diff --git a/examples/doc_signalO.hs b/examples/doc_signalO.hs
new file mode 100644
--- /dev/null
+++ b/examples/doc_signalO.hs
@@ -0,0 +1,27 @@
+#!/usr/bin/env stack
+-- stack runghc --package reanimate
+module Main(main) where
+
+import Reanimate
+import Reanimate.Scene
+import Reanimate.Builtin.Documentation
+import Control.Lens
+import Control.Monad
+
+main :: IO ()
+main = reanimate $ docEnv $ scene $ do
+  objs <- waitOn $ replicateM 3 $ do
+    obj1 <- newObject $ mkCircle 1
+    oModifyS obj1 $ oEasing .= id
+    oModifyS obj1 $ oRightX .= screenRight
+    fork $ oShowWith obj1 oFadeIn
+    fork $ oTweenS obj1 2 $ \t ->
+      oLeftX %= \origin -> fromToS origin screenLeft t
+    wait 1
+    oHideWith obj1 oFadeOut
+    wait (-1.5)
+    return obj1
+  dur <- queryNow
+  wait (-dur)
+  forM_ objs $ \obj -> do
+    signalO obj dur $ curveS 2
diff --git a/examples/doc_signalS.hs b/examples/doc_signalS.hs
new file mode 100644
--- /dev/null
+++ b/examples/doc_signalS.hs
@@ -0,0 +1,13 @@
+#!/usr/bin/env stack
+-- stack runghc --package reanimate
+module Main(main) where
+
+import Reanimate
+import Reanimate.Builtin.Documentation
+
+main :: IO ()
+main = reanimate $ docEnv $ playThenReverseA $ scene $ do
+  sprite <- fork $ newSpriteA $ drawCircle
+  signalS sprite 1 (curveS 2)
+  wait 1
+  signalS sprite 1 (powerS 2)
diff --git a/examples/tut_glue_povray.hs b/examples/tut_glue_povray.hs
--- a/examples/tut_glue_povray.hs
+++ b/examples/tut_glue_povray.hs
@@ -53,6 +53,8 @@
       transZ_ = T.pack $ show transZ
       rotZ_ = T.pack $ show rotZ
   in [text|
+#version 3.7;
+//Files with predefined colors and textures
 #include "colors.inc"
 
 //Place the camera
@@ -64,6 +66,8 @@
   right x*16/9
 }
 
+global_settings { assumed_gamma 1.0 }
+
 //Ambient light to "brighten up" darker pictures
 global_settings { ambient_light White*3 }
 
@@ -71,15 +75,15 @@
 background { color rgbt <0, 0, 0, 1> } // transparent
 
 polygon {
-  4,
-  <0, 0>, <0, 1>, <1, 1>, <1, 0>
+  5,
+  <0, 0>, <0, 1>, <1, 1>, <1, 0>, <0, 0>
   texture {
     pigment{
       image_map{ png "${png_}" }
     }
   }
   translate <-1/2,-1/2>
-  scale <16,9>
+  scale <16,9,1>
   rotate <0,${rotX_},${rotZ_}>
   translate <0,0,${transZ_}>
 }
diff --git a/examples/tut_glue_povray_ortho.hs b/examples/tut_glue_povray_ortho.hs
--- a/examples/tut_glue_povray_ortho.hs
+++ b/examples/tut_glue_povray_ortho.hs
@@ -60,6 +60,7 @@
       rotY_ = T.pack $ show rotY
       rotZ_ = T.pack $ show rotZ
   in [text|
+# version 3.7;
 //Files with predefined colors and textures
 #include "colors.inc"
 
@@ -74,6 +75,7 @@
   right x*16
 }
 
+global_settings { assumed_gamma 1.0 }
 
 //Ambient light to "brighten up" darker pictures
 global_settings { ambient_light White*3 }
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:             1.1.4.0
+version:             1.1.5.0
 -- synopsis:
 -- description:
 license:             PublicDomain
@@ -111,6 +111,7 @@
                       Reanimate.Scene.Sprite
                       Reanimate.Scene.Object
                       Detach
+                      POVRay
   autogen-modules:    Paths_reanimate
   if flag(no-hgeometry)
     cpp-options: -DNO_HGEOMETRY
@@ -121,7 +122,7 @@
   build-depends:
     base                 >=4.10 && <5,
     JuicyPixels          >=3.3.3,
-    aeson                >=1.3.0.0,
+    aeson                >=1.3.0.0 && <2,
     ansi-terminal        >=0.8.0.4,
     array                >=0.5.2.0,
     attoparsec           >=0.13.2.0,
diff --git a/src/Reanimate.hs b/src/Reanimate.hs
--- a/src/Reanimate.hs
+++ b/src/Reanimate.hs
@@ -103,6 +103,7 @@
   , unVar             -- :: Var s a -> Frame s a
   , spriteT           -- :: Frame s Time
   , spriteDuration    -- :: Frame s Duration
+  , signalS           -- :: Sprite s -> Duration -> Signal -> Scene s ()
   , newSprite         -- :: Frame s SVG -> Scene s (Sprite s)
   , newSprite_        -- :: Frame s SVG -> Scene s ()
   , newSpriteA        -- :: Animation -> Scene s (Sprite s)
diff --git a/src/Reanimate/Animation.hs b/src/Reanimate/Animation.hs
--- a/src/Reanimate/Animation.hs
+++ b/src/Reanimate/Animation.hs
@@ -17,6 +17,7 @@
   , Animation
   -- * Creating animations
   , mkAnimation
+  , unsafeMkAnimation
   , animate
   , staticFrame
   , pause
@@ -72,17 +73,26 @@
 -- | Animations are SVGs over a finite time.
 data Animation = Animation Duration (Time -> SVG)
 
--- | Construct an animation with a given duration.
+-- | Construct an animation with a given duration. If the duration is not
+--   positive throws 'Prelude.error'.
 mkAnimation :: Duration -> (Time -> SVG) -> Animation
-mkAnimation = Animation
+mkAnimation d f
+  | d > 0     = unsafeMkAnimation d f
+  | otherwise = error $ "Animation duration (" ++ show d ++ ") is not positive."
 
