{-# LANGUAGE EmptyDataDecls #-}
module Graphics.GD (
                    -- * Types
                    Image, Size, Point, Color, PCREOption(..),
                    -- * Creating and copying images
                    newImage, copyImage, 
                    copyRegion, copyRegionScaled,
                    -- * Memory management
                    withImage,
                    -- * Loading images
                    -- ** JPEG
                    loadJpegFile, loadJpegData, loadJpegByteString,
                    -- ** PNG
                    loadPngFile, loadPngData, loadPngByteString,
                    -- ** GIF
                    loadGifFile, loadGifData, loadGifByteString,
                    -- * Saving images
                    -- ** JPEG
                    saveJpegFile, saveJpegByteString,
                    -- ** PNG
                    savePngFile, savePngByteString,
                    -- ** GIF
                    saveGifFile, saveGifByteString,
                    -- * Getting image information
                    imageSize,
                    -- * Querying
                    getPixel,
                    -- * Manipulating images
                    resizeImage, rotateImage,
                    -- * Drawing
                    brushed,
                    setBrush,
                    fillImage,
                    drawFilledRectangle,
                    drawFilledEllipse,
                    drawLine,
                    drawArc,
                    antiAliased,
                    setPixel,
                    colorAllocate,
                    -- * Text
                    useFontConfig,
                    drawString, measureString,
                    drawStringCircle,
                    -- * Colors
                    rgb, rgba, toRGBA,
                    -- * Misc
                    saveAlpha,
                    alphaBlending,

                   ) where

import           Control.Exception        (bracket)
import           Control.Monad            (liftM, unless)
import           Data.Bits
import qualified Data.ByteString.Internal as B
import           Foreign                  (Ptr,FunPtr,ForeignPtr)
import           Foreign                  (peekByteOff)
import qualified Foreign                  as F
import           Foreign.C                (CString)
import qualified Foreign.C                as C
import           Foreign.C.Types

data CFILE

foreign import ccall "stdio.h fopen" c_fopen
    :: CString -> CString -> IO (Ptr CFILE)
foreign import ccall "stdio.h fclose" c_fclose
    :: Ptr CFILE -> IO CInt

fopen :: FilePath -> String -> IO (Ptr CFILE)
fopen file mode = 
    C.throwErrnoIfNull file $ C.withCString file $ 
                         \f -> C.withCString mode $ \m -> c_fopen f m

fclose :: Ptr CFILE -> IO ()
fclose p = C.throwErrnoIf_ (== #{const EOF}) "fclose" $ c_fclose p

withCFILE :: FilePath -> String -> (Ptr CFILE -> IO a) -> IO a
withCFILE file mode = bracket (fopen file mode) fclose

#include <gd.h>
#include "gd-extras.h"

data GDImage

-- JPEG format

foreign import ccall "gd.h gdImageCreateFromJpeg" gdImageCreateFromJpeg
    :: Ptr CFILE -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImageCreateFromJpegPtr" gdImageCreateFromJpegPtr
    :: CInt -> Ptr a -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImageJpeg" gdImageJpeg
    :: Ptr GDImage -> Ptr CFILE -> CInt -> IO ()

foreign import ccall "gd.h gdImageJpegPtr" gdImageJpegPtr
    :: Ptr GDImage -> Ptr CInt -> CInt -> IO (Ptr a)

-- PNG format

foreign import ccall "gd.h gdImageCreateFromPng" gdImageCreateFromPng
    :: Ptr CFILE -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImageCreateFromPngPtr" gdImageCreateFromPngPtr
    :: CInt -> Ptr a -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImagePng" gdImagePng
    :: Ptr GDImage -> Ptr CFILE -> IO ()

foreign import ccall "gd.h gdImagePngPtr" gdImagePngPtr
    :: Ptr GDImage -> Ptr CInt -> IO (Ptr a)

-- GIF format

foreign import ccall "gd.h gdImageCreateFromGif" gdImageCreateFromGif
    :: Ptr CFILE -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImageCreateFromGifPtr" gdImageCreateFromGifPtr
    :: CInt -> Ptr a -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImageGif" gdImageGif
    :: Ptr GDImage -> Ptr CFILE -> IO ()

foreign import ccall "gd.h gdImageGifPtr" gdImageGifPtr
    :: Ptr GDImage -> Ptr CInt -> IO (Ptr a)

