PNG to BMP in Haskell (for Gloss)

I have PNG files and the Gloss library has a Bitmap constructor for Picture. I can't use loadBMP :: FilePath -> IO Picture because of the filetype, so I'm searching how to load a PNG file, convert it to BMP, and feed it to either bitmapOfBMP :: BMP -> Picture, bitmapOfForeignPtr :: Int -> Int -> ForeignPtr Word8 -> Bool -> Picture or bitmapOfByteString :: Int -> Int -> ByteString -> Bool -> Picture.

Test with JuicyPixels

import Data.ByteString as B
import System.IO as A

import Codec.Picture.Png
import Graphics.Gloss.Interface.Pure.Game


main = do
    png <- B.readFile "samus.png"
    let img = decodePng png
    case img of
        Left x -> A.putStrLn x
        Right x -> do
            let bmp = encodeDynamicPng x
            case bmp of
                Left x -> A.putStrLn x
                Right x -> do
                    let pic = bitmapOfByteString 29 52 x True
                    game pic

game pic
    =  play
        (InWindow "Test" (700, 500) (10, 10))
        white
        30
        pic
        draw
        (const id)
        (const id)

draw bmp
    = bmp

Everything succeeds but the image is not the same at all.

Solution

Even though I didn't come with the answer alone, but thanks to Thomas' answer, I will post it here instead of inside the question.

As a reminder, the goal is to convert a BMP file into a Gloss' Picture, so I wrote a function called bmpToPic. I put it in a module, because it uses two others functions, and needs many imports. Also, repaToPicture from Thomas' answer is slighty different here.

module PngToPic
    (pngToPic)
    where

import Data.ByteString as B
import Data.Word

import Codec.Picture.Png
import Codec.Picture.Repa
import qualified Data.ByteString.Internal as BI
import Data.Array.Repa ((:.)(..), Z, Z(..), extent, DIM3, Array)
import qualified Data.Array.Repa as R
import qualified Data.Array.Repa.Repr.ForeignPtr as F
import Graphics.Gloss.Data.Picture


pngToPic :: ByteString -> Picture
pngToPic png
    = let
        Right img -- unsafe
            = decodePng png
        repa
            = imgData (convertImage img :: Img RGBA)
    in repaToPicture True repa

repaToPicture :: Bool -> Array F.F DIM3 Word8 -> Picture
repaToPicture b arr
    = bitmapOfByteString row col bs b
    where
        bs
            = BI.fromForeignPtr fptr 0 (R.size sh)
        fptr
            = F.toForeignPtr arr'
        sh@(Z :. col :. row :. depth)
            = extent arr'
        arr'
            = flipVert arr

flipVert :: Array F.F DIM3 Word8 -> Array F.F DIM3 Word8
flipVert g
    = R.computeS $ R.backpermute e flop g
    where
        e@(Z :. x :. y :. _)
            = extent g
        flop (Z :. i         :. j         :. k)
            = Z :. x - i - 1 :. j :. k

You use it like that:

import Data.ByteString as B

import Graphics.Gloss.Interface.Pure.Game

import PngToPic

main = do
    png <- B.readFile "someImage.png"
    game $ pngToPic png

game pic
    = play
        (InWindow "Test" (700, 500) (10, 10))
        white
        30
        pic
        id
        (const id)
        (const id)

and the image will show up in the middle of the window.