+-- | Construct an animation with a given duration, without checking that the
+--   duration is positive.
+unsafeMkAnimation :: Duration -> (Time -> SVG) -> Animation
+unsafeMkAnimation = Animation
+
 -- | Construct an animation with a duration of @1@.
 animate :: (Time -> SVG) -> Animation
 animate = Animation 1
 
--- | Create an animation with provided @duration@, which consists of stationary frame displayed for its entire duration.
+-- | Create an animation with provided @duration@, which consists of stationary
+--   frame displayed for its entire duration.
 staticFrame :: Duration -> SVG -> Animation
-staticFrame d svg = Animation d (const svg)
+staticFrame d svg = mkAnimation d (const svg)
 
 -- | Query the duration of an animation.
 duration :: Animation -> Duration
@@ -162,7 +172,8 @@
   where
     totalD = max d1 d2
 
--- | Empty animation (no SVG output) with a fixed duration.
+-- | Empty animation (no SVG output) with a fixed duration. If the duration is
+--   not positive throws 'Prelude.error'.
 --
 --   Example:
 --
@@ -170,7 +181,7 @@
 --
 --   <<docs/gifs/doc_pause.gif>>
 pause :: Duration -> Animation
-pause d = Animation d (const None)
+pause d = mkAnimation d (const None)
 
 -- | Play left animation and freeze on the last frame, then play the right
 --   animation. New duration is '@duration lhs + duration rhs@'.
@@ -183,9 +194,10 @@
 andThen :: Animation -> Animation -> Animation
 andThen a b = a `parA` (pause (duration a) `seqA` b)
 
--- | Calculate the frame that would be displayed at given point in @time@ of running @animation@.
+-- | Calculate the frame that would be displayed at given point in @time@ of
+--   running @animation@.
 --
--- The provided time parameter is clamped between 0 and animation duration.
+--   The provided time parameter is clamped between 0 and animation duration.
 frameAt :: Time -> Animation -> SVG
 frameAt t (Animation d f) = f t'
   where
@@ -196,8 +208,12 @@
 renderTree t = maybe "" ppElement $ xmlOfTree t
 
 -- | Helper function for pretty-printing SVG nodes as SVG documents.
-renderSvg :: Maybe Number -- ^ The number to use as value of the @width@ attribute of the resulting top-level svg element. If @Nothing@, the width attribute won't be rendered.
-          -> Maybe Number -- ^ Similar to previous argument, but for @height@ attribute.
+renderSvg :: Maybe Number -- ^ The number to use as value of the @width@
+                          --   attribute of the resulting top-level svg element.
+                          --   If @Nothing@, the width attribute won't be
+                          --   rendered.
+          -> Maybe Number -- ^ Similar to previous argument, but for @height@
+                          --   attribute.
           -> SVG          -- ^ SVG to render
           -> String       -- ^ String representation of SVG XML markup
 renderSvg w h t = ppDocument doc
@@ -226,7 +242,8 @@
 mapA :: (SVG -> SVG) -> Animation -> Animation
 mapA fn (Animation d f) = Animation d (fn . f)
 
--- | Freeze the last frame for @t@ seconds at the end of the animation.
+-- | Freeze the last frame for @t@ seconds at the end of the animation. If the
+--   duration is negative throws 'Prelude.error'.
 --
 --   Example:
 --
@@ -234,9 +251,12 @@
 --
 --   <<docs/gifs/doc_pauseAtEnd.gif>>
 pauseAtEnd :: Duration -> Animation -> Animation
-pauseAtEnd t a = a `andThen` pause t
+pauseAtEnd t a
+  | t == 0    = a
+  | otherwise = a `andThen` pause t
 
 -- | Freeze the first frame for @t@ seconds at the beginning of the animation.
+--   If the duration is negative throws 'Prelude.error'.
 --
 --   Example:
 --
@@ -244,10 +264,12 @@
 --
 --   <<docs/gifs/doc_pauseAtBeginning.gif>>
 pauseAtBeginning :: Duration -> Animation -> Animation
-pauseAtBeginning t a =
-    Animation t (freezeFrame 0 a) `seqA` a
+pauseAtBeginning t a
+  | t == 0    = a
+  | otherwise = mkAnimation t (freezeFrame 0 a) `seqA` a
 
--- | Freeze the first and the last frame of the animation for a specified duration.
+-- | Freeze the first and the last frame of the animation for a specified
+--   duration. If a duration is negative throws 'Prelude.error'.
 --
 --   Example:
 --
@@ -262,13 +284,14 @@
 freezeFrame t (Animation d f) = const $ f (t/d)
 
 -- | Change the duration of an animation. Animates are stretched or squished
---   (rather than truncated) to fit the new duration.
+--   (rather than truncated) to fit the new duration. If a changed duration is
+--   not positive throws 'Prelude.error'.
 adjustDuration :: (Duration -> Duration) -> Animation -> Animation
-adjustDuration fn (Animation d gen) =
-  Animation (fn d) gen
+adjustDuration fn (Animation d gen) = mkAnimation (fn d) gen
 
 -- | Set the duration of an animation by adjusting its playback rate. The
---   animation is still played from start to finish without being cropped.
+--   animation is still played from start to finish without being cropped. If
+--   the duration is not positive throws 'Prelude.error'.
 setDuration :: Duration -> Animation -> Animation
 setDuration newD = adjustDuration (const newD)
 
@@ -295,8 +318,8 @@
 playThenReverseA a = a `seqA` reverseA a
 
 -- | Loop animation @n@ number of times. This number may be fractional and it
---   may be less than 1. It must be greater than or equal to 0, though.
---   New duration is @n*duration input@.
+--   may be less than 1. It must be greater than 0, though. New duration is
+--   @n*duration input@. If the duration is not positive throws 'Prelude.error'.
 --
 --   Example:
 --
@@ -304,16 +327,21 @@
 --
 --   <<docs/gifs/doc_repeatA.gif>>
 repeatA :: Double -> Animation -> Animation
-repeatA n (Animation d f) = Animation (d*n) $ \t ->
-  f ((t*n) `mod'` 1)
-
+repeatA n (Animation d f) = mkAnimation (d*n) $ \t -> f ((t*n) `mod'` 1)
 