-- Creating and destroying images

foreign import ccall "gd.h gdImageCreateTrueColor" gdImageCreateTrueColor 
    :: CInt -> CInt -> IO (Ptr GDImage)

foreign import ccall "gd.h gdImageDestroy" gdImageDestroy
    :: Ptr GDImage -> IO ()

foreign import ccall "gd-extras.h &gdImagePtrDestroyIfNotNull"
        ptr_gdImagePtrDestroyIfNotNull 
    :: FunPtr (Ptr (Ptr GDImage) -> IO ())


-- Copying image parts

foreign import ccall "gd.h gdImageCopy" gdImageCopy
    :: Ptr GDImage -> Ptr GDImage 
    -> CInt -> CInt -> CInt -> CInt
    -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageCopyResampled" gdImageCopyResampled
    :: Ptr GDImage -> Ptr GDImage 
    -> CInt -> CInt -> CInt -> CInt
    -> CInt -> CInt -> CInt -> CInt -> IO ()

{-
foreign import ccall "gd.h gdImageCopyRotated" gdImageCopyRotated
    :: Ptr GDImage -> Ptr GDImage 
    -> CDouble -> CDouble -> CInt -> CInt
    -> CInt -> CInt -> CInt -> IO ()
-}

foreign import ccall "gd-extras.h gdImageCopyRotated90" gdImageCopyRotated90
    :: Ptr GDImage -> Ptr GDImage 
    -> CInt -> CInt -> CInt -> CInt
    -> CInt -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageGetPixel" gdImageGetPixel
    :: Ptr GDImage -> CInt -> CInt -> IO CInt

-- Drawing functions

foreign import ccall "gd.h gdImageSetBrush" gdImageSetBrush
    :: Ptr GDImage -> Ptr GDImage -> IO ()

foreign import ccall "gd.h gdImageFilledRectangle" gdImageFilledRectangle
    :: Ptr GDImage -> CInt -> CInt -> CInt -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageFilledEllipse" gdImageFilledEllipse
    :: Ptr GDImage -> CInt -> CInt -> CInt -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageLine" gdImageLine
    :: Ptr GDImage -> CInt -> CInt -> CInt -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageArc" gdImageArc
    :: Ptr GDImage -> CInt -> CInt -> CInt -> CInt
    -> CInt -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageSetAntiAliased" gdImageSetAntiAliased
    :: Ptr GDImage -> CInt -> IO ()

foreign import ccall "gd.h gdImageSetPixel" gdImageSetPixel
    :: Ptr GDImage -> CInt -> CInt -> CInt -> IO ()

foreign import ccall "gd.h gdImageColorAllocate" gdImageColorAllocate
    :: Ptr GDImage -> CInt -> CInt -> CInt -> CInt -> IO CInt

-- Text functions

foreign import ccall "gd.h gdFTUseFontConfig" gdFTUseFontConfig
    :: CInt -> IO CInt

foreign import ccall "gd.h gdImageStringFT" gdImageStringFT
    :: Ptr GDImage -> Ptr CInt -> CInt -> CString -> CDouble -> CDouble ->
       CInt -> CInt -> CString -> IO CString

foreign import ccall "gd.h gdImageStringFTCircle" gdImageStringFTCircle
    :: Ptr GDImage -> CInt -> CInt -> CDouble -> CDouble -> CDouble -> CString
       -> CDouble -> CString -> CString -> CInt -> IO CString

-- Miscellaneous functions

foreign import ccall "gd.h &gdFree" gdFree
    :: FunPtr (Ptr a -> IO ())
    
toRGBA :: Color -> (Int, Int, Int, Int) 
toRGBA c = (fromIntegral r, fromIntegral g, fromIntegral b, fromIntegral a)
 where
   b = c `mod` byte
   g = shiftR c 8 `mod` byte
   r = shiftR c 16 `mod` byte
   a = shiftR c 24 `mod` byte
   byte = 2 ^ (8::Int)

foreign import ccall "gd.h gdImageSaveAlpha" gdImageSaveAlpha
    :: Ptr GDImage -> CInt -> IO ()

foreign import ccall "gd.h gdImageAlphaBlending" gdImageAlphaBlending
    :: Ptr GDImage -> CInt -> IO ()

