.7 $ (powerS 2 . reverseS) t)
+  wait (wordDuration flashWord)
+  destroySprite flash
+
+--------------------------------------------------------------------------
+-- Helpers and sprites
+
+textHandler :: Scene s (Var s T.Text)
+textHandler = simpleVar render T.empty
+ where
+  render txt =
+    let txtSvg      = translate 0 (-0.25) $ centerX $ latex txt
+        activeWidth = svgWidth txtSvg + 0.5
+    in  mkGroup
+          [ withStrokeWidth 0 $ withFillColorPixel rtfdBackgroundColor $ mkRect
+            activeWidth
+            1
+          , txtSvg
+          , withStrokeColor "black"
+            $ mkLine (activeWidth / 2, 0.5) (activeWidth / 2, -0.5)
+          , withStrokeColor "black"
+            $ mkLine (-activeWidth / 2, 0.5) (-activeWidth / 2, -0.5)
+          ]
+
+circleSprite :: Double -> QuadBezier Double -> Scene s (Sprite s)
+circleSprite radius path = newSprite $ do
+  t <- spriteT
+  d <- spriteDuration
+  pure
+    $ let Point x y = evalBezier path (t / d)
+      in  mkGroup
+            [ mkClipPath "circle-mask"
+            $ removeGroups
+            $ translate 0 (screenHeight / 2)
+            $ withFillColorPixel rtfdBackgroundColor
+            $ mkRect screenWidth screenHeight
+            , withClipPathRef (Ref "circle-mask") $ translate x y $ mkCircle
+              radius
+            ]
+
+squareSprite :: Double -> QuadBezier Double -> Scene s (Sprite s)
+squareSprite size path = newSprite $ do
+  t <- spriteT
+  d <- spriteDuration
+  pure
+    $ let Point x y = evalBezier path (t / d)
+      in  mkGroup
+            [ mkClipPath "square-mask"
+            $ removeGroups
+            $ translate 0 (screenHeight / 2)
+            $ withFillColorPixel rtfdBackgroundColor
+            $ mkRect screenWidth screenHeight
+            , withClipPathRef (Ref "square-mask")
+            $ translate x y
+            $ rotate (t / d * 360)
+            $ withFillOpacity 0
+            $ withStrokeColor "black"
+            $ mkRect size size
+            ]
diff --git a/examples/voice_fake.hs b/examples/voice_fake.hs
new file mode 100644
--- /dev/null
+++ b/examples/voice_fake.hs
@@ -0,0 +1,56 @@
+#!/usr/bin/env stack
+-- stack --resolver lts-15.04 runghc --package reanimate
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ApplicativeDo #-}
+module Main where
+
+import           Control.Monad
+import qualified Data.Text                     as T
+import           Reanimate
+import           Reanimate.Voice
+import           Reanimate.Builtin.Documentation
+import           Graphics.SvgTree                         ( ElementRef(..) )
+
+transcript :: Transcript
+transcript =
+  fakeTranscript
+    "There is no audio\n\n\
+    \for this transcript....\n\n\n\
+    \Timings are fake,\n\n\
+    \which is quite useful\n\n\
+    \during development"
+
+main :: IO ()
+main = reanimate $ sceneAnimation $ do
+  newSpriteSVG_ $ mkBackgroundPixel rtfdBackgroundColor
+  waitOn $ forM_ (splitTranscript transcript) $ \(svg, tword) -> do
+    highlighted <- newVar 0
+    void $ newSprite $ do
+      v <- unVar highlighted
+      pure $ centerUsing (latex $ transcriptText transcript) $ masked
+        (wordKey tword)
+        v
+        svg
+        (withFillColor "grey" $ mkRect 1 1)
+        (withFillColor "black" $ mkRect 1 1)
+    fork $ do
+      wait (wordStart tword)
+      let dur = wordEnd tword - wordStart tword
+      tweenVar highlighted dur $ \v -> fromToS v 1
+  wait 2
+ where
+  wordKey tword =
+    T.unpack (wordReference tword) ++ show (wordStartOffset tword)
+
+{-# INLINE masked #-}
+masked :: String -> Double -> SVG -> SVG -> SVG -> SVG
+masked key t maskSVG srcSVG dstSVG = mkGroup
+  [ mkClipPath label $ removeGroups maskSVG
+  , withClipPathRef (Ref label)
+    $ translate (x - w / 2 + w * t) y (scaleToSize w screenHeight dstSVG)
+  , withClipPathRef (Ref label)
+    $ translate (x + w / 2 + w * t) y (scaleToSize w screenHeight srcSVG)
+  ]
+ where
+  label         = "word-mask-" ++ key
+  (x, y, w, _h) = boundingBox maskSVG
diff --git a/examples/voice_transcript.hs b/examples/voice_transcript.hs
--- a/examples/voice_transcript.hs
+++ b/examples/voice_transcript.hs
@@ -2,213 +2,48 @@
 -- stack --resolver lts-15.04 runghc --package reanimate
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ApplicativeDo #-}
-{-# LANGUAGE RecordWildCards #-}
 module Main where
 
-import           Codec.Picture.Types
 import           Control.Monad
-import           Data.Hashable
-import           Data.Aeson
-import           Data.Char
-import           System.IO.Unsafe
-import           Data.Function
-import           Data.List
-import           Data.Maybe
-import           Data.Ratio
 import qualified Data.Text                     as T
-import           Data.Tuple
-import qualified Data.Vector                   as V
-import           Debug.Trace
 import           Reanimate
-import           Reanimate.Animation
-import           Reanimate.Interpolate
-import           Reanimate.Svg
-import           Graphics.SvgTree                         ( Texture(..)
-                                                          , ElementRef(..)
-                                                          )
-
-data Transcript = Transcript
-  { transcriptText  :: T.Text
-  , transcriptWords :: [TWord]
-  } deriving (Show)
-
-instance FromJSON Transcript where
-  parseJSON =
-    withObject "transcript" $ \o -> Transcript <$> o .: "transcript" <*> o .: "words"
-
-data TWord = TWord
-  { wordAligned     :: T.Text
-  , wordCase        :: T.Text
-  , wordStart       :: Double
-  , wordStartOffset :: Int
-  , wordEnd         :: Double
-  , wordEndOffset   :: Int
-  , wordPhones      :: [Phone]
-  , wordReference   :: T.Text
-  } deriving (Show)
-
-instance FromJSON TWord where
-  parseJSON = withObject "word" $ \o ->
-    TWord
-      <$> o
-      .:? "alignedWord"
-      .!= T.empty
-      <*> o
-      .:  "case"
-      <*> o
-      .:? "start"
-      .!= 0
-      <*> o
-      .:  "startOffset"
-      <*> o
-      .:? "end"
-      .!= 0
-      <*> o
-      .:  "endOffset"
-      <*> o
-      .:? "phones"
-      .!= []
-      <*> o
-      .:  "word"
-
-data Phone = Phone
-  { phoneDuration :: Double
-  , phoneType     :: T.Text
-  } deriving (Show)
-
-instance FromJSON Phone where
-  parseJSON = withObject "phone" $ \o -> Phone <$> o .: "duration" <*> o .: "phone"
-
--- transcript :: Transcript
--- transcript = case unsafePerformIO (decodeFileStrict "voice_transcript.json") of
---   Nothing -> error "bad json"
---   Just t  -> t
+import           Reanimate.Voice
+import           Reanimate.Builtin.Documentation
+import           Graphics.SvgTree                         ( ElementRef(..) )
 