--- | @freezeAtPercentage time animation@ creates an animation consisting of stationary frame,
--- that would be displayed in the provided @animation@ at given @time@.
--- The duration of the new animation is the same as the duration of provided @animation@.
-freezeAtPercentage :: Time  -- ^ value between 0 and 1. The frame displayed at this point in the original animation will be displayed for the duration of the new animation
-                   -> Animation -- ^ original animation, from which the frame will be taken
-                   -> Animation -- ^ new animation consisting of static frame displayed for the duration of the original animation
+-- | @freezeAtPercentage time animation@ creates an animation consisting of
+--   stationary frame, that would be displayed in the provided @animation@ at
+--   given @time@. The duration of the new animation is the same as the duration
+--   of provided @animation@.
+freezeAtPercentage :: Time      -- ^ value between 0 and 1. The frame displayed
+                                --   at this point in the original animation
+                                --   will be displayed for the duration of the
+                                --   new animation
+                   -> Animation -- ^ original animation, from which the frame
+                                --   will be taken
+                   -> Animation -- ^ new animation consisting of static frame
+                                --   displayed for the duration of the original
+                                --   animation
 freezeAtPercentage frac (Animation d genFrame) =
   Animation d $ const $ genFrame frac
 
@@ -337,31 +365,35 @@
 signalA :: Signal -> Animation -> Animation
 signalA fn (Animation d gen) = Animation d $ gen . fn
 
--- | @takeA duration animation@ creates a new animation consisting of initial segment of
---   @animation@ of given @duration@, played at the same rate as the original animation.
+-- | @takeA duration animation@ creates a new animation consisting of initial
+--   segment of @animation@ of given @duration@, played at the same rate as the
+--   original animation. If the duration is not positive throws 'Prelude.error'.
 --
---  The @duration@ parameter is clamped to be between 0 and @animation@'s duration.
---  New animation duration is equal to (eventually clamped) @duration@.
+--   The @duration@ parameter is clamped to be up to the @animation@'s duration.
+--   New animation duration is equal to (eventually clamped) @duration@.
 takeA :: Duration -> Animation -> Animation
-takeA len (Animation d gen) = Animation len' $ \t ->
-    gen (t * len'/d)
-  where
-    len' = clamp 0 d len
+takeA len a@(Animation d gen)
+  | len >= d  = a
+  | otherwise = mkAnimation len $ \t -> gen (t * len/d)
 
--- | @dropA duration animation@ creates a new animation by dropping initial segment
---   of length @duration@ from the provided @animation@, played at the same rate as the original animation.
+-- | @dropA duration animation@ creates a new animation by dropping initial
+--   segment of length @duration@ from the provided @animation@, played at the
+--   same rate as the original animation. If the duration greater than or equal
+--   to the @animation@'s duration throws 'Prelude.error'.
 --
---  The @duration@ parameter is clamped to be between 0 and @animation@'s duration.
---  The duration of the resulting animation is duration of provided @animation@ minus (eventually clamped) @duration@.
+--   The @duration@ parameter is clamped to be non-negative. The duration of the
+--   resulting animation is duration of provided @animation@ minus @duration@.
 dropA :: Duration -> Animation -> Animation
-dropA len (Animation d gen) = Animation len' $ \t ->
-    gen (t * len'/d + len/d)
+dropA len a@(Animation d gen)
+  | len <= 0  = a
+  | otherwise = mkAnimation rest $ \t -> gen (t * rest/d + len/d)
   where
-    len' = d - clamp 0 d len
+    rest = d - len
 
--- | @lastA duration animation@ return the last @duration@ seconds of the animation.
+-- | @lastA duration animation@ return the last @duration@ seconds of the
+--   animation. If the duration is not positive throws 'Prelude.error'.
 lastA :: Duration -> Animation -> Animation
-lastA len a = dropA (duration a - len) a
+lastA len a@(Animation d _) = dropA (d - len) a
 
 clamp :: Double -> Double -> Double -> Double
 clamp a b number
diff --git a/src/Reanimate/Cache.hs b/src/Reanimate/Cache.hs
--- a/src/Reanimate/Cache.hs
+++ b/src/Reanimate/Cache.hs
@@ -104,6 +104,6 @@
     worker key sh
       | sh < 0 = []
       | otherwise =
-        case (key `shiftR` sh) `mod` 64 of
-          idx -> alphabet !! fromIntegral idx : worker key (sh-6)
-    alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+$"
+        case (key `shiftR` sh) `mod` 32 of
+          idx -> alphabet !! fromIntegral idx : worker key (sh-5)
+    alphabet = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"
diff --git a/src/Reanimate/Driver/Check.hs b/src/Reanimate/Driver/Check.hs
--- a/src/Reanimate/Driver/Check.hs
+++ b/src/Reanimate/Driver/Check.hs
@@ -11,6 +11,7 @@
 import           Control.Monad                (forM_)
 import           Data.Maybe                   (fromMaybe, listToMaybe)
 import           Data.Version                 (Version (Version), parseVersion, showVersion)
+import           POVRay                       (povrayApp)
 import           Reanimate.Driver.Magick      (magickCmd)
 import           Reanimate.Misc               (runCmd_)
 import           System.Console.ANSI.Codes
@@ -88,7 +89,7 @@
 hasDvisvgm = hasProgram "dvisvgm"
 
 hasPovray :: IO (Either String String)
-hasPovray = hasProgram "povray"
+hasPovray = hasProgram povrayApp
 
 hasFFmpeg :: IO (Either String String)
 hasFFmpeg = checkMinVersion minVersion <$> ffmpegVersion
@@ -120,7 +121,7 @@
 hasInkscape :: IO (Either String String)
 hasInkscape = checkMinVersion minVersion <$> inkscapeVersion
   where
-    minVersion = Version [0,92] []
+    minVersion = Version [1,0] []
 