-- | To access the definition 'gdBrush' from GD
newtype PCREOption = PCREOption { unPCREOption :: CInt }
    deriving (Eq,Show)

-- We use a second level of indirection to allow storing a null pointer
-- when the image has already been freed. This allows 'withImage' to 
-- free the @gdImage@ early.
newtype Image = Image (ForeignPtr (Ptr GDImage))

type Size = (Int,Int)

type Point = (Int,Int)

type Color = CInt

mkImage :: Ptr GDImage -> IO Image
mkImage img = do fp <- F.mallocForeignPtr
                 F.withForeignPtr fp $ \p -> F.poke p img
                 F.addForeignPtrFinalizer ptr_gdImagePtrDestroyIfNotNull fp
                 return $ Image fp

-- | Creates an image, performs an operation on the image, and
-- frees it.
-- This function allows block scoped management of 'Image'
-- objects.  If you are handling large images, the delay before
-- the finalizer which frees the image runs may cause significant
-- temporary extra memory use.  Use this function to force the
-- image to be freed as soons as you are done with it.  Note that
-- it is unsafe to hold on to the 'Image' after the function is
-- done.
withImage :: IO Image        -- ^ Image creation action.
          -> (Image -> IO b) -- ^ Some operation on the image. The result should
                             -- not reference the 'Image'.
          -> IO b
withImage ini f = bracket ini freeImage f

-- | Overwrites the pointer with a null pointer, and frees the @gdImage@. 
-- Safe to call twice. Doesn't free the 'ForeignPtr', we rely on the
-- GC to do that.
freeImage :: Image -> IO ()
freeImage (Image fp) = F.withForeignPtr fp $ 
  \pp -> do p <- F.peek pp
            F.poke pp F.nullPtr
            unless (p == F.nullPtr) $ gdImageDestroy p

withImagePtr :: Image -> (Ptr GDImage -> IO a) -> IO a
withImagePtr (Image fp) f = F.withForeignPtr fp $
  \pp -> F.peek pp >>= \p ->
       if p == F.nullPtr then fail "Image has been freed." else f p

-- | Create a new empty image.
newImage :: Size -> IO Image
newImage (w,h) = newImage_ (int w) (int h)

newImage_  :: CInt -> CInt -> IO Image
newImage_ w h = do p <- F.throwIfNull "gdImageCreateTrueColor" $
                        gdImageCreateTrueColor w h
                   mkImage p

-- | Create a new empty image and apply a function to it.
onNewImage :: CInt -> CInt -> (Ptr GDImage -> IO a) -> IO Image
onNewImage w h f = newImage_ w h >>= \i -> withImagePtr i f >> return i

-- | Make a copy of an image.
copyImage :: Image -> IO Image
copyImage i = withImagePtr i f
  where f p = do (w,h) <- imageSize_ p
                 onNewImage w h (\p' -> gdImageCopy p' p 0 0 0 0 w h)

-- | Copy a region of one image into another
copyRegion :: Point    -- ^ Source upper left-hand corner
              -> Size  -- ^ Size of copied region
              -> Image -- ^ Source image
              -> Point -- ^ Destination upper left-hand corner
              -> Image -- ^ Destination image
              -> IO ()
copyRegion (srcX, srcY) (w, h) srcIPtr (dstX, dstY) dstIPtr
    = withImagePtr dstIPtr $
      \dstImg -> withImagePtr srcIPtr $
      \srcImg -> gdImageCopy dstImg srcImg
                    (int dstX) (int dstY) (int srcX) (int srcY) (int w) (int h)

-- | Copy a region of one image into another, rescaling the region 
copyRegionScaled :: Point -- ^ Source upper left-hand corner
                 -> Size  -- ^ Size of source region
                 -> Image -- ^ Source image
                 -> Point -- ^ Destination upper left-hand corner
                 -> Size  -- ^ Size of destination region
                 -> Image -- ^ Destination image
                 -> IO ()
copyRegionScaled (srcX,srcY) (srcW,srcH) srcIPtr (dstX,dstY) (dstW,dstH) dstIPtr
    = withImagePtr dstIPtr $
      \dstImg -> withImagePtr srcIPtr $
      \srcImg -> gdImageCopyResampled dstImg srcImg
                   (int dstX) (int dstY) (int srcX) (int srcY)
                   (int dstW) (int dstH) (int srcW) (int srcH)

