I'm trying to experiment with image processing in haskell (with which
I haven't much experience). I've written some FFI code to talk to
the ImageMagick library which provokes a few questions (environment is
ghc 6.2.1 on debian):
1. Speed: I'm reading in a 2000x1500 pixel image, and have defined a
Pixel type like this:
data Pixel a = Pixel !a !a !a deriving Show
I use ImageMagick to load the image, then build an Array of Pixel
Floats. Building the array takes 45 seconds on a 2.5Ghz P4 with
code compiled -O2, which seems slow to me - are my expectations
unrealistic? I've tried various UNPACK things which didn't make
much difference.
2. How do I convert a CFloat into a Float?
3. I get the wrong answer ;-) I expect the C and haskell code below
to produce the same pixel data, but they don't (the C code is right).
C code:
#include
#include
int main(int argc,char *argv[])
{
MagickWand *wand = NewMagickWand();
int width,height;
float *pixels;
int i,j;
if (argc!=2)
{
fprintf(stderr,"bad args\n");
exit(100);
}
if (!MagickReadImage(wand,argv[1]))
{
fprintf(stderr,"can't read file\n");
exit(1);
}
height = MagickGetImageHeight(wand);
width = MagickGetImageWidth(wand);
printf("dimension: %dx%d\n",width,height);
pixels = (float*) malloc(3*width*height*sizeof(float));
if (!MagickGetImagePixels(wand,0,0,width,height,"RGB",FloatPixel,(unsigned char*) pixels))
{
fprintf(stderr,"can't get pixels\n");
exit(1);
}
printf("%f %f %f\n",pixels[0],pixels[1],pixels[2]);
printf("%f %f %f\n",pixels[4],pixels[5],pixels[6]);
printf("%f %f %f\n",pixels[7],pixels[8],pixels[9]);
}
haskell code:
-- requires flags: -fffi -fglasgow-exts
import Foreign
import Foreign.C
import MarshalArray
import Char
import List
import Array
import Bits
data Pixel a = Pixel !a !a !a deriving Show
data MagickWand -- opaque
foreign import ccall "wand/magick_wand.h NewMagickWand" newMagickWand :: IO (Ptr MagickWand)
foreign import ccall "wand/magick_wand.h DestroyMagickWand" destroyMagickWand :: (Ptr MagickWand) -> IO ()
foreign import ccall "wand/magick_wand.h MagickSetFilename" setFilename :: Ptr MagickWand -> CString -> IO ()
foreign import ccall "wand/magick_wand.h MagickReadImage" readImage :: Ptr MagickWand -> CString -> IO CInt
foreign import ccall "wand/magick_wand.h MagickGetImageWidth" getWidth :: Ptr MagickWand -> CULong
foreign import ccall "wand/magick_wand.h MagickGetImageHeight" getHeight :: Ptr MagickWand -> CULong
foreign import ccall "wand/magick_wand.h MagickGetImagePixels" getPixels :: Ptr MagickWand -> CULong -> CULong -> CULong -> CULong -> CString -> CInt -> (Ptr a) -> IO CInt
-- StorageType: enum Char=1,Short,Integer,Long,Float=5,Double
type Frame a = Array (Int,Int) (Pixel a)
getPixel :: ([a],Int) -> Pixel a
getPixel (floats,i) = Pixel (floats!!(i*3)) (floats!!(i*3+1)) (floats!!(i*3+2))
getFrame :: (Storable a) => (Ptr a) -> Int -> Int -> IO (Frame a)
getFrame arr h w = do
putStr ("getFrame: "++(show (w,h))++"\n")
floats <- peekArray (3*h*w) arr
return (array ((1,1),(h,w)) [((i,j),getPixel (floats,((w*i)+j))) | i<-[1..h],j<-[1..w]])
loadFrameFloat :: FilePath -> IO (Frame CFloat)
loadFrameFloat filename = do
wand <- newMagickWand
f <- (newCString filename)
ok <- readImage wand f
let w = (fromInteger.toInteger) (getWidth wand)
h = (fromInteger.toInteger) (getHeight wand) in
do
arr <- mallocArray (w*h*3) :: (IO (Ptr CFloat))
rgb <- (newCString "RGB")
ok1 <- getPixels wand 0 0 (getHeight wand) (getWidth wand) rgb 5 arr -- PixelFloat==5
frame <- getFrame arr h w
free arr
destroyMagickWand wand
return frame
thanks,
John