 hasMagick :: IO (Either String String)
 hasMagick = checkMinVersion minVersion <$> magickVersion
diff --git a/src/Reanimate/Math/Triangulate.hs b/src/Reanimate/Math/Triangulate.hs
--- a/src/Reanimate/Math/Triangulate.hs
+++ b/src/Reanimate/Math/Triangulate.hs
@@ -46,7 +46,7 @@
 import           Data.Ext
 import           Data.Geometry.PlanarSubdivision                      (PolygonFaceData)
 import           Data.Geometry.Point
-import           Data.Geometry.Polygon
+import           Data.Geometry.Polygon                                (SimplePolygon, fromPoints)
 import qualified Data.IntSet                                          as ISet
 import qualified Data.PlaneGraph as Geo
 import           Data.Proxy
diff --git a/src/Reanimate/Misc.hs b/src/Reanimate/Misc.hs
--- a/src/Reanimate/Misc.hs
+++ b/src/Reanimate/Misc.hs
@@ -7,6 +7,7 @@
     withTempFile,
     renameOrCopyFile,
     getReanimateCacheDirectory,
+    fileUri
   )
 where
 
@@ -118,3 +119,12 @@
     -- Incrementing this value invalidates all cached results.
     cacheVersion :: Int
     cacheVersion = 0
+
+-- | A valid file URI is file://<hostname>/<path>. If <hostname> is absent, it
+--   is file:///<path>. On Windows, absolute paths begin (for example) "C:\".
+fileUri :: FilePath -> String
+fileUri path = "file://" <> path'
+ where
+  path' = case path of
+    '/' : _ -> path
+    _ -> '/' : path
diff --git a/src/Reanimate/Povray.hs b/src/Reanimate/Povray.hs
--- a/src/Reanimate/Povray.hs
+++ b/src/Reanimate/Povray.hs
@@ -1,7 +1,25 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-|
-  [Povray](http://povray.org/) is a scriptable raytracer. All povray functions
-  are cached and will reuse images when scripts stay the same.
+  [POV-Ray](http://povray.org/) (the Persistance of Vision Raytracer) is a
+  scriptable raytracer. All POV-Ray functions are cached and will reuse images
+  when scripts stay the same.
+
+  The functions come in two versions, for example 'povray' and 'povray''. The
+  versions without an apostrophe character yield a 'Tree'. The versions with a
+  final apostrophe character yield a 'FilePath' of a PNG file containing the
+  resulting image.
+
+  The functions differ in their use of the POV-Ray @+W@ (width), @+H@ (height)
+  and @+A@ or @-A@ (anti-aliasing) switches, as set out below. Resolutions are
+  \'width x height\'.
+
+  Example of use:
+
+  > povray args script
+
+  where @args@ is a list of other POV-Ray command-line switches (if any), and
+  @script@ is a POV-Ray scene description specified in the POV-Ray /scene/
+  /description language/.
 -}
 module Reanimate.Povray
   ( povray
@@ -16,18 +34,20 @@
 
 import           Data.Hashable              (Hashable (hash))
 import           Data.Text                  (Text)
-import qualified Data.Text                  as T
-import qualified Data.Text.IO               as T
+import qualified Data.Text                  as T (concat, pack)
+import qualified Data.Text.IO               as T (writeFile)
 import           Graphics.SvgTree           (Tree)
+import           POVRay                     (runPOVRay)
 import           Reanimate.Cache            (cacheFile, encodeInt)
 import           Reanimate.Constants        (screenHeight, screenWidth)
-import           Reanimate.Misc             (requireExecutable, runCmd)
 import           Reanimate.Parameters       (pNoExternals)
 import           Reanimate.Raster           (mkImage)
 import           Reanimate.Svg.Constructors (mkText)
 import           System.FilePath            (replaceExtension, (<.>))
 import           System.IO.Unsafe           (unsafePerformIO)
 
+
+
 povrayRaw :: [String] -> Text -> Tree
 povrayRaw args script =
   unsafePerformIO $ mkPovrayImage args script
@@ -36,53 +56,53 @@
 povrayRaw' args script =
   unsafePerformIO $ mkPovrayImage' args script
 
--- | Run the povray raytracer with a default resolution of 320x180
+-- | Run the POV-Ray raytracer with a default resolution of 320x180
 --   and antialiasing enabled. The resulting image is scaled to fit
 --   the screen exactly.
 povray :: [String] -> Text -> Tree
-povray args = povrayRaw (["+H180","+W320", "+A"] ++ args)
+povray args = povrayRaw (["+H180", "+W320", "+A"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 320x180
+-- | Run the POV-Ray raytracer with a default resolution of 320x180
 --   and antialiasing enabled. The FilePath points to a PNG file
 --   containing the resulting image.
 povray' :: [String] -> Text -> FilePath
-povray' args = povrayRaw' (["+H180","+W320", "+A"] ++ args)
+povray' args = povrayRaw' (["+H180", "+W320", "+A"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 320x180
+-- | Run the POV-Ray raytracer with a default resolution of 320x180
 --   but without antialiasing. The resulting image is scaled to fit
 --   the screen exactly.
 povrayQuick :: [String] -> Text -> Tree
-povrayQuick args = povrayRaw (["+H180","+W320"] ++ args)
+povrayQuick args = povrayRaw (["+H180", "+W320"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 320x180
+-- | Run the POV-Ray raytracer with a default resolution of 320x180
 --   but without antialiasing. The FilePath points to a PNG file
 --   containing the resulting image.
 povrayQuick' :: [String] -> Text -> FilePath
-povrayQuick' args = povrayRaw' (["+H180","+W320"] ++ args)
+povrayQuick' args = povrayRaw' (["+H180", "+W320"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 1440x2560
---   and antialiasing enabled. The FilePath points to a PNG file
---   containing the resulting image.
+-- | Run the POV-Ray raytracer with a default resolution of 2560x1440
+--   and antialiasing enabled. The resulting image is scaled to fit
+--   the screen exactly.
 povraySlow :: [String] -> Text -> Tree
-povraySlow args = povrayRaw (["+H1440","+W2560", "+A"] ++ args)
+povraySlow args = povrayRaw (["+H1440", "+W2560", "+A"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 1440x2560
+-- | Run the POV-Ray raytracer with a default resolution of 2560x1440
 --   and antialiasing enabled. The FilePath points to a PNG file
 --   containing the resulting image.
 povraySlow' :: [String] -> Text -> FilePath
-povraySlow' args = povrayRaw' (["+H1440","+W2560", "+A"] ++ args)
+povraySlow' args = povrayRaw' (["+H1440", "+W2560", "+A"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 2160x3840
---   and antialiasing enabled. The FilePath points to a PNG file
---   containing the resulting image.
+-- | Run the POV-Ray raytracer with a default resolution of 3840x2160
+--   and antialiasing enabled. The resulting image is scaled to fit
+--   the screen exactly.
 povrayExtreme :: [String] -> Text -> Tree
-povrayExtreme args = povrayRaw (["+H2160","+W3840", "+A"] ++ args)
+povrayExtreme args = povrayRaw (["+H2160", "+W3840", "+A"] ++ args)
 