 transcript :: Transcript
-transcript = fakeTranscript
-  "This is a fake transcript.\n\n\n\
-  \No audio has been recorded\n\n\
-  \and the timings are guessed."
-
-data Token = TokenWord Int Int T.Text | TokenComma | TokenPeriod | TokenParagraph
-  deriving (Show)
-
-lexText :: T.Text -> [Token]
-lexText = worker 0
- where
-  worker offset txt = case T.uncons txt of
-    Nothing -> []
-    Just (c, cs)
-      | isSpace c
-      -> let (w, rest) = T.span (== '\n') txt
-         in  if T.length w >= 3
-               then TokenParagraph : worker (offset + T.length w) rest
-               else worker (offset + 1) cs
-      | c == '.'
-      -> TokenPeriod : worker (offset + 1) cs
-      | c == ','
-      -> TokenComma : worker (offset + 1) cs
-      | isAlphaNum c
-      -> let (w, rest) = T.span isAlphaNum txt
-             newOffset = offset + T.length w
-         in  TokenWord offset newOffset w : worker newOffset rest
-      | otherwise
-      -> worker (offset + 1) cs
-
-fakeTranscript :: T.Text -> Transcript
-fakeTranscript input = Transcript { transcriptText  = input
-                                  , transcriptWords = worker 0 (lexText input)
-                                  }
- where
-  worker now []             = []
-  worker now (token : rest) = case token of
-    TokenWord start end w ->
-      let duration = realToFrac (end-start) * 0.1
-      in  TWord { wordAligned     = T.toLower w
-                , wordCase        = "success"
-                , wordStart       = now
-                , wordStartOffset = start
-                , wordEnd         = now + duration
-                , wordEndOffset   = end
-                , wordPhones      = []
-                , wordReference   = w
-                }
-            : worker (now + duration) rest
-    TokenComma     -> worker (now + commaPause) rest
-    TokenPeriod    -> worker (now + periodPause) rest
-    TokenParagraph -> worker (now + paragraphPause) rest
-  wpm            = 130
-  paragraphPause = 0.5
-  commaPause     = 0.1
-  periodPause    = 0.2
-
--- tweenVar :: Var s a -> Duration -> (a -> Time -> a) -> Scene s ()
--- interpolateRGBA8 :: ColorComponents -> PixelRGBA8 -> PixelRGBA8 -> (Double -> PixelRGBA8)
-toColor :: String -> PixelRGBA8
-toColor c = case mkColor c of
-  ColorRef pixel -> pixel
+transcript = loadTranscript "voice_transcript.txt"
 
 main :: IO ()
 main = reanimate $ sceneAnimation $ do
-  newSpriteSVG_ $ mkBackground "black"
-  waitOn $ forM_ (transcriptGlyphs transcript) $ \(svg, tword) -> do
+  newSpriteSVG_ $ mkBackgroundPixel rtfdBackgroundColor
+  waitOn $ forM_ (splitTranscript transcript) $ \(svg, tword) -> fork $ do
     highlighted <- newVar 0
-    s <- newSprite $ do
+    newSprite_ $ do
       v <- unVar highlighted
-      pure $ translate (-2) 2 $ scale 0.5 $ mkGroup
-        [ maskedIn v svg (withFillColor "white" $ mkRect (svgWidth svg) screenHeight)
-        , maskedOut v svg (withFillColor "grey" $ mkRect (svgWidth svg) screenHeight)
-        ]
-    fork $ do
-      wait (wordStart tword)
-      let dur = wordEnd tword - wordStart tword
-      tweenVar highlighted dur $ \v -> fromToS v 1
+      pure $ centerUsing (latex $ transcriptText transcript) $ masked
+        (wordKey tword)
+        v
+        svg
+        (withFillColor "grey" $ mkRect 1 1)
+        (withFillColor "black" $ mkRect 1 1)
+    wait (wordStart tword)
+    let dur = wordEnd tword - wordStart tword
+    tweenVar highlighted dur $ \v -> fromToS v 1
   wait 2
-
-maskedIn :: Double -> SVG -> SVG -> SVG
-maskedIn t maskSVG targetSVG = mkGroup
-  [ mkClipPath label $ removeGroups maskSVG
-  , withClipPathRef (Ref label) $ translate (x-w/2 + w * t) y targetSVG
-  ]
-  where
-    label = "word-mask-" ++ show (hash $ renderTree maskSVG)
-    (x, y, w, _h) = boundingBox maskSVG
+ where
+  wordKey tword =
+    T.unpack (wordReference tword) ++ show (wordStartOffset tword)
 
