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." wrote:
> 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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