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'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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