-maskedOut :: Double -> SVG -> SVG -> SVG
-maskedOut t maskSVG targetSVG = mkGroup
+{-# INLINE masked #-}
+masked :: String -> Double -> SVG -> SVG -> SVG -> SVG
+masked key t maskSVG srcSVG dstSVG = mkGroup
   [ mkClipPath label $ removeGroups maskSVG
-  , withClipPathRef (Ref label) $ translate (x+w/2 + w * t) y targetSVG
+  , withClipPathRef (Ref label)
+    $ translate (x - w / 2 + w * t) y (scaleToSize w screenHeight dstSVG)
+  , withClipPathRef (Ref label)
+    $ translate (x + w / 2 + w * t) y (scaleToSize w screenHeight srcSVG)
   ]
-  where
-    label = "word-mask-" ++ show (hash (renderTree maskSVG, renderTree targetSVG))
-    (x, y, w, _h) = boundingBox maskSVG
-
--- svgGlyphs :: Tree -> [(Tree -> Tree, DrawAttributes, Tree)]
-transcriptGlyphs :: Transcript -> [(SVG, TWord)]
-transcriptGlyphs Transcript {..}
-  | T.length textSymbols /= length gls
-  = error "Bad size"
-  | otherwise
-  = [ ( mkGroup $ take (wordEndOffset - wordStartOffset) $ drop
-        (wordStartOffset - spaces)
-        gls
-      , tword
-      )
-    | tword@TWord {..} <- transcriptWords
-    , let spaces = nSpaces wordStartOffset
-    ]
  where
-  nSpaces limit = T.length (T.filter isSpace (T.take limit transcriptText))
-  textSymbols = T.filter (not . isSpace) transcriptText
-  total       = center $ simplify $ latex transcriptText
-  gls         = [ ctx g | (ctx, _attr, g) <- svgGlyphs total ]
-
-forceLayout txt =
-  fst $ splitGlyphs [0, 1, 2, 3] (latex $ "\\fbox{\\phantom{TyhILW}}" <> txt)
-
-alignText :: SVG -> SVG
-alignText txt = translate 0 (svgHeight ref / 2) $ centerX txt
-  where ref = latex "\\fbox{Thy}"
-
--- abc, width=14.92 height=7.02
--- Thy, width=32.73 height=8.95
+  label         = "word-mask-" ++ key
+  (x, y, w, _h) = boundingBox maskSVG
diff --git a/examples/voice_triggers.hs b/examples/voice_triggers.hs
new file mode 100644
--- /dev/null
+++ b/examples/voice_triggers.hs
@@ -0,0 +1,67 @@
+#!/usr/bin/env stack
+-- stack --resolver lts-15.04 runghc --package reanimate
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ApplicativeDo #-}
+module Main where
+
+import           Control.Monad
+import qualified Data.Text                     as T
+import           Reanimate
+import           Reanimate.Voice
+import           Reanimate.Builtin.Documentation
+import           Graphics.SvgTree                         ( ElementRef(..) )
+
+transcript :: Transcript
+transcript = loadTranscript "voice_triggers.txt"
+
+transformer :: SVG -> SVG
+transformer = scale 1 . translate (-4) 0 . centerUsing (latex $ transcriptText transcript)
+
+main :: IO ()
+main = reanimate $ sceneAnimation $ do
+  newSpriteSVG_ $ mkBackgroundPixel rtfdBackgroundColor
+  waitOn $ forM_ (splitTranscript transcript) $ \(svg, tword) -> do
+    highlighted <- newVar 0
+    void $ newSprite $ do
+      v <- unVar highlighted
+      pure $ transformer $ masked (wordKey tword)
+                                  v
+                                  svg
+                                  (withFillColor "grey" $ mkRect 1 1)
+                                  (withFillColor "black" $ mkRect 1 1)
+    let dur = wordEnd tword - wordStart tword
+    fork $ do
+      wait (wordStart tword)
+      tweenVar highlighted dur $ \v -> fromToS v 1
+    fork $ do
+      wait (wordStart tword)
+      case wordReference tword of
+        "one"   -> highlight dur $ latex "1"
+        "two"   -> highlight dur $ latex "2"
+        "three" -> highlight dur $ latex "3"
+        "red"   -> highlight dur $ withFillColor "red" $ mkCircle 1
+        "green" -> highlight dur $ withFillColor "green" $ mkCircle 1
+        "blue"  -> highlight dur $ withFillColor "blue" $ mkCircle 1
+        _       -> return ()
+  wait 2
+ where
+  wordKey tword = T.unpack (wordReference tword) ++ show (wordStartOffset tword)
+  highlight dur img =
+    play
+      $ animate
+          (\t -> translate (screenWidth / 4) 0 $ scale t $ scaleToHeight 4 $ center img)
+      # signalA (bellS 2)
+      # setDuration dur
+
+{-# INLINE masked #-}
+masked :: String -> Double -> SVG -> SVG -> SVG -> SVG
+masked key t maskSVG srcSVG dstSVG = mkGroup
+  [ mkClipPath label $ removeGroups maskSVG
+  , withClipPathRef (Ref label)
+    $ translate (x - w / 2 + w * t) y (scaleToSize w screenHeight dstSVG)
+  , withClipPathRef (Ref label)
+    $ translate (x + w / 2 + w * t) y (scaleToSize w screenHeight srcSVG)
+  ]
+ where
+  label         = "word-mask-" ++ key
+  (x, y, w, _h) = boundingBox maskSVG
diff --git a/reanimate.cabal b/reanimate.cabal
--- a/reanimate.cabal
+++ b/reanimate.cabal
@@ -3,7 +3,7 @@
 --  see http://haskell.org/cabal/users-guide/
 
 name:                reanimate
-version:             0.3.2.3
+version:             0.3.3.0
 -- synopsis:
 -- description:
 license:             PublicDomain
@@ -52,7 +52,7 @@
   default-extensions: PackageImports
   exposed-modules:    Reanimate
                       Reanimate.Animation
-                      Reanimate.Signal
+                      Reanimate.Ease
                       Reanimate.Render
                       Reanimate.LaTeX
                       Reanimate.Svg
@@ -80,9 +80,9 @@
                       Reanimate.Morph.LineBend
                       Reanimate.Morph.Cache
                       Reanimate.Raster
+                      Reanimate.ColorComponents
                       Reanimate.ColorMap
                       Reanimate.ColorSpace
-                      Reanimate.Interpolate
                       Reanimate.Memo
                       Reanimate.Scene
                       Reanimate.Povray
@@ -99,6 +99,7 @@
                       Reanimate.GeoProjection
                       Reanimate.Builtin.Documentation
                       Reanimate.Builtin.Images
+                      Reanimate.Voice
   other-modules:      Reanimate.Cache
                       Reanimate.Driver
                       Reanimate.Driver.Check
@@ -111,7 +112,7 @@
                        containers, reanimate-svg >= 0.9.7.0, xml, bytestring, lens, linear, mtl, matrix,
                        JuicyPixels, attoparsec, parallel,
                        cubicbezier, websockets >= 0.12.7.0,
-                       hashable, fsnotify, open-browser, random-shuffle, base64-bytestring,
+                       hashable, fsnotify, open-browser, random, random-shuffle, base64-bytestring,
                        vector >= 0.12.0.0, colour, cassava, ansi-wl-pprint, here, temporary,
                        optparse-applicative, chiphunk >= 0.1.2.1,
                        geojson, aeson >= 1.3.0.0,
diff --git a/src/Reanimate.hs b/src/Reanimate.hs
--- a/src/Reanimate.hs
+++ b/src/Reanimate.hs
@@ -63,7 +63,7 @@
     freezeAtPercentage,
     addStatic,
     signalA,
-    -- ** Signals
+    -- ** Easing functions
     Signal,
     constantS,
     fromToS,
@@ -197,7 +197,7 @@
 import           Reanimate.Raster
 import           Reanimate.Effect
 import           Reanimate.Scene
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Reanimate.Svg
 import           Reanimate.Svg.BoundingBox
 import           Reanimate.Svg.Constructors
diff --git a/src/Reanimate/Animation.hs b/src/Reanimate/Animation.hs
--- a/src/Reanimate/Animation.hs
+++ b/src/Reanimate/Animation.hs
@@ -51,7 +51,7 @@
                                              Tree (..), xmlOfTree)
 import           Graphics.SvgTree.Printer
 import           Reanimate.Constants
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Reanimate.Svg.Constructors
 import           Text.XML.Light.Output
 
diff --git a/src/Reanimate/Builtin/Flip.hs b/src/Reanimate/Builtin/Flip.hs
--- a/src/Reanimate/Builtin/Flip.hs
+++ b/src/Reanimate/Builtin/Flip.hs
@@ -14,7 +14,7 @@
 import           Reanimate.Blender
 import           Reanimate.Raster
 import           Reanimate.Scene
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Reanimate.Transition
 import           Reanimate.Svg.Constructors
 
