I'm playing a bit with Repa library and its DevIL bindings. I tried to modify one of the examples from tutorial on HaskellWiki. I want to load an image, rotate it and save it to disk. I managed to write something like this: import Foreign.Ptr import System.Environment import Data.Array.Repa as R hiding ((++)) import qualified Data.Array.Repa.Repr.ForeignPtr as RFP import Data.Array.Repa.IO.DevIL main = do [f] <- getArgs (RGB v) <- runIL $ readImage f RFP.computeIntoP (RFP.toForeignPtr v) (rot180 v) runIL $ writeImage ("flip-"++f) (RGB v) return () rot180 g = backpermute e flop g where e@(Z :. x :. y :. _) = extent g flop (Z :. i :. j :. k) = (Z :. x - i - 1 :. y - j - 1 :. k) This is obviously wrong, because the foreign pointer used as a data source is at the same time used as destination, so the data gets overwritten before it is used. Does this mean that I have to allocate foreign memory buffers on my own? If so, than it feels kind of painfull to go through the hassle of allocating foreign pointers, converting between many different representations and so on. Am I doing something wrong and if not is there a more painless way of working with images and repa in Haskell? Jan
You do not have to use computeIntoP. You can just use computeP followed by
toForeignPtr (i don't remember the exact name for that and am on my phone
so it would be awkward to look up). So Repa can create the buffer for you.
Coincidentally, I didn't realize computeIntoP even existed, and I want it
for what I'm doing!
On Oct 10, 2012 7:55 AM, "Janek S."
I'm playing a bit with Repa library and its DevIL bindings. I tried to modify one of the examples from tutorial on HaskellWiki. I want to load an image, rotate it and save it to disk. I managed to write something like this:
import Foreign.Ptr import System.Environment import Data.Array.Repa as R hiding ((++)) import qualified Data.Array.Repa.Repr.ForeignPtr as RFP import Data.Array.Repa.IO.DevIL
main = do [f] <- getArgs (RGB v) <- runIL $ readImage f RFP.computeIntoP (RFP.toForeignPtr v) (rot180 v) runIL $ writeImage ("flip-"++f) (RGB v) return ()
rot180 g = backpermute e flop g where e@(Z :. x :. y :. _) = extent g flop (Z :. i :. j :. k) = (Z :. x - i - 1 :. y - j - 1 :. k)
This is obviously wrong, because the foreign pointer used as a data source is at the same time used as destination, so the data gets overwritten before it is used. Does this mean that I have to allocate foreign memory buffers on my own? If so, than it feels kind of painfull to go through the hassle of allocating foreign pointers, converting between many different representations and so on. Am I doing something wrong and if not is there a more painless way of working with images and repa in Haskell?
Jan
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You do not have to use computeIntoP. You can just use computeP followed by toForeignPtr (i don't remember the exact name for that and am on my phone so it would be awkward to look up). So Repa can create the buffer for you. Coincidentally, I didn't realize computeIntoP even existed, and I want it for what I'm doing! I managed to fix my code - thanks! The main function now reads:
main = do [f] <- getArgs (RGB v) <- runIL $ readImage f rotated <- (computeP $ rot180 v) :: IO (Array RFP.F DIM3 Word8) runIL $ writeImage ("flip-"++f) (RGB rotated) Jan
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