CompressJpg.hs

-- |
-- Module      : Hakyll.Images.CompressJpg
-- Description : Hakyll compiler to compress Jpeg images
-- Copyright   : (c) Laurent P René de Cotret, 2019
-- License     : BSD3
-- Maintainer  : laurent.decotret@outlook.com
-- Stability   : unstable
-- Portability : portable
--
-- This module defines a Hakyll compiler, 'compressJpgCompiler', which can be used to
-- re-encode Jpeg images at a lower quality during website compilation. Original images are
-- left unchanged, but compressed images can be up to 10x smaller.
--
-- The @compressJpgCompiler@ is expected to be used like this:
--
-- @
--     import Hakyll
--     import Hakyll.Images        ( loadImage
--                                 , compressJpgCompiler
--                                 )
--
--     hakyll $ do
--
--         -- Compress all source Jpegs to a Jpeg quality of 50
--         match "images/**.jpg" $ do
--             route idRoute
--             compile $ loadImage
--                 >>= compressJpgCompiler 50
--
--         (... omitted ...)
-- @
module Hakyll.Images.CompressJpg
  ( JpgQuality,
    compressJpgCompiler,
    compressJpg,
  )
where

import Codec.Picture.Jpg (decodeJpeg)
import Codec.Picture.Saving (imageToJpg)
import Data.ByteString.Lazy (toStrict)
import Hakyll.Core.Compiler (Compiler)
import Hakyll.Core.Item (Item (..))
import Hakyll.Images.Common
  ( Image (..),
    ImageFormat (..),
    format,
    image,
  )

-- | Jpeg encoding quality, from 0 (lower quality) to 100 (best quality).
type JpgQuality = Int

-- | Compress a JPG bytestring to a certain quality setting.
-- The quality should be between 0 (lowest quality) and 100 (best quality).
-- An error is raised if the image cannot be decoded, or if the
-- encoding quality is out-of-bounds
compressJpg :: JpgQuality -> Image -> Image
compressJpg quality src =
  if (format src) /= Jpeg
    then error $ "Image is not a JPEG."
    else case decodeJpeg $ image src of
      Left _ -> error $ "Loading the image failed."
      Right dynImage ->
        if (quality < 0 || quality > 100)
          then error $ "JPEG encoding quality should be between 0 and 100."
          else Image Jpeg $ (toStrict $ imageToJpg quality dynImage)

-- | Compiler that compresses a JPG image to a certain quality setting.
-- The quality should be between 0 (lowest quality) and 100 (best quality).
-- An error is raised if the image cannot be decoded.
--
-- @
-- match "*.jpg" $ do
--     route idRoute
--     compile $ loadImage
--         >>= compressJpgCompiler 50
-- @
compressJpgCompiler :: JpgQuality -> Item Image -> Compiler (Item Image)
compressJpgCompiler quality = return . fmap (compressJpg quality)