diff --git a/src/Reanimate/ColorComponents.hs b/src/Reanimate/ColorComponents.hs
new file mode 100644
--- /dev/null
+++ b/src/Reanimate/ColorComponents.hs
@@ -0,0 +1,99 @@
+{-# LANGUAGE RecordWildCards #-}
+module Reanimate.ColorComponents where
+
+import           Codec.Picture
+import           Codec.Picture.Types
+import           Data.Colour
+import           Data.Colour.CIE
+import           Data.Colour.CIE.Illuminant (d65)
+import           Data.Colour.RGBSpace
+import           Data.Colour.RGBSpace.HSV
+import           Data.Colour.SRGB
+import           Data.Fixed
+import           Reanimate.Ease
+
+data ColorComponents = ColorComponents
+  { colorUnpack :: Colour Double -> (Double, Double, Double)
+  , colorPack   :: Double -> Double -> Double -> Colour Double }
+
+-- | > interpolate rgbComponents yellow blue
+--
+--   <<docs/gifs/doc_rgbComponents.gif>>
+rgbComponents :: ColorComponents
+rgbComponents = ColorComponents rgbUnpack sRGB
+  where
+    rgbUnpack :: Colour Double -> (Double, Double, Double)
+    rgbUnpack c =
+      case toSRGB c of
+        RGB r g b -> (r,g,b)
+
+-- | > interpolate hsvComponents yellow blue
+--
+--   <<docs/gifs/doc_hsvComponents.gif>>
+hsvComponents :: ColorComponents
+hsvComponents = ColorComponents unpack pack
+  where
+    unpack = hsvView.toSRGB
+    pack a b c = uncurryRGB sRGB $ hsv a b c
+
+-- | > interpolate labComponents yellow blue
+--
+--   <<docs/gifs/doc_labComponents.gif>>
+labComponents :: ColorComponents
+labComponents = ColorComponents unpack pack
+  where
+    unpack = cieLABView d65
+    pack = cieLAB d65
+
+-- | > interpolate xyzComponents yellow blue
+--
+--   <<docs/gifs/doc_xyzComponents.gif>>
+xyzComponents :: ColorComponents
+xyzComponents = ColorComponents cieXYZView cieXYZ
+
+-- | > interpolate lchComponents yellow blue
+--
+--   <<docs/gifs/doc_lchComponents.gif>>
+lchComponents :: ColorComponents
+lchComponents = ColorComponents unpack pack
+  where
+    toDeg,toRad :: Double -> Double
+    toRad deg = deg/180 * pi
+    toDeg rad = rad/pi * 180
+    unpack :: Colour Double -> (Double, Double, Double)
+    unpack color =
+      let (l,a,b) = cieLABView d65 color
+          c = sqrt (a*a + b*b)
+          h :: Double
+          h = (toDeg(atan2 b a) + 360) `mod'` 360
+          isZero = round (c*10000) == (0::Integer)
+      in (l, c, if isZero then 0/0 else h)
+    pack l c h =
+      cieLAB d65 l (cos (toRad h) * c) (sin (toRad h) * c)
+
+interpolate :: ColorComponents -> Colour Double -> Colour Double -> (Double -> Colour Double)
+interpolate ColorComponents{..} from to = \d ->
+    colorPack (a1 + (a2-a1)*d) (b1 + (b2-b1)*d) (c1 + (c2-c1)*d)
+  where
+    (a1,b1,c1) = colorUnpack from
+    (a2,b2,c2) = colorUnpack to
+
+interpolateRGB8 :: ColorComponents -> PixelRGB8 -> PixelRGB8 -> (Double -> PixelRGB8)
+interpolateRGB8 comps from to = toRGB8 . interpolate comps (fromRGB8 from) (fromRGB8 to)
+
+interpolateRGBA8 :: ColorComponents -> PixelRGBA8 -> PixelRGBA8 -> (Double -> PixelRGBA8)
+interpolateRGBA8 comps from to = \t ->
+  case interp t of
+    PixelRGB8 r g b ->
+      let alpha = fromToS (fromIntegral $ pixelOpacity from) (fromIntegral $ pixelOpacity to) t
+      in PixelRGBA8 r g b (round alpha)
+  where
+    interp = interpolateRGB8 comps (dropTransparency from) (dropTransparency to)
+
+toRGB8 :: Colour Double -> PixelRGB8
+toRGB8 c = PixelRGB8 r g b
+  where
+    RGB r g b = toSRGBBounded c
+
+fromRGB8 :: PixelRGB8 -> Colour Double
+fromRGB8 (PixelRGB8 r g b) = sRGB24 r g b
diff --git a/src/Reanimate/Ease.hs b/src/Reanimate/Ease.hs
new file mode 100644
--- /dev/null
+++ b/src/Reanimate/Ease.hs
@@ -0,0 +1,107 @@
+module Reanimate.Ease
+  ( Signal
+  , constantS
+  , fromToS
+  , reverseS
+  , curveS
+  , powerS
+  , bellS
+  , oscillateS
+  , fromListS
+  , cubicBezierS
+  ) where
+
+-- | Signals are time-varying variables. Signals can be composed using function
+--   composition.
+type Signal = Double -> Double
+
+fromListS :: [(Double, Signal)] -> Signal
+fromListS fns t = worker 0 fns
+  where
+    worker _ [] = 0
+    worker now [(len, fn)] = fn (min 1 ((t-now) / min (1-now) len))
+    worker now ((len, fn):rest)
+      | now+len < t = worker (now+len) rest
+      | otherwise = fn ((t-now) / len)
+
+-- | Constant signal.
+--
+--   Example:
+--
+--   > signalA (constantS 0.5) drawProgress
+--
+--   <<docs/gifs/doc_constantS.gif>>
+constantS :: Double -> Signal
+constantS = const
+
+-- | Signal with new starting and end values.
+--
+--   Example:
+--
+--   > signalA (fromToS 0.8 0.2) drawProgress
+--
+--   <<docs/gifs/doc_fromToS.gif>>
+fromToS :: Double -> Double -> Signal
+fromToS from to t = from + (to-from)*t
+
+-- | Reverse signal order.
+--
+--   Example:
+--
+--   > signalA reverseS drawProgress
+--
+--   <<docs/gifs/doc_reverseS.gif>>
+reverseS :: Signal
+reverseS t = 1-t
+
+-- | S-curve signal. Takes a steepness parameter. 2 is a good default.
+--
+--   Example:
+--
+--   > signalA (curveS 2) drawProgress
+--
+--   <<docs/gifs/doc_curveS.gif>>
+curveS :: Double -> Signal
+curveS steepness s =
+  if s < 0.5
+    then 0.5 * (2*s)**steepness
+    else 1-0.5 * (2 - 2*s)**steepness
+
+powerS :: Double -> Signal
+powerS steepness s = s**steepness
+
+-- | Oscillate signal.
+--
+--   Example:
+--
+--   > signalA oscillateS drawProgress
+--
+--   <<docs/gifs/doc_oscillateS.gif>>
+oscillateS :: Signal
+oscillateS t =
+  if t < 1/2
+    then t*2
+    else 2-t*2
+
+-- | Bell-curve signal. Takes a steepness parameter. 2 is a good default.
+--
+--   Example:
+--
+--   > signalA (bellS 2) drawProgress
+--
+--   <<docs/gifs/doc_bellS.gif>>
+bellS :: Double -> Signal
+bellS steepness = curveS steepness . oscillateS
+
+-- | Cubic Bezier signal. Gives you a fair amount of control over how the
+--   signal will 'curve'.
+--
+--   Example:
+--
+--   > signalA (cubicBezierS (0.0, 0.8, 0.9, 1.0)) drawProgress
+--   
+--   <<docs/gifs/doc_cubicBezierS.gif>>
+cubicBezierS :: (Double, Double, Double, Double) -> Signal
+cubicBezierS (x1, x2, x3, x4) s = 
+  let ms = 1-s
+  in x1*ms^(3::Int) + 3*x2*ms^(2::Int)*s + 3*x3*ms*s^(2::Int) + x4*s^(3::Int)
diff --git a/src/Reanimate/Interpolate.hs b/src/Reanimate/Interpolate.hs
deleted file mode 100644
--- a/src/Reanimate/Interpolate.hs
+++ /dev/null
@@ -1,99 +0,0 @@
-{-# LANGUAGE RecordWildCards #-}
-module Reanimate.Interpolate where
-
-import           Codec.Picture
-import           Codec.Picture.Types
-import           Data.Colour
-import           Data.Colour.CIE
-import           Data.Colour.CIE.Illuminant (d65)
-import           Data.Colour.RGBSpace
-import           Data.Colour.RGBSpace.HSV
-import           Data.Colour.SRGB
-import           Data.Fixed
-import           Reanimate.Signal
-
-data ColorComponents = ColorComponents
-  { colorUnpack :: Colour Double -> (Double, Double, Double)
-  , colorPack   :: Double -> Double -> Double -> Colour Double }
-
--- | > interpolate rgbComponents yellow blue
---
---   <<docs/gifs/doc_rgbComponents.gif>>
-rgbComponents :: ColorComponents
-rgbComponents = ColorComponents rgbUnpack sRGB
-  where
-    rgbUnpack :: Colour Double -> (Double, Double, Double)
-    rgbUnpack c =
-      case toSRGB c of
-        RGB r g b -> (r,g,b)
-
--- | > interpolate hsvComponents yellow blue
---
---   <<docs/gifs/doc_hsvComponents.gif>>
-hsvComponents :: ColorComponents
-hsvComponents = ColorComponents unpack pack
-  where
-    unpack = hsvView.toSRGB
-    pack a b c = uncurryRGB sRGB $ hsv a b c
-
--- | > interpolate labComponents yellow blue
---
---   <<docs/gifs/doc_labComponents.gif>>
-labComponents :: ColorComponents
-labComponents = ColorComponents unpack pack
-  where
-    unpack = cieLABView d65
-    pack = cieLAB d65
-
--- | > interpolate xyzComponents yellow blue
---
---   <<docs/gifs/doc_xyzComponents.gif>>
-xyzComponents :: ColorComponents
-xyzComponents = ColorComponents cieXYZView cieXYZ
-
--- | > interpolate lchComponents yellow blue
---
---   <<docs/gifs/doc_lchComponents.gif>>
-lchComponents :: ColorComponents
-lchComponents = ColorComponents unpack pack
-  where
-    toDeg,toRad :: Double -> Double
-    toRad deg = deg/180 * pi
-    toDeg rad = rad/pi * 180
-    unpack :: Colour Double -> (Double, Double, Double)
-    unpack color =
-      let (l,a,b) = cieLABView d65 color
-          c = sqrt (a*a + b*b)
-          h :: Double
-          h = (toDeg(atan2 b a) + 360) `mod'` 360
-          isZero = round (c*10000) == (0::Integer)
-      in (l, c, if isZero then 0/0 else h)
-    pack l c h =
-      cieLAB d65 l (cos (toRad h) * c) (sin (toRad h) * c)
-
-interpolate :: ColorComponents -> Colour Double -> Colour Double -> (Double -> Colour Double)
-interpolate ColorComponents{..} from to = \d ->
-    colorPack (a1 + (a2-a1)*d) (b1 + (b2-b1)*d) (c1 + (c2-c1)*d)
-  where
-    (a1,b1,c1) = colorUnpack from
-    (a2,b2,c2) = colorUnpack to
-
-interpolateRGB8 :: ColorComponents -> PixelRGB8 -> PixelRGB8 -> (Double -> PixelRGB8)
-interpolateRGB8 comps from to = toRGB8 . interpolate comps (fromRGB8 from) (fromRGB8 to)
-
-interpolateRGBA8 :: ColorComponents -> PixelRGBA8 -> PixelRGBA8 -> (Double -> PixelRGBA8)
-interpolateRGBA8 comps from to = \t ->
-  case interp t of
-    PixelRGB8 r g b ->
-      let alpha = fromToS (fromIntegral $ pixelOpacity from) (fromIntegral $ pixelOpacity to) t
-      in PixelRGBA8 r g b (round alpha)
-  where
-    interp = interpolateRGB8 comps (dropTransparency from) (dropTransparency to)
-
-toRGB8 :: Colour Double -> PixelRGB8
-toRGB8 c = PixelRGB8 r g b
-  where
-    RGB r g b = toSRGBBounded c
-
-fromRGB8 :: PixelRGB8 -> Colour Double
-fromRGB8 (PixelRGB8 r g b) = sRGB24 r g b
diff --git a/src/Reanimate/LaTeX.hs b/src/Reanimate/LaTeX.hs
--- a/src/Reanimate/LaTeX.hs
+++ b/src/Reanimate/LaTeX.hs
@@ -1,18 +1,29 @@
 {-# LANGUAGE OverloadedStrings   #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-module Reanimate.LaTeX (latex,xelatex,latexAlign) where
+module Reanimate.LaTeX
+  ( latex
+  , latexChunks
+  , xelatex
+  , latexAlign
+  )
+where
 