--
-- * Loading images
--

-- | Load a JPEG image from a file.
loadJpegFile :: FilePath -> IO Image
loadJpegFile = loadImageFile gdImageCreateFromJpeg

-- | Load a JPEG image from a buffer.
loadJpegData :: Int   -- ^ Buffer size.
             -> Ptr a -- ^ Buffer with image data.
             -> IO Image
loadJpegData = loadImageData gdImageCreateFromJpegPtr

-- | Load a JPEG image from a ByteString
loadJpegByteString :: B.ByteString -> IO Image
loadJpegByteString = onByteStringData loadJpegData


-- | Load a PNG image from a file.
loadPngFile :: FilePath -> IO Image
loadPngFile = loadImageFile gdImageCreateFromPng

-- | Load a PNG image from a buffer.
loadPngData :: Int   -- ^ Buffer size.
            -> Ptr a -- ^ Buffer with image data.
            -> IO Image
loadPngData = loadImageData gdImageCreateFromPngPtr

-- | Load a PNG image from a ByteString
loadPngByteString :: B.ByteString -> IO Image
loadPngByteString = onByteStringData loadPngData

-- | Load a GIF image from a file.
loadGifFile :: FilePath -> IO Image
loadGifFile = loadImageFile gdImageCreateFromGif

-- | Load a GIF image from a buffer.
loadGifData :: Int   -- ^ Buffer size.
            -> Ptr a -- ^ Buffer with image data.
            -> IO Image
loadGifData = loadImageData gdImageCreateFromGifPtr

-- | Load a GIF image from a ByteString
loadGifByteString :: B.ByteString -> IO Image
loadGifByteString = onByteStringData loadGifData


loadImageFile :: (Ptr CFILE -> IO (Ptr GDImage)) -> FilePath -> IO Image
loadImageFile f file = do
    p <- F.throwIfNull ("Loading image from " ++ file) $ withCFILE file "rb" f
    mkImage p

loadImageData :: (CInt -> Ptr a -> IO (Ptr GDImage)) -> Int -> Ptr a -> IO Image
loadImageData f sz buf = do
    p <- F.throwIfNull ("Loading image") $ f (fromIntegral sz) buf
    mkImage p

onByteStringData :: (Int -> Ptr a -> IO b) -> B.ByteString -> IO b
onByteStringData f bstr =
    case B.toForeignPtr bstr of
      (fptr, start, sz) -> F.withForeignPtr fptr $
                           \ptr -> f sz (F.plusPtr ptr start)

--
-- * Saving images
--

-- | Save an image as a JPEG file.
saveJpegFile :: Int -- ^ quality: 0-95, or negative for default quality.
             -> FilePath -> Image -> IO ()
saveJpegFile q = saveImageFile (\p h -> gdImageJpeg p h (fromIntegral q))

-- | Write a JPEG format ByteString of an image.
saveJpegByteString :: Int -> Image -> IO B.ByteString
saveJpegByteString q =
  saveImageByteString (\p h -> gdImageJpegPtr p h (fromIntegral q))


-- | Save an image as a PNG file.
savePngFile :: FilePath -> Image -> IO ()
savePngFile = saveImageFile gdImagePng

-- | Write a PNG format ByteString of an image.
savePngByteString :: Image -> IO B.ByteString
savePngByteString = saveImageByteString gdImagePngPtr


-- | Save an image as a GIF file.
saveGifFile :: FilePath -> Image -> IO ()
saveGifFile = saveImageFile gdImageGif

-- | Write a GIF format ByteString of an image.
saveGifByteString :: Image -> IO B.ByteString
saveGifByteString = saveImageByteString gdImageGifPtr

saveImageFile :: (Ptr GDImage -> Ptr CFILE -> IO ()) 
                 -> FilePath -> Image -> IO ()
saveImageFile f file i = withImagePtr i (\p -> withCFILE file "wb" (f p))

saveImageByteString :: (Ptr GDImage -> Ptr CInt -> IO (Ptr a)) -> Image
                       -> IO (B.ByteString)
saveImageByteString f img = withImagePtr img (\p -> dataByteString (f p))

dataByteString :: (Ptr CInt -> IO (Ptr a)) -> IO B.ByteString
dataByteString f = F.alloca $ \szPtr -> do
    datPtr <- f szPtr >>= F.newForeignPtr gdFree . F.castPtr
    liftM (B.fromForeignPtr datPtr 0 . fromIntegral) (F.peek szPtr)
                  
