有没有办法读取画布上特定点的颜色?
类似于:
getColor :: Canvas -> Point -> Color
我在Graphics.UI.Threepenny.Canvas上查看了文档,但找不到任何函数。也许我只是没看到,因为我用Haskell的时间不长。
如果你对我有任何提示,请告诉我。
提前感谢,Clem
编辑:多亏了Heinrich Apfelmus的回答,我能够编写一个有效的解决方案,并希望在有人需要相同功能的情况下分享它。当然,如果你使用它并进行调整,可以随意分享:)
import qualified Graphics.UI.Threepenny as UI
import Graphics.UI.Threepenny.Core
import Codec.Picture.Types
-- to UI (PixelRGB8) is also possible just change from fst to snd after the return
getCanvCol :: UI.Canvas -> UI.Point -> UI (UI.Color)
getCanvCol canvas (x,y) = do
-- str returns a string with comma separated values i.e. "255,0,255"
str <- callFunction $ ffi ("(%1.getContext('2d').getImageData(%2,%3,1,1).data[0])+
\","+(%1.getContext('2d').getImageData(%2,%3,1,1).data[1])+
\","+(%1.getContext('2d').getImageData(%2,%3,1,1).data[2])")
canvas x y
return $ fst $ tripleToCol $ lsToRGB $ wordsWhen (==',') str
where
-- could also use splitOn
wordsWhen :: (Char -> Bool) -> String -> [String]
wordsWhen p s = case dropWhile p s of
"" -> []
s' -> w : wordsWhen p s''
where (w, s'') = break p s'
-- take a list of strings and make a triple of ints
lsToRGB :: [String] -> (Int,Int,Int)
lsToRGB (a:b:c:xs) = (read a, read b, read c)
lsToRGB _ = (0,0,0)
-- make a triple of Int to Color needed
tripleToCol :: (Int,Int,Int) -> (UI.Color, PixelRGB8)
tripleToCol (r,g,b) = ((UI.RGB r g b),(PixelRGB8 r' g' b'))
where (r',g',b') = (fromIntegral r,fromIntegral g,fromIntegral b)
(作者在此)
截至threepenny-gui--0.5.0,目前还没有预定义的函数可以做到这一点。但是,您可以使用包含的JavaScriptFFI来调用返回所需值的JavaScript函数。例如,这里是drawImage
函数的源代码:
drawImage :: Element -> Vector -> Canvas -> UI ()
drawImage image (x,y) canvas =
runFunction $ ffi "%1.getContext('2d').drawImage(%2,%3,%4)" canvas image x y
ffi
函数允许您调用任意的JavaScript函数。唯一的问题是,您将不得不将结果封送为类型Color
;目前,只有像Int
或String
这样的几个类型被支持作为返回值。请查看源代码以获取示例。