--- | Run the povray raytracer with a default resolution of 2160x3840
+-- | Run the POV-Ray raytracer with a default resolution of 3840x2160
 --   and antialiasing enabled. The FilePath points to a PNG file
 --   containing the resulting image.
 povrayExtreme' :: [String] -> Text -> FilePath
-povrayExtreme' args = povrayRaw' (["+H2160","+W3840", "+A"] ++ args)
+povrayExtreme' args = povrayRaw' (["+H2160", "+W3840", "+A"] ++ args)
 
 mkPovrayImage :: [String] -> Text -> IO Tree
 mkPovrayImage _ script | pNoExternals = pure $ mkText script
@@ -92,10 +112,10 @@
 mkPovrayImage' :: [String] -> Text -> IO FilePath
 mkPovrayImage' _ _ | pNoExternals = pure "/povray/has/been/disabled"
 mkPovrayImage' args script = cacheFile template $ \target -> do
-    exec <- requireExecutable "povray"
-    let pov_file = replaceExtension target "pov"
-    T.writeFile pov_file script
-    runCmd exec (args ++ ["-D","+UA", pov_file, "+o"++target])
-  where
-    template = encodeInt (hash key) <.> "png"
-    key = T.concat (script:map T.pack args)
+  let pov_file = replaceExtension target "pov"
+      otherArgs = ["-D", "+UA", "+I" ++ pov_file, "+O" ++ target]
+  T.writeFile pov_file script
+  runPOVRay $ args ++ otherArgs
+ where
+  template = encodeInt (hash key) <.> "png"
+  key = T.concat (script:map T.pack args)
diff --git a/src/Reanimate/Raster.hs b/src/Reanimate/Raster.hs
--- a/src/Reanimate/Raster.hs
+++ b/src/Reanimate/Raster.hs
@@ -43,8 +43,9 @@
 import           Reanimate.Cache             (cacheFile, encodeInt)
 import           Reanimate.Constants         (screenHeight, screenWidth)
 import           Reanimate.Driver.Magick     (magickCmd)
-import           Reanimate.Misc              (getReanimateCacheDirectory, renameOrCopyFile,
-                                              requireExecutable, runCmd)
+import           Reanimate.Misc              (fileUri, getReanimateCacheDirectory,
+                                              renameOrCopyFile, requireExecutable,
+                                              runCmd)
 import           Reanimate.Parameters        (Height, Raster (RasterNone), Width, pHeight,
                                               pNoExternals, pRaster, pRootDirectory, pWidth)
 import           Reanimate.Render            (applyRaster, requireRaster)
@@ -122,7 +123,7 @@
     &  Svg.imageHeight
     .~ Svg.Num height
     &  Svg.imageHref
-    .~ ("file://" ++ target)
+    .~ (fileUri target)
     &  Svg.imageCornerUpperLeft
     .~ (Svg.Num (-width / 2), Svg.Num (-height / 2))
     &  Svg.imageAspectRatio
diff --git a/src/Reanimate/Render.hs b/src/Reanimate/Render.hs
--- a/src/Reanimate/Render.hs
+++ b/src/Reanimate/Render.hs
@@ -413,9 +413,8 @@
 applyRaster RasterAuto     _    = return ()
 applyRaster RasterInkscape path = runCmd
   "inkscape"
-  [ "--without-gui"
-  , "--file=" ++ path
-  , "--export-png=" ++ replaceExtension path "png"
+  ["--export-type=png"
+  , path
   ]
 applyRaster RasterRSvg path = runCmd
   "rsvg-convert"
diff --git a/src/Reanimate/Scene.hs b/src/Reanimate/Scene.hs
--- a/src/Reanimate/Scene.hs
+++ b/src/Reanimate/Scene.hs
@@ -20,6 +20,7 @@
     waitOn, -- :: Scene s a -> Scene s a
     adjustZ, -- :: (ZIndex -> ZIndex) -> Scene s a -> Scene s a
     withSceneDuration, -- :: Scene s () -> Scene s Duration
+    signalS, -- Signal -> Scene s a -> Scene s a
 
     -- * Variables
     Var,
@@ -56,6 +57,7 @@
     -- * Object API
     Object,
     ObjectData,
+    signalO,
     oNew,
     newObject,
     oModify,
diff --git a/src/Reanimate/Scene/Core.hs b/src/Reanimate/Scene/Core.hs
--- a/src/Reanimate/Scene/Core.hs
+++ b/src/Reanimate/Scene/Core.hs
@@ -73,6 +73,24 @@
             )
     )
 
