Jeff Briggs wrote:
Hello,
I am attempting to process images captured from a webcam. My aim is to do so, in real time, at the frame rate of the camera. I'm using GHC 6.4.2 with -O3. A frame consists of ~100k 24bit colour values.
The webcam is interfaced through FFI bindings to some C++ -- these are all labelled 'unsafe'. The image is passed to Haskell as a Ptr Word8.
To blit this to the screen (via Gtk2Hs) I do the following:
data Cam = Cam { snap_width :: !Int , snap_height :: !Int , snap_bytespp :: !Int , snap_size :: !Int , cam_img :: Ptr Word8 , cam_obj :: ForeignPtr () }
do (PixbufData _ dst _) <- (pixbufGetPixels pixbuf :: IO (PixbufData Int Word8)) copyBytes dst (cam_img cam)
This achieves the desired throughput (25-29fps.) However, I am at a bit of a loss how to do something similar for preprocessing the data in Haskell before blitting the data (whilst also retaining some semblance of functional programming...)
Currently, I have:
cam_snap cam f x = do let loop (r:g:b:rest) n x = f r g b n x >>= loop rest (n+3) loop _ _ x = return x px <- peekArray (snap_size cam) (cam_img cam) loop px 0 x
cam_snap2 cam f x = let loop ptr n x | n >= snap_size cam = return x | otherwise = do let ptrs = scanl plusPtr ptr [1,1] [r,g,b] <- mapM peek ptrs f r g b n x >>= loop (ptr `plusPtr` 3) (n+3) in loop (cam_img cam) 0 x
do ... let sum_px r g b _ (sr,sg,sb) = return (sr+r,sg+g,sb+b) sum <- cam_snap (cam ui) sum_px (0.0,0.0,0.0) print sum
cam_snap only processes at 5 fps, whereas cam_snap2 operates at 6fps.
Any suggestions?
I suggest trying something, using "/usr/bin/ghc -O3 -optc-O3" like this:
{-# OPTIONS_GHC -funbox-strict-fields #-}
import Foreign import Control.Monad
data Cam = Cam { snap_width :: !Int , snap_height :: !Int , snap_bytespp :: !Int , snap_size :: !Int , cam_img :: Ptr Word8 , cam_obj :: ForeignPtr () }
type F = Word8 -> Word8 -> Word8 -> Int -> Int -> Int
{-# INLINE cam_snap_3 #-} cam_snap_3 :: Cam -> F -> Int -> IO Int cam_snap_3 cam f x = let end = snap_size cam loop ptr n x | ptr `seq` n `seq` x `seq` False = undefined | n >= end = return x | otherwise = do r <- peek ptr g <- peek (advancePtr ptr 1) b <- peek (advancePtr ptr 2) loop (advancePtr ptr 3) (n+3) (f r g b n x) in loop (cam_img cam) 0 x