Library/PNG
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Revision as of 05:41, 19 February 2010
[[Bool]]
This code requires the Zlib package.
{- A small library for creating monochrome PNG files. This file is placed into the public domain. Dependencies: Zlib. -} module Png (png) where import Data.Array import Data.Bits import Data.List import Data.Word import qualified Codec.Compression.Zlib as Z import qualified Data.ByteString.Lazy as B be8 :: Word8 -> B.ByteString be8 x = B.singleton x be32 :: Word32 -> B.ByteString be32 x = B.pack [fromIntegral (x `shiftR` sh) | sh <- [24,16,8,0]] pack :: String -> B.ByteString pack xs = B.pack $ map (fromIntegral.fromEnum) xs unpack :: B.ByteString -> String unpack xs = map (toEnum.fromIntegral) (B.unpack xs) hdr, iHDR, iDAT, iEND :: B.ByteString hdr = pack "\137\80\78\71\13\10\26\10" iHDR = pack "IHDR" iDAT = pack "IDAT" iEND = pack "IEND" chunk :: B.ByteString -> B.ByteString -> [B.ByteString] chunk tag xs = [be32 (fromIntegral $ B.length xs), dat, be32 (crc dat)] where dat = B.append tag xs -- | Return a monochrome PNG file from a two dimensional bitmap -- stored in a list of lines represented as a list of booleans. png :: [[Bool]] -> String png dat = unpack $ B.concat $ hdr : concat [ihdr, imgdat, iend] where height = fromIntegral $ length dat width = fromIntegral $ length (head dat) ihdr = chunk iHDR (B.concat [ be32 width, be32 height, be8 1, be8 0, be8 0, be8 0, be8 0]) imgdat = chunk iDAT (Z.compress imgbits) imgbits = B.concat $ map scanline dat iend = chunk iEND B.empty scanline :: [Bool] -> B.ByteString scanline dat = 0 `B.cons` bitpack dat bitpack' :: [Bool] -> Word8 -> Word8 -> B.ByteString bitpack' [] n b = if b /= 0x80 then B.singleton n else B.empty bitpack' (x:xs) n b = if b == 1 then v `B.cons` bitpack' xs 0 0x80 else bitpack' xs v (b `shiftR` 1) where v = if x then n else n .|. b bitpack :: [Bool] -> B.ByteString bitpack xs = bitpack' xs 0 0x80 crc :: B.ByteString -> Word32 crc xs = updateCrc 0xffffffff xs `xor` 0xffffffff updateCrc :: Word32 -> B.ByteString -> Word32 updateCrc = B.foldl' crcStep crcStep :: Word32 -> Word8 -> Word32 crcStep crc ch = (crcTab ! n) `xor` (crc `shiftR` 8) where n = fromIntegral (crc `xor` fromIntegral ch) crcTab :: Array Word8 Word32 crcTab = array (0,255) $ zip [0..255] $ flip map [0..255] (\n -> foldl' (\c k -> if c .&. 1 == 1 then 0xedb88320 `xor` (c `shiftR` 1) else c `shiftR` 1) n [0..7])
1 8-bit Grayscale
This code is relatively easy to extend to handle other PNG color formats. For instance, a version that produced 8-bit grayscale instead of 1-bit monochrome would only need to change the png and bitpack functions, as follows:
white, black :: Int white = 255 black = 0 -- | Produces a single grayscale bit given a percent black gray :: Int -> Int gray percent = 255 - floor (fromIntegral percent * 2.55) -- | Return a grayscale PNG file from a two dimensional bitmap stored in a list -- of lines represented as a list of 0-255 integer values. png :: [[Int]] -> String png dat = unpack $ B.concat $ hdr : concat [ihdr, imgdat, iend] where height = fromIntegral $ length dat width = fromIntegral $ length (head dat) ihdr = chunk iHDR $ B.concat [ be32 width , be32 height , be8 8 -- bits per pixel , be8 0 -- color type , be8 0 -- compression method , be8 0 -- filter method , be8 0 ] -- interlace method imgdat = chunk iDAT (Z.compress imgbits) imgbits = B.concat $ map scanline dat iend = chunk iEND B.empty scanline :: [Int] -> B.ByteString scanline dat = B.pack (0 : map fromIntegral dat) bitpack :: [Int] -> B.ByteString bitpack = B.pack . map fromIntegral
Since one pixel fits into one byte, we no longer need the bitpack' function in this version.
2 24 Bit RGB
The code can be extended to full RGB with minor changes:
png :: [[(Int,Int,Int)]] -> B.ByteString png dat = B.concat $ hdr : concat [ihdr, imgdat ,iend] where height = fromIntegral $ length dat width = fromIntegral $ length (head dat) ihdr = chunk iHDR $ B.concat [ be32 height , be32 width , be8 8 -- bits per sample (8 for r, 8 for g, 8 for b) , be8 2 -- color type (2=rgb) , be8 0 -- compression method , be8 0 -- filter method , be8 0 ] -- interlace method imgdat = chunk iDAT (Z.compress imagedata) imagedata = B.concat $ map scanline dat iend = chunk iEND B.empty scanline :: [(Int,Int,Int)] -> B.ByteString scanline dat = B.pack (0 : (map fromIntegral $ concatMap (\(r,g,b) -> [r,g,b]) dat))
By appropriatly changing the color type and adding another Int to the tuple you could have an alpha channel too.