+-- -- | Apply easing function to all render elements created by the scene
+-- --   in the timespan from now to the scene duration.
+-- --
+-- --   Note that this does not affect time as seen by `queryNow` or any
+-- --   time-dependent variables or object properties.
+-- signalS :: Signal -> Scene s a -> Scene s a
+-- signalS signal (M action) = M $ \now -> do
+--   (a, s, p, gens) <- action now
+--   let action_dur = max s p
+--       modify_t t
+--         | t < now            = t
+--         | t > now+action_dur = t
+--         | otherwise          = now + signal ((t-now) / action_dur) * action_dur
+--   let applyS gen = do
+--         fn <- gen
+--         return $ \dur t -> fn dur (modify_t t)
+--   return (a, s, p, map applyS gens)
+
 -- | Execute actions in a scene without advancing the clock. Note that scenes do not end before
 --   all forked actions have completed.
 --
diff --git a/src/Reanimate/Scene/Object.hs b/src/Reanimate/Scene/Object.hs
--- a/src/Reanimate/Scene/Object.hs
+++ b/src/Reanimate/Scene/Object.hs
@@ -18,7 +18,7 @@
 import           Reanimate.Svg
 
 import           Reanimate.Scene.Core   (Scene, fork, scene, wait)
-import           Reanimate.Scene.Sprite (Sprite, newSprite, newSpriteA', play, spriteModify, unVar)
+import           Reanimate.Scene.Sprite (Sprite, newSprite, newSpriteA', play, spriteModify, unVar, signalS)
 import           Reanimate.Scene.Var    (Var, modifyVar, newVar, readVar, tweenVar)
 
 -------------------------------------------------------
@@ -244,6 +244,10 @@
 
 -------------------------------------------------------------------------------
 -- Object modifiers
+
+-- | Apply easing function before rendering object.
+signalO :: Object s a -> Duration -> Signal -> Scene s ()
+signalO obj dur signal = signalS (objectSprite obj) dur signal
 
 -- | Modify object properties.
 oModify :: Object s a -> (ObjectData a -> ObjectData a) -> Scene s ()
diff --git a/src/Reanimate/Scene/Sprite.hs b/src/Reanimate/Scene/Sprite.hs
--- a/src/Reanimate/Scene/Sprite.hs
+++ b/src/Reanimate/Scene/Sprite.hs
@@ -16,6 +16,7 @@
                                        wait)
 import           Reanimate.Scene.Var  (Var (..), newVar, readVar, unpackVar)
 import           Reanimate.Transition (Transition, overlapT)
+import           Reanimate.Ease       (Signal)
 
 -- | Create and render a variable. The rendering will be born at the current timestamp
 --   and will persist until the end of the scene.
@@ -57,7 +58,7 @@
 
 -- | Sprites are animations with a given time of birth as well as a time of death.
 --   They can be controlled using variables, tweening, and effects.
-data Sprite s = Sprite Time (STRef s (Duration, ST s (Duration -> Time -> SVG -> (SVG, ZIndex))))
+data Sprite s = Sprite Time (STRef s (Time -> Time)) (STRef s (Duration, ST s (Duration -> Time -> SVG -> (SVG, ZIndex))))
 
 -- | Sprite frame generator. Generates frames over time in a stateful environment.
 newtype Frame s a = Frame {unFrame :: ST s (Time -> Duration -> Time -> a)}
@@ -113,13 +114,16 @@
 newSprite :: Frame s SVG -> Scene s (Sprite s)
 newSprite render = do
   now <- queryNow
+  tmod <- liftST $ newSTRef id
   ref <- liftST $ newSTRef (-1, return $ \_d _t svg -> (svg, 0))
   addGen $ do
     fn <- unFrame render
+    time_fn <- readSTRef tmod
     (spriteDur, spriteEffectGen) <- readSTRef ref
     spriteEffect <- spriteEffectGen
-    return $ \d absT ->
-      let relD = (if spriteDur < 0 then d else spriteDur) - now
+    return $ \d absT_ ->
+      let absT = time_fn absT_
+          relD = (if spriteDur < 0 then d else spriteDur) - now
           relT = absT - now
           -- Sprite is live [now;duration[
           -- If we're at the end of a scene, sprites
@@ -131,7 +135,7 @@
        in if inTimeSlice || isLastFrame
             then spriteEffect relD relT (fn absT relD relT)
             else (None, 0)
-  return $ Sprite now ref
+  return $ Sprite now tmod ref
 
 -- | Create new sprite defined by a frame generator. The sprite will die at
 --   the end of the scene.
@@ -224,7 +228,7 @@
 --
 --   <<docs/gifs/doc_destroySprite.gif>>
 destroySprite :: Sprite s -> Scene s ()
-destroySprite (Sprite _ ref) = do
+destroySprite (Sprite _ _tmod ref) = do
   now <- queryNow
   liftST $
     modifySTRef ref $ \(ttl, render) ->
@@ -232,7 +236,7 @@
 
 -- | Low-level frame modifier.
 spriteModify :: Sprite s -> Frame s ((SVG, ZIndex) -> (SVG, ZIndex)) -> Scene s ()
-spriteModify (Sprite born ref) modFn = liftST $
+spriteModify (Sprite born _tmod ref) modFn = liftST $
   modifySTRef ref $ \(ttl, renderGen) ->
     ( ttl,
       do
@@ -242,6 +246,19 @@
           let absT = relT + born in modRender absT relD relT . render relD relT
     )
 