-import qualified Data.ByteString   as B
-import           Data.Text         (Text)
-import qualified Data.Text         as T
-import qualified Data.Text.IO      as T
-import           Graphics.SvgTree  (Tree (..), parseSvgFile)
+import qualified Data.ByteString               as B
+import           Data.Text                                ( Text )
+import qualified Data.Text                     as T
+import qualified Data.Text.IO                  as T
+import           Graphics.SvgTree                         ( Tree(..)
+                                                          , parseSvgFile
+                                                          )
 import           Reanimate.Cache
 import           Reanimate.Misc
 import           Reanimate.Svg
 import           Reanimate.Parameters
-import           System.FilePath   (replaceExtension, takeFileName, (</>))
-import           System.IO.Unsafe  (unsafePerformIO)
+import           System.FilePath                          ( replaceExtension
+                                                          , takeFileName
+                                                          , (</>)
+                                                          )
+import           System.IO.Unsafe                         ( unsafePerformIO )
 
 -- | Invoke latex and import the result as an SVG object. SVG objects are
 --   cached to improve performance.
@@ -24,12 +35,25 @@
 --   <<docs/gifs/doc_latex.gif>>
 latex :: T.Text -> Tree
 latex tex | pNoExternals = mkText tex
-latex tex = (unsafePerformIO . (cacheMem . cacheDiskSvg) (latexToSVG "dvi" exec args)) script
-  where
-    exec = "latex"
-    args = []
-    script = mkTexScript exec args [] tex
+latex tex =
+  (unsafePerformIO . (cacheMem . cacheDiskSvg) (latexToSVG "dvi" exec args))
+    script
+ where
+  exec   = "latex"
+  args   = []
+  script = mkTexScript exec args [] tex
 
+latexChunks :: [T.Text] -> [Tree]
+latexChunks chunks | pNoExternals = map mkText chunks
+latexChunks chunks                = worker (svgGlyphs $ latex $ T.concat chunks) chunks
+ where
+  merge lst = mkGroup [ fmt svg | (fmt, _, svg) <- lst ]
+  worker [] [] = []
+  worker _ [] = error "latex chunk mismatch"
+  worker everything (x : xs) =
+    let width = length $ svgGlyphs (latex x)
+    in merge (take width everything) : worker (drop width everything) xs
+
 -- | Invoke xelatex and import the result as an SVG object. SVG objects are
 --   cached to improve performance. Xelatex has support for non-western scripts.
 --
@@ -40,12 +64,14 @@
 --   <<docs/gifs/doc_xelatex.gif>>
 xelatex :: Text -> Tree
 xelatex tex | pNoExternals = mkText tex
-xelatex tex = (unsafePerformIO . (cacheMem . cacheDiskSvg) (latexToSVG "xdv" exec args)) script
-  where
-    exec = "xelatex"
-    args = ["-no-pdf"]
-    headers = ["\\usepackage[UTF8]{ctex}"]
-    script = mkTexScript exec args headers tex
+xelatex tex =
+  (unsafePerformIO . (cacheMem . cacheDiskSvg) (latexToSVG "xdv" exec args))
+    script
+ where
+  exec    = "xelatex"
+  args    = ["-no-pdf"]
+  headers = ["\\usepackage[UTF8]{ctex}"]
+  script  = mkTexScript exec args headers tex
 
