I want to access the pixel data of what is being displayed to the window, but I have not had any luck finding such a function in gloss, nor by attempting to call OpenGL readPixels in a keyboard event callback. It looks like gloss renders a Picture to the window without exposing the rendered bitmap.
If this is hard to do in gloss, is there an alternative which has realtime high-level bitmap manipulation (translation, rotation, transparency)?
It turns out readPixels can be used in this case. I found this snippet while digging through #haskell chat logs:
-- save a screenshot to a handle as binary PPM
snapshotWith :: (BS.ByteString -> IO b) -> Position -> Size -> IO b
snapshotWith f p0 vp@(Size vw vh) = do
let fi q = fromIntegral q
p6 = "P6\n" ++ show vw ++ " " ++ show vh ++ " 255\n"
allocaBytes (fi (vw*vh*3)) $ \ptr -> do
readPixels p0 vp $ PixelData RGB UnsignedByte ptr
px <- BSI.create (fi $ vw * vh * 3) $ \d -> forM_ [0..vh-1] $ \y ->
BSI.memcpy
(d`plusPtr`fi(y*vw*3))
(ptr`plusPtr`fi ((vh-1-y)*vw*3))
(fi(vw*3))
f $ BS.pack (map (toEnum . fromEnum) p6) `BS.append` px
writeSnapshot :: FilePath -> Position -> Size -> IO ()
writeSnapshot f = snapshotWith (BS.writeFile f)
Recently a new package gloss-export was released with the appropriate purpose:
Export gloss pictures as PNG, Bitmap, TGA, TIFF, Gif
I once had the same problem and couldn't find a good solution, so my answer probably won't be appropriate. My workaround was to use bmp package, deal with the BMP content manually (via ByteString manipulation), and then convert it to a glossy bitmap with bitmapOfBMP. For example, this was a function for blending a bitmap with a color:
recolor :: (Float, Float, Float) -> BMP -> BMP
recolor (rc, gc, bc) bmp@BMP {bmpRawImageData = raw} =
bmp {bmpRawImageData = B.pack $ process $ B.unpack raw} where
process (b:g:r:a:xs) = (mul b bc):(mul g gc):(mul r rc):a:process xs
process xs = xs
mul c cc = round $ cc * fromIntegral c
This was enough for me in that time, so I stopped finding a better solution. If you'll find something, please share.