+-- | Apply easing function before rendering sprite.
+signalS :: Sprite s -> Duration -> Signal -> Scene s ()
+signalS (Sprite _born tmod _ref) dur signal = do
+  now <- queryNow
+  let modify_t t
+        | t < now     = t
+        | t > now+dur = t
+        | otherwise   = now + signal ((t-now) / dur) * dur
+  liftST $
+    modifySTRef tmod $ \fn -> modify_t . fn
+    
+
+
 -- | Map the SVG output of a sprite.
 --
 --   Example:
@@ -254,7 +271,7 @@
 --
 --   <<docs/gifs/doc_spriteMap.gif>>
 spriteMap :: Sprite s -> (SVG -> SVG) -> Scene s ()
-spriteMap sprite@(Sprite born _) fn = do
+spriteMap sprite@(Sprite born _ _) fn = do
   now <- queryNow
   let tDelta = now - born
   spriteModify sprite $ do
@@ -272,7 +289,7 @@
 --
 --   <<docs/gifs/doc_spriteTween.gif>>
 spriteTween :: Sprite s -> Duration -> (Double -> SVG -> SVG) -> Scene s ()
-spriteTween sprite@(Sprite born _) dur fn = do
+spriteTween sprite@(Sprite born _ _) dur fn = do
   now <- queryNow
   let tDelta = now - born
   spriteModify sprite $ do
@@ -314,7 +331,7 @@
 --
 --   <<docs/gifs/doc_spriteE.gif>>
 spriteE :: Sprite s -> Effect -> Scene s ()
-spriteE (Sprite born ref) effect = do
+spriteE (Sprite born _tmod ref) effect = do
   now <- queryNow
   liftST $
     modifySTRef ref $ \(ttl, renderGen) ->
@@ -340,7 +357,7 @@
 --
 --   <<docs/gifs/doc_spriteZ.gif>>
 spriteZ :: Sprite s -> ZIndex -> Scene s ()
-spriteZ (Sprite born ref) zindex = do
+spriteZ (Sprite born _tmod ref) zindex = do
   now <- queryNow
   liftST $
     modifySTRef ref $ \(ttl, renderGen) ->
diff --git a/src/Reanimate/Svg/BoundingBox.hs b/src/Reanimate/Svg/BoundingBox.hs
--- a/src/Reanimate/Svg/BoundingBox.hs
+++ b/src/Reanimate/Svg/BoundingBox.hs
@@ -29,7 +29,9 @@
 --
 --  Note: Bounding boxes are computed on a best-effort basis and will not work
 --        in all cases. The only supported SVG nodes are: path, circle, polyline,
---        ellipse, line, rectangle, image. All other nodes return (0,0,0,0).
+--        ellipse, line, rectangle, image and svg. All other nodes return (0,0,0,0).
+--        The box for the svg node is based on the document's width and height
+--        (if both are present).
 boundingBox :: Tree -> (Double, Double, Double, Double)
 boundingBox t =
     case svgBoundingPoints t of
@@ -76,36 +78,36 @@
 svgBoundingPoints :: Tree -> [RPoint]
 svgBoundingPoints t = map (Transform.transformPoint m) $
     case t of
-      None            -> []
-      UseTree{}       -> []
-      GroupTree g     -> concatMap svgBoundingPoints (g^.groupChildren)
-      SymbolTree g    -> concatMap svgBoundingPoints (g^.groupChildren)
-      FilterTree{}    -> []
+      None             -> []
+      UseTree{}        -> []
+      GroupTree g      -> concatMap svgBoundingPoints (g ^. groupChildren)
+      SymbolTree g     -> concatMap svgBoundingPoints (g ^. groupChildren)
+      FilterTree{}     -> []
       DefinitionTree{} -> []
-      PathTree p      -> linePoints $ toLineCommands (p^.pathDefinition)
-      CircleTree c    -> circleBoundingPoints c
-      PolyLineTree pl -> pl ^. polyLinePoints
-      EllipseTree e   -> ellipseBoundingPoints e
-      LineTree line   -> map pointToRPoint [line^.linePoint1, line^.linePoint2]
-      RectangleTree rect ->
-        case pointToRPoint (rect^.rectUpperLeftCorner) of
-          V2 x y -> V2 x y :
-            case mapTuple (fmap $ toUserUnit defaultDPI) (rect^.rectWidth, rect^.rectHeight) of
-              (Just (Num w), Just (Num h)) -> [V2 (x+w) (y+h)]
-              _                            -> []
-      TextTree{}      -> []
-      ImageTree img   ->
-        case (img^.imageCornerUpperLeft, img^.imageWidth, img^.imageHeight) of
-          ((Num x, Num y), Num w, Num h) ->
-            [V2 x y, V2 (x+w) (y+h)]
-          _ -> []
+      PathTree p       -> linePoints $ toLineCommands (p ^. pathDefinition)
+      CircleTree c     -> circleBoundingPoints c
+      PolyLineTree pl  -> pl ^. polyLinePoints
+      EllipseTree e    -> ellipseBoundingPoints e
+      LineTree l       -> map pointToRPoint [l ^. linePoint1, l ^. linePoint2]
+      RectangleTree r  ->
+        let p = pointToRPoint (r ^. rectUpperLeftCorner)
+            mDims = (r ^. rectWidth, r ^. rectHeight)
+        in  rectPoints p mDims
+      TextTree{}       -> []
+      ImageTree img    ->
+        let p = pointToRPoint (img ^. imageCornerUpperLeft)
+            dims = (img ^. imageWidth, img ^. imageHeight)
+        in  rectPoints' p dims
       MeshGradientTree{} -> []
-      _ -> []
+      SvgTree d        -> let mDims = (d ^. documentWidth, d ^. documentHeight)
+                          in  rectPoints (V2 0 0) mDims
+      _                -> []
   where
-    m = Transform.mkMatrix (t^.transform)
+    m = Transform.mkMatrix (t ^. transform)
     mapTuple f = f *** f
+    toUserUnit' = toUserUnit defaultDPI
     pointToRPoint p =
-      case mapTuple (toUserUnit defaultDPI) p of
+      case mapTuple toUserUnit' p of
         (Num x, Num y) -> V2 x y
         _              -> error "Reanimate.Svg.svgBoundingPoints: Unrecognized number format."
 