 -- | Invoke latex and import the result as an SVG object. SVG objects are
 --   cached to improve performance. This wraps the TeX code in an 'align*'
@@ -60,39 +86,58 @@
 latexAlign tex = latex $ T.unlines ["\\begin{align*}", tex, "\\end{align*}"]
 
 postprocess :: Tree -> Tree
-postprocess = lowerTransformations . scaleXY 1 (-1) . scale 0.1 . pathify
+postprocess = simplify
 
 -- executable, arguments, header, tex
 latexToSVG :: String -> String -> [String] -> Text -> IO Tree
 latexToSVG dviExt latexExec latexArgs tex = do
   latexBin <- requireExecutable latexExec
-  dvisvgm <- requireExecutable "dvisvgm"
-  withTempDir $ \tmp_dir -> withTempFile "tex" $ \tex_file -> withTempFile "svg" $ \svg_file -> do
-    let dvi_file = tmp_dir </> replaceExtension (takeFileName tex_file) dviExt
-    T.writeFile tex_file tex
-    runCmd latexBin (latexArgs ++ ["-interaction=nonstopmode", "-halt-on-error", "-output-directory="++tmp_dir, tex_file])
-    runCmd dvisvgm [ dvi_file, "--precision=5"
-                   , "--exact"    -- better bboxes.
-                   -- , "--bbox=1,1" -- increase bbox size.
-                   , "--no-fonts" -- use glyphs instead of fonts.
-                   ,"--verbosity=0", "-o",svg_file]
-    svg_data <- B.readFile svg_file
-    case parseSvgFile svg_file svg_data of
-      Nothing  -> error "Malformed svg"
-      Just svg -> return $ postprocess $ unbox $ replaceUses svg
+  dvisvgm  <- requireExecutable "dvisvgm"
+  withTempDir $ \tmp_dir -> withTempFile "tex" $ \tex_file ->
+    withTempFile "svg" $ \svg_file -> do
+      let dvi_file =
+            tmp_dir </> replaceExtension (takeFileName tex_file) dviExt
+      T.writeFile tex_file tex
+      runCmd
+        latexBin
+        (  latexArgs
+        ++ [ "-interaction=nonstopmode"
+           , "-halt-on-error"
+           , "-output-directory=" ++ tmp_dir
+           , tex_file
+           ]
+        )
+      runCmd
+        dvisvgm
+        [ dvi_file
+        , "--precision=5"
+        , "--exact"    -- better bboxes.
+        , "--no-fonts" -- use glyphs instead of fonts.
+        , "--scale=0.1,-0.1"
+        , "--verbosity=0"
+        , "-o"
+        , svg_file
+        ]
+      svg_data <- B.readFile svg_file
+      case parseSvgFile svg_file svg_data of
+        Nothing  -> error "Malformed svg"
+        Just svg -> return $ postprocess $ unbox $ replaceUses svg
 
 mkTexScript :: String -> [String] -> [Text] -> Text -> Text
-mkTexScript latexExec latexArgs texHeaders tex = T.unlines $
-  [ "% " <> T.pack (unwords (latexExec:latexArgs))
-  , "\\documentclass[preview]{standalone}"
-  , "\\usepackage{amsmath}"
-  , "\\usepackage{gensymb}"
-  ] ++ texHeaders ++
-  [ "\\usepackage[english]{babel}"
-  , "\\linespread{1}"
-  , "\\begin{document}"
-  , tex
-  , "\\end{document}" ]
+mkTexScript latexExec latexArgs texHeaders tex =
+  T.unlines
+    $  [ "% " <> T.pack (unwords (latexExec : latexArgs))
+       , "\\documentclass[preview]{standalone}"
+       , "\\usepackage{amsmath}"
+       , "\\usepackage{gensymb}"
+       ]
+    ++ texHeaders
+    ++ [ "\\usepackage[english]{babel}"
+       , "\\linespread{1}"
+       , "\\begin{document}"
+       , tex
+       , "\\end{document}"
+       ]
 
 {- Packages used by manim.
 
diff --git a/src/Reanimate/Morph/Common.hs b/src/Reanimate/Morph/Common.hs
--- a/src/Reanimate/Morph/Common.hs
+++ b/src/Reanimate/Morph/Common.hs
@@ -24,14 +24,14 @@
                                          strokeOpacity)
 import           Linear.V2
 import           Reanimate.Animation
-import           Reanimate.Interpolate
+import           Reanimate.ColorComponents
 import           Reanimate.Math.EarClip
 import           Reanimate.Math.Polygon (Polygon, mkPolygon, pAddPoints,
                                          pCentroid, pRing, pSize, pdualPolygons,
                                          polygonPoints)
 import           Reanimate.Math.SSSP
 import           Reanimate.PolyShape
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Reanimate.Svg
 
 -- Correspondence
diff --git a/src/Reanimate/Morph/Linear.hs b/src/Reanimate/Morph/Linear.hs
--- a/src/Reanimate/Morph/Linear.hs
+++ b/src/Reanimate/Morph/Linear.hs
@@ -8,7 +8,7 @@
 import           Data.Hashable
 import qualified Data.Vector            as V
 import           Linear.Vector
-import           Reanimate.Interpolate
+import           Reanimate.ColorComponents
 import           Reanimate.Math.Common
 import           Reanimate.Math.Polygon
 import           Reanimate.Morph.Cache
diff --git a/src/Reanimate/Morph/Rotational.hs b/src/Reanimate/Morph/Rotational.hs
--- a/src/Reanimate/Morph/Rotational.hs
+++ b/src/Reanimate/Morph/Rotational.hs
@@ -8,7 +8,7 @@
 import           Linear.V2
 import           Linear.Metric
 
-import           Reanimate.Signal
+import           Reanimate.Ease
 import           Reanimate.Morph.Common
 import           Reanimate.Math.Polygon
 