--
-- * Querying
--
                                       
-- | Retrieves the color index or the color values of a particular pixel.
getPixel :: (Int,Int) -> Image -> IO Color
getPixel (x,y) i = withImagePtr i f
    where f p' = gdImageGetPixel p' (int x) (int y)
                     
--
-- * Getting information about images.
--

-- | Get the size of an image.
imageSize :: Image -> IO (Int,Int) -- ^ (width, height)
imageSize i = liftM f $ withImagePtr i imageSize_
    where f = (\ (w,h) -> (fromIntegral w, fromIntegral h))

imageSize_ :: Ptr GDImage -> IO (CInt,CInt)
imageSize_ p = do w <- #{peek gdImage, sx} p
                  h <- #{peek gdImage, sy} p
                  return (w, h)

--
-- * Transforming images.
--

-- | Resize an image to a give size.
resizeImage :: Int -- ^ width in pixels of output image
            -> Int -- ^ height in pixels of output image
            -> Image 
            -> IO Image
resizeImage w h i = withImagePtr i f
    where 
      f p = do let (outW,outH) = (fromIntegral w, fromIntegral h)
               (inW, inH) <- imageSize_ p
               onNewImage outW outH $ \p' -> 
                   gdImageCopyResampled p' p 0 0 0 0 outW outH inW inH

-- | Rotate an image by a multiple of 90 degrees counter-clockwise.
rotateImage :: Int -- ^ 1 for 90 degrees counter-clockwise, 
                   -- 2 for 180 degrees, etc.
            -> Image 
            -> IO Image
rotateImage r i = withImagePtr i f
    where f p = do (inW,inH) <- imageSize_ p
                   let q = fromIntegral (r `mod` 4)
                       (outW,outH) | r `mod` 2 == 0 = (inW,inH) 
                                   | otherwise      = (inH,inW)
                       srcX = if q == 1 || q == 2 then inW-1 else 0;
                       srcY = if q == 2 || q == 3 then inH-1 else 0;
                   onNewImage outW outH (\p' ->
                       gdImageCopyRotated90 p' p 0 0 srcX srcY inW inH q)

--
-- * Drawing
--

-- | Special character for gdBrushed
brushed :: PCREOption
brushed = PCREOption (#const gdBrushed)

-- | Set an @Image@ as a brush for an @Image@
setBrush :: Image -- ^ Source image
         -> Image -- ^ Brush
         -> IO ()
setBrush i b = 
    withImagePtr b $
    \brushImg -> withImagePtr i $
    \srcImg -> gdImageSetBrush srcImg brushImg

-- | Fill the entire image with the given color.
fillImage :: Color -> Image -> IO ()
fillImage c i = do sz <- imageSize i
                   drawFilledRectangle (0,0) sz c i

drawFilledRectangle :: Point -- ^ Upper left corner
                    -> Point -- ^ Lower right corner
                    -> Color -> Image -> IO ()
drawFilledRectangle (x1,y1) (x2,y2) c i =
    withImagePtr i $ \p -> 
        gdImageFilledRectangle p (int x1) (int y1) (int x2) (int y2) c

drawFilledEllipse :: Point -- ^ Center
                  -> Size  -- ^ Width and height
                  -> Color -> Image -> IO ()
drawFilledEllipse (cx,cy) (w,h) c i =
    withImagePtr i $ \p -> 
        gdImageFilledEllipse p (int cx) (int cy) (int w) (int h) c

drawLine :: Point -- ^ Start
         -> Point -- ^ End
         -> Color -> Image -> IO ()
drawLine (x1,y1) (x2,y2) c i =
    withImagePtr i $ \p ->
        gdImageLine p (int x1) (int y1) (int x2) (int y2) c

drawArc :: Point -- ^ Center
        -> Size  -- ^ Width and height
        -> Int   -- ^ Starting position (degrees)
        -> Int   -- ^ Ending position (degrees)
        -> Color -> Image -> IO ()
drawArc (cx,cy) (w,h) sp ep c i =
    withImagePtr i $ \p ->
        gdImageArc p (int cx) (int cy) (int w) (int h) (int sp) (int ep) c

-- | Use anti-aliasing when performing the given drawing function.
--   This can cause a segault with some gd versions.
antiAliased :: (Color -> Image -> IO a) -> Color -> Image -> IO a
antiAliased f c i = 
    do withImagePtr i (\p -> gdImageSetAntiAliased p c)
       f (#{const gdAntiAliased}) i

setPixel :: Point -> Color -> Image -> IO ()
setPixel (x,y) c i =
    withImagePtr i $ \p ->
        gdImageSetPixel p (int x) (int y) c

colorAllocate :: CInt -> CInt -> CInt -> CInt -> Image -> IO Color
colorAllocate r g b a i =
    withImagePtr i $ \p ->
        gdImageColorAllocate p r g b a

--
-- * Text
--

-- | Globally switch from using font file names to fontconfig paths
-- | for fonts in drawString (and measureString).
useFontConfig :: Bool -> IO Bool
useFontConfig use = liftM (/= 0) $ gdFTUseFontConfig $ if use then 1 else 0

-- | Draw a string using the FreeType 2.x library
drawString :: String -- ^ Font name
           -> Double -- ^ Font point size
           -> Double -- ^ Angle in counterclockwise radians
           -> Point -- ^ Origin
           -> String -- ^ Text, including HTML entities
           -> Color -> Image
           -> IO (Point, Point, Point, Point) -- ^ Bounding box
                                              -- of the drawn
                                              -- text.
drawString fontName ptSize angle (oriX, oriY) txt color img
    = withImagePtr img $
        drawStringImagePtr color fontName ptSize angle (oriX, oriY) txt

-- | Measure a string using the FreeType 2.x library.  This computes
-- the bounding box but does not actually draw the string to any
-- image.
measureString :: String -- ^ Font name
              -> Double -- ^ Font point size
              -> Double -- ^ Angle in counterclockwise radians
              -> Point -- ^ Origin
              -> String -- ^ Text, including HTML entities
              -> Color
              -> IO (Point, Point, Point, Point) -- ^ Bounding
                                                 -- box of the
                                                 -- drawn text
measureString fontName ptSize angle (oriX, oriY) txt color
    = drawStringImagePtr color fontName ptSize angle (oriX, oriY) txt F.nullPtr

drawStringImagePtr :: Color -> String -> Double -> Double -> Point -> String ->
                      Ptr GDImage -> IO (Point, Point, Point, Point)
drawStringImagePtr color fontName ptSize angle (oriX, oriY) txt imgPtr
    = F.allocaArray 8 $
      \bboxPtr -> C.withCAString fontName $
      \cFontName -> C.withCAString txt $
      \cTxt -> do res <- gdImageStringFT imgPtr bboxPtr color cFontName
                                         (double ptSize) (double angle)
                                         (int oriX) (int oriY) cTxt
                  if res == F.nullPtr
                     then F.peekArray 8 bboxPtr >>= parseBBox
                     else C.peekCAString res >>= ioError . userError
    where parseBBox l =
            case map int l of
              [llx, lly, lrx, lry, urx, ury, ulx, uly] ->
                return ((llx, lly), (lrx, lry), (urx, ury), (ulx, uly))
              _ -> ioError $ userError $
                     "parseBBox with /= 8 elements: " ++ show l

-- | Draw strings around the top and bottom of a torus
drawStringCircle :: Point -- ^ Center of text path circle
                 -> Double -- ^ Outer radius of text
                 -> Double -- ^ Fraction of radius occupied by text
                 -> Double -- ^ Portion of circle arc filled by text
                 -> String -- ^ Font name
                 -> Double -- ^ Font size hint
                 -> String -- ^ Text to write on the top of the circle
                 -> String -- ^ Text to write on the bottom of the circle
                 -> Color -- ^ Text color
                 -> Image -> IO ()
drawStringCircle (ctrX, ctrY) rad textRad textFill fontName
                 fontSize topTxt bottomTxt color img
    = C.withCAString fontName $ 
      \cFontName -> C.withCAString topTxt $
      \cTopTxt -> C.withCAString bottomTxt $ 
      \cBottomTxt -> withImagePtr img $