@@ -113,7 +115,7 @@
       let (xnum, ynum) = circ ^. circleCenter
           rnum = circ ^. circleRadius
       in case mapMaybe unpackNumber [xnum, ynum, rnum] of
-        [x, y, r] -> [ V2 (x + r * cos angle) (y + r * sin angle) | angle <- [0, pi/10 .. 2 * pi]]
+        [x, y, r] -> ellipsePoints x y r r
         _         -> []
 
     ellipseBoundingPoints e =
@@ -121,10 +123,23 @@
           xrnum = e ^. ellipseXRadius
           yrnum = e ^. ellipseYRadius
       in case mapMaybe unpackNumber [xnum, ynum, xrnum, yrnum] of
-        [x,y,xr,yr] -> [V2 (x + xr * cos angle) (y + yr * sin angle) | angle <- [0, pi/10 .. 2 * pi]]
-        _ -> []
+        [x, y, xr, yr] -> ellipsePoints x y xr yr
+        _              -> []
 
+    ellipsePoints x y xr yr = [ V2 (x + xr * cos angle) (y + yr * sin angle)
+                              | angle <- [0, pi/10 .. 2 * pi] ]
+
+    rectPoints p mDims = case mDims of
+                           (Just w, Just h) -> rectPoints' p (w, h)
+                           _ -> [p]
+
+    rectPoints' p@(V2 x y) dims =
+      p : case mapTuple toUserUnit' dims of
+            ((Num w), (Num h)) -> let (x', y') = (x + w, y + h)
+                                  in  [V2 x' y, V2 x' y', V2 x y']
+            _ -> []
+
     unpackNumber n =
-      case toUserUnit defaultDPI n of
+      case toUserUnit' n of
         Num d -> Just d
         _     -> Nothing
diff --git a/src/Reanimate/Svg/Constructors.hs b/src/Reanimate/Svg/Constructors.hs
--- a/src/Reanimate/Svg/Constructors.hs
+++ b/src/Reanimate/Svg/Constructors.hs
@@ -20,6 +20,7 @@
   , withId
   , withStrokeColor
   , withStrokeColorPixel
+  , withStrokeOpacity
   , withStrokeDashArray
   , withStrokeLineJoin
   , withFillColor
@@ -192,10 +193,12 @@
   where
     (x, y, w, h) = boundingBox a
 
--- | Create 'Texture' based on SVG color name.
---   See <https://en.wikipedia.org/wiki/Web_colors#X11_color_names> for the list of available names.
---   If the provided name doesn't correspond to valid SVG color name, white-ish color is used.
+-- | Create 'Texture' based on a SVG color name or @\"none\"@. If the provided
+--   'String' is not a valid value, a white-ish color is used.
+--   See <https://en.wikipedia.org/wiki/Web_colors#X11_color_names> for the list
+--   of available color names.
 mkColor :: String -> Texture
+mkColor "none" = FillNone
 mkColor name =
   case Map.lookup (T.pack name) svgNamedColors of
     Nothing -> ColorRef (PixelRGBA8 240 248 255 255)
@@ -209,6 +212,10 @@
 withStrokeColorPixel :: PixelRGBA8 -> Tree -> Tree
 withStrokeColorPixel color = strokeColor .~ pure (ColorRef color)
 
+-- | See <https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-opacity>
+withStrokeOpacity :: Double -> Tree -> Tree
+withStrokeOpacity opacity = strokeOpacity ?~ realToFrac opacity
+
 -- | See <https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-dasharray>
 withStrokeDashArray :: [Double] -> Tree -> Tree
 withStrokeDashArray arr = strokeDashArray .~ pure (map Num arr)
@@ -217,7 +224,10 @@
 withStrokeLineJoin :: LineJoin -> Tree -> Tree
 withStrokeLineJoin ljoin = strokeLineJoin .~ pure ljoin
 
--- | See <https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/fill>
+-- | See <https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/fill>.
+--
+-- @withFillColor color@ takes the same approach to @color@ as 'mkColor'. For
+-- RGB color values, use 'withFillColorPixel'.
 withFillColor :: String -> Tree -> Tree
 withFillColor color = fillColor .~ pure (mkColor color)
 
diff --git a/unix/POVRay.hs b/unix/POVRay.hs
new file mode 100644
--- /dev/null
+++ b/unix/POVRay.hs
@@ -0,0 +1,16 @@
+module POVRay
+  ( povrayApp
+  , runPOVRay
+  ) where
+
+import Reanimate.Misc (requireExecutable, runCmd)
+
+-- | Name of the POV-Ray executable
+povrayApp :: String
+povrayApp = "povray"
+
+-- | Run the POV-Ray executable with arguments
+runPOVRay :: [String] -> IO ()
+runPOVRay args = do
+  exec <- requireExecutable povrayApp
+  runCmd exec args
diff --git a/windows/POVRay.hs b/windows/POVRay.hs
new file mode 100644
--- /dev/null
+++ b/windows/POVRay.hs
@@ -0,0 +1,50 @@
+module POVRay
+  ( povrayApp
+  , runPOVRay
+  ) where
+
+import           Reanimate.Misc       (requireExecutable, runCmd)
+import           System.IO            (hClose, hPutStrLn)
+import           System.IO.Temp       (withSystemTempFile)
+
+-- | Name of the POV-Ray executable
+povrayApp :: String
+povrayApp = "pvengine64"  -- Assumes 64 bit Windows
+
+-- | Run the POV-Ray executable with arguments
+runPOVRay :: [String] -> IO ()
+runPOVRay args = do
+  exec <- requireExecutable povrayApp
+  let exec' = '"' : exec ++ "\""  -- Wrap exec in "" because the path is likely
+                                  -- to includes spaces
+      args' = "/EXIT" : args  -- Note [/EXIT special command-line option]
+      command = concatMap (' ':) (exec' : args')
+  -- Note [Use of a batch file]
+  withSystemTempFile "pvcommand.bat" $ \path h -> do
+    hPutStrLn h command
+    hClose h
+    runCmd path []
+
+{-
+Note [/EXIT special command-line option]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+"The /EXIT command tells POV-Ray to perform the render given by the other
+command-line options, combined with previously-set options such as internal
+command-line settings, INI file, source file, and so forth, and then to exit. By
+default, if this switch is not present, POV-Ray for Windows will remain running
+after the render is complete. This switch only applies to renders started by
+other options on the command-line. It will not affect renders started manually
+from within POV-Ray for Windows itself."
+source: https://www.povray.org/documentation/view/3.6.1/603/
+
+Note [Use of batch file]
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+POV-Ray for Windows assumes that anything on the command-line that is not a
+switch is the name of an INI file. Switches are preceded by a plus or minus.
+However, System.Process.createProcess_ wraps all arguments in "". POV_Ray for
+Windows appears to interpret a switch wrapped in "" as the name of an INI file.
+
+The work around used here is to write the POV-Ray command line to a batch file.
+-}