diff --git a/src/Reanimate/Signal.hs b/src/Reanimate/Signal.hs
deleted file mode 100644
--- a/src/Reanimate/Signal.hs
+++ /dev/null
@@ -1,107 +0,0 @@
-module Reanimate.Signal
-  ( Signal
-  , constantS
-  , fromToS
-  , reverseS
-  , curveS
-  , powerS
-  , bellS
-  , oscillateS
-  , fromListS
-  , cubicBezierS
-  ) where
-
--- | Signals are time-varying variables. Signals can be composed using function
---   composition.
-type Signal = Double -> Double
-
-fromListS :: [(Double, Signal)] -> Signal
-fromListS fns t = worker 0 fns
-  where
-    worker _ [] = 0
-    worker now [(len, fn)] = fn (min 1 ((t-now) / min (1-now) len))
-    worker now ((len, fn):rest)
-      | now+len < t = worker (now+len) rest
-      | otherwise = fn ((t-now) / len)
-
--- | Constant signal.
---
---   Example:
---
---   > signalA (constantS 0.5) drawProgress
---
---   <<docs/gifs/doc_constantS.gif>>
-constantS :: Double -> Signal
-constantS = const
-
--- | Signal with new starting and end values.
---
---   Example:
---
---   > signalA (fromToS 0.8 0.2) drawProgress
---
---   <<docs/gifs/doc_fromToS.gif>>
-fromToS :: Double -> Double -> Signal
-fromToS from to t = from + (to-from)*t
-
--- | Reverse signal order.
---
---   Example:
---
---   > signalA reverseS drawProgress
---
---   <<docs/gifs/doc_reverseS.gif>>
-reverseS :: Signal
-reverseS t = 1-t
-
--- | S-curve signal. Takes a steepness parameter. 2 is a good default.
---
---   Example:
---
---   > signalA (curveS 2) drawProgress
---
---   <<docs/gifs/doc_curveS.gif>>
-curveS :: Double -> Signal
-curveS steepness s =
-  if s < 0.5
-    then 0.5 * (2*s)**steepness
-    else 1-0.5 * (2 - 2*s)**steepness
-
-powerS :: Double -> Signal
-powerS steepness s = s**steepness
-
--- | Oscillate signal.
---
---   Example:
---
---   > signalA oscillateS drawProgress
---
---   <<docs/gifs/doc_oscillateS.gif>>
-oscillateS :: Signal
-oscillateS t =
-  if t < 1/2
-    then t*2
-    else 2-t*2
-
--- | Bell-curve signal. Takes a steepness parameter. 2 is a good default.
---
---   Example:
---
---   > signalA (bellS 2) drawProgress
---
---   <<docs/gifs/doc_bellS.gif>>
-bellS :: Double -> Signal
-bellS steepness = curveS steepness . oscillateS
-
--- | Cubic Bezier signal. Gives you a fair amount of control over how the
---   signal will 'curve'.
---
---   Example:
---
---   > signalA (cubicBezierS (0.0, 0.8, 0.9, 1.0)) drawProgress
---   
---   <<docs/gifs/doc_cubicBezierS.gif>>
-cubicBezierS :: (Double, Double, Double, Double) -> Signal
-cubicBezierS (x1, x2, x3, x4) s = 
-  let ms = 1-s
-  in x1*ms^(3::Int) + 3*x2*ms^(2::Int)*s + 3*x3*ms*s^(2::Int) + x4*s^(3::Int)
diff --git a/src/Reanimate/Svg.hs b/src/Reanimate/Svg.hs
--- a/src/Reanimate/Svg.hs
+++ b/src/Reanimate/Svg.hs
@@ -174,6 +174,7 @@
   where
     worker acc attr =
       \case
+        None -> []
         GroupTree g ->
           let acc' sub = acc (GroupTree $ g & groupChildren .~ [sub])
               attr' = (g^.drawAttributes) `mappend` attr
diff --git a/src/Reanimate/Svg/Unuse.hs b/src/Reanimate/Svg/Unuse.hs
--- a/src/Reanimate/Svg/Unuse.hs
+++ b/src/Reanimate/Svg/Unuse.hs
@@ -37,12 +37,12 @@
         Nothing  -> m
         Just tid -> Map.insert tid tree m
 
+-- FIXME: the viewbox is ignored. Can we use the viewbox as a mask?
 -- Transform out viewbox. defs and CSS rules are discarded.
 unbox :: Document -> Tree
-unbox doc@Document{_viewBox = Just (minx, minw, _width, _height)} =
+unbox doc@Document{_viewBox = Just (_minx, _minw, _width, _height)} =
   GroupTree $ defaultSvg
           & groupChildren .~ doc^.elements
-          & transform ?~ [Translate (-minx) (-minw)]
 unbox doc =
   GroupTree $ defaultSvg
     & groupChildren .~ doc^.elements
diff --git a/src/Reanimate/Transition.hs b/src/Reanimate/Transition.hs
--- a/src/Reanimate/Transition.hs
+++ b/src/Reanimate/Transition.hs
@@ -9,7 +9,7 @@
   ) where
 
 import Reanimate.Animation
-import Reanimate.Signal
+import Reanimate.Ease
 import Reanimate.Effect
 
