如何从(大多数)图片中提取RGB值

Question

我想从Haskell可能的图片中提取每个RGB值。

获得原始值(0-255)的最简单方法是什么?

我已经用Juicy Pixels库得到了一些结果，但不知何故我总是得到异常:

*** Exception: ./Data/Vector/Generic.hs:249 ((!)): index out of bounds (660000,660000)

对应的代码。

{-# LANGUAGE TypeSynonymInstances #-}
module Main where
import Codec.Picture         (readImage, pixelAt, PixelRGB8(..))
import Codec.Picture.Types
import System.FilePath.Posix (splitExtension)
toRGBRaw :: FilePath -> IO ()
toRGBRaw fp = do
    image <- readImage fp
    case image of
      Left _ -> putStrLn $ "Sorry, not a supported codec for " ++ fp
      Right dynimg -> do
        let imgrgba8 = fromDynamicImage dynimg
        let (name, _) = splitExtension fp
        writeFile (name ++ ".txt") (concat $ accumPixels imgrgba8)
accumPixels :: Image PixelRGBA8 -> [String]
accumPixels img@(Image w h _) = [ format (pixelAt img x y) x y | x <- [0..w], y <- [0..h]]
  where format (PixelRGBA8 r g b _) j k = "#" ++ show r ++ "$"
                                              ++ show g ++ "$"
                                              ++ show b ++ "$"
                                              ++ show j ++ "$"
                                              ++ show k ++ "*n"

-- Copied from
-- See http://hackage.haskell.org/package/JuicyPixels-util-0.2/docs/Codec-Picture-RGBA8.html
class ToPixelRGBA8 a where
    toRGBA8 :: a -> PixelRGBA8
instance ToPixelRGBA8 Pixel8 where
    toRGBA8 b = PixelRGBA8 b b b 255
instance ToPixelRGBA8 PixelYA8 where
    toRGBA8 (PixelYA8 l a) = PixelRGBA8 l l l a
instance ToPixelRGBA8 PixelRGB8 where
    toRGBA8 (PixelRGB8 r g b) = PixelRGBA8 r g b 255
instance ToPixelRGBA8 PixelRGBA8 where
    toRGBA8 = id
fromDynamicImage :: DynamicImage -> Image PixelRGBA8
fromDynamicImage (ImageY8 img) = pixelMap toRGBA8 img
fromDynamicImage (ImageYA8 img) = pixelMap toRGBA8 img
fromDynamicImage (ImageRGB8 img) = pixelMap toRGBA8 img
fromDynamicImage (ImageRGBA8 img) = img
-- end of Codec.Picture.RGBA8

这是一个示例图像:Link

代码目前有点无序，因为我必须从Codec.Picture.RGBA8复制一些定义，这些定义无法通过LTS获得。不要介意字符串表示，我通过带有WiFi屏蔽的Arduino解析这些。

Answer 1

您在accumPixels中的列表理解是0索引的，但也包括边界。这可能就是问题所在。在Haskell中，[0..3]是列表[0,1,2,3]，所以您可能指的是[0..(w-1)]和[0..(h-1)]。