 -- | A transition transforms one animation into another.
diff --git a/src/Reanimate/Voice.hs b/src/Reanimate/Voice.hs
new file mode 100644
--- /dev/null
+++ b/src/Reanimate/Voice.hs
@@ -0,0 +1,274 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ApplicativeDo #-}
+{-# LANGUAGE RecordWildCards #-}
+{-|
+  Reanimate can automatically synchronize animations to your voice if you have
+  a transcript and an audio recording. This works with the help of Gentle
+  (<https://lowerquality.com/gentle/>). Accuracy is not perfect but it is pretty
+  close, and it is by far the easiest way of adding narration to an animation.
+-}
+module Reanimate.Voice
+  ( Transcript(..)
+  , TWord(..)
+  , findWord        -- :: Transcript -> [Text] -> Text -> TWord
+  , findWords       -- :: Transcript -> [Text] -> Text -> [TWord]
+  , loadTranscript  -- :: FilePath -> Transcript
+  , fakeTranscript  -- :: Text -> Transcript
+  , splitTranscript -- :: Transcript -> SVG -> [(SVG, TWord)]
+  )
+where
+
+import           Data.Aeson
+import           Data.Char
+import           System.IO.Unsafe                         ( unsafePerformIO )
+import           System.Directory
+import           System.FilePath
+import           System.Process
+import           System.Exit
+import           Data.List
+import           Data.Maybe
+import qualified Data.Map                      as Map
+import           Data.Map                                 ( Map )
+import           Data.Text                                ( Text )
+import qualified Data.Text                     as T
+import qualified Data.Text.IO                  as T
+import           Reanimate.Animation                      ( SVG )
+import           Reanimate.Misc
+import           Reanimate.LaTeX
+
+data Transcript = Transcript
+  { transcriptText  :: Text
+  , transcriptKeys  :: Map Text Int
+  , transcriptWords :: [TWord]
+  } deriving (Show)
+
+instance FromJSON Transcript where
+  parseJSON = withObject "transcript" $ \o ->
+    Transcript <$> o .: "transcript" <*> pure Map.empty <*> o .: "words"
+
+data TWord = TWord
+  { wordAligned     :: Text
+  , wordCase        :: Text
+  , wordStart       :: Double -- ^ Start of pronunciation in seconds
+  , wordStartOffset :: Int    -- ^ Character index of word in transcript
+  , wordEnd         :: Double -- ^ End of pronunciation in seconds
+  , wordEndOffset   :: Int    -- ^ Last character index of word in transcript
+  , wordPhones      :: [Phone]
+  , wordReference   :: Text   -- ^ The word being pronounced.
+  } deriving (Show)
+
+instance FromJSON TWord where
+  parseJSON = withObject "word" $ \o ->
+    TWord
+      <$> o
+      .:? "alignedWord"
+      .!= T.empty
+      <*> o
+      .:  "case"
+      <*> o
+      .:? "start"
+      .!= 0
+      <*> o
+      .:  "startOffset"
+      <*> o
+      .:? "end"
+      .!= 0
+      <*> o
+      .:  "endOffset"
+      <*> o
+      .:? "phones"
+      .!= []
+      <*> o
+      .:  "word"
+
+data Phone = Phone
+  { phoneDuration :: Double
+  , phoneType     :: Text
+  } deriving (Show)
+
+instance FromJSON Phone where
+  parseJSON =
+    withObject "phone" $ \o -> Phone <$> o .: "duration" <*> o .: "phone"
+
+-- | Locate the first word that occurs after all the given keys.
+--   An error is thrown if no such word exists. An error is thrown
+--   if the keys do not exist in the transcript.
+findWord :: Transcript -> [Text] -> Text -> TWord
+findWord t keys w = case listToMaybe (findWords t keys w) of
+  Nothing    -> error $ "Word not in transcript: " ++ show (keys, w)
+  Just tword -> tword
+
+-- | Locate all words that occur after all the given keys.
+--   May return an empty list. An error is thrown
+--   if the keys do not exist in the transcript.
+findWords :: Transcript -> [Text] -> Text -> [TWord]
+findWords t [] wd =
+  [ tword | tword <- transcriptWords t, wordReference tword == wd ]
+findWords t (key : keys) wd =
+  [ tword
+  | tword <- findWords t keys wd
+  , wordStartOffset tword > Map.findWithDefault badKey key (transcriptKeys t)
+  ]
+  where badKey = error $ "Missing transcript key: " ++ show key
+
+-- | Loading a transcript does three things depending on which files are available
+--   with the same basename as the input argument:
+--   1. If a JSON file is available, it is parsed and returned.
+--   2. If an audio file is available, reanimate tries to align it by calling out to
+--      Gentle on localhost:8765/. If Gentle is not running, an error will be thrown.
+--   3. If only the text transcript is available, a fake transcript is returned,
+--      with timings roughly at 120 words per minute.
+loadTranscript :: FilePath -> Transcript
+loadTranscript path = unsafePerformIO $ do
+  rawTranscript <- T.readFile path
+  let keys           = parseTranscriptKeys rawTranscript
+      trimTranscript = cutoutKeys keys rawTranscript
+  hasJSON    <- doesFileExist jsonPath
+  transcript <- if hasJSON
+    then do
+      mbT <- decodeFileStrict jsonPath
+      case mbT of
+        Nothing -> error "bad json"
+        Just t  -> pure t
+    else do
+      hasAudio <- findWithExtension path audioExtensions
+      case hasAudio of
+        Nothing        -> return $ fakeTranscript' trimTranscript
+        Just audioPath -> withTempFile "txt" $ \txtPath -> do
+          T.writeFile txtPath trimTranscript
+          runGentleForcedAligner audioPath txtPath
+          mbT <- decodeFileStrict jsonPath
+          case mbT of
+            Nothing -> error "bad json"
+            Just t  -> pure t
+  pure $ transcript { transcriptKeys = keys }
+ where
+  jsonPath        = replaceExtension path "json"
+  audioExtensions = ["mp3", "m4a", "flac"]
+
+parseTranscriptKeys :: Text -> Map Text Int
+parseTranscriptKeys = worker Map.empty 0
+ where
+  worker keys offset txt = case T.uncons txt of
+    Nothing -> keys
+    Just ('[', cs) ->
+      let key       = T.takeWhile (/= ']') cs
+          newOffset = T.length key + 2
+      in  worker (Map.insert key offset keys)
+                 (offset + newOffset)
+                 (T.drop newOffset txt)
+    Just (_, cs) -> worker keys (offset + 1) cs
+
+cutoutKeys :: Map Text Int -> Text -> Text
+cutoutKeys keys = T.concat . worker 0 (sortOn snd (Map.toList keys))
+ where
+  worker _offset [] txt = [txt]
+  worker offset ((key, at) : xs) txt =
+    let keyLen          = T.length key + 2
+        (before, after) = T.splitAt (at - offset) txt
+    in  before : worker (at + keyLen) xs (T.drop keyLen after)
+
+findWithExtension :: FilePath -> [String] -> IO (Maybe FilePath)
+findWithExtension _path []       = return Nothing
+findWithExtension path  (e : es) = do
+  let newPath = replaceExtension path e
+  hasFile <- doesFileExist newPath
+  if hasFile then return (Just newPath) else findWithExtension path es
+
+runGentleForcedAligner :: FilePath -> FilePath -> IO ()
+runGentleForcedAligner audioFile transcriptFile = do
+  ret <- rawSystem prog args
+  case ret of
+    ExitSuccess -> return ()
+    ExitFailure e ->
+      error
+        $  "Gentle forced aligner failed with: "
+        ++ show e
+        ++ "\nIs it running locally on port 8765?"
+        ++ "\nCommand: "
+        ++ showCommandForUser prog args
+ where
+  prog = "curl"
+  args =
+    [ "--silent"
+    , "--form"
+    , "audio=@" ++ audioFile
+    , "--form"
+    , "transcript=@" ++ transcriptFile
+    , "--output"
+    , replaceExtension audioFile "json"
+    , "http://localhost:8765/transcriptions?async=false"
+    ]
+
+data Token = TokenWord Int Int Text | TokenComma | TokenPeriod | TokenParagraph
+  deriving (Show)
+
+lexText :: Text -> [Token]
+lexText = worker 0
+ where
+  worker offset txt = case T.uncons txt of
+    Nothing -> []
+    Just (c, cs)
+      | isSpace c
+      -> let (w, rest) = T.span (== '\n') txt
+         in  if T.length w >= 3
+               then TokenParagraph : worker (offset + T.length w) rest
+               else worker (offset + 1) cs
+      | c == '.'
+      -> TokenPeriod : worker (offset + 1) cs
+      | c == ','
+      -> TokenComma : worker (offset + 1) cs
+      | isAlphaNum c
+      -> let (w, rest) = T.span (\elt -> isAlphaNum elt || elt == '\'') txt
+             newOffset = offset + T.length w
+         in  TokenWord offset newOffset w : worker newOffset rest
+      | otherwise
+      -> worker (offset + 1) cs
+
+-- | Fake transcript timings at roughly 120 words per minute.
+fakeTranscript :: Text -> Transcript
+fakeTranscript rawTranscript =
+  let keys = parseTranscriptKeys rawTranscript
+      t    = fakeTranscript' (cutoutKeys keys rawTranscript)
+  in  t { transcriptKeys = keys }
+
+fakeTranscript' :: Text -> Transcript
+fakeTranscript' input = Transcript { transcriptText  = input
+                                   , transcriptKeys  = Map.empty
+                                   , transcriptWords = worker 0 (lexText input)
+                                   }
+ where
+  worker _now []             = []
+  worker now  (token : rest) = case token of
+    TokenWord start end w ->
+      let duration = realToFrac (end - start) * 0.1
+      in  TWord { wordAligned     = T.toLower w
+                , wordCase        = "success"
+                , wordStart       = now
+                , wordStartOffset = start
+                , wordEnd         = now + duration
+                , wordEndOffset   = end
+                , wordPhones      = []
+                , wordReference   = w
+                }
+            : worker (now + duration) rest
+    TokenComma     -> worker (now + commaPause) rest
+    TokenPeriod    -> worker (now + periodPause) rest
+    TokenParagraph -> worker (now + paragraphPause) rest
+  paragraphPause = 0.5
+  commaPause     = 0.1
+  periodPause    = 0.2
+
+-- | Convert the transcript text to an SVG image using LaTeX and associate
+--   each word image with its timing information.
+splitTranscript :: Transcript -> [(SVG, TWord)]
+splitTranscript Transcript {..} =
+  [ (svg, tword)
+  | tword@TWord {..} <- transcriptWords
+  , let wordLength  = wordEndOffset - wordStartOffset
+        [_, svg, _] = latexChunks
+          [ T.take wordStartOffset transcriptText
+          , T.take wordLength (T.drop wordStartOffset transcriptText)
+          , T.drop wordEndOffset transcriptText
+          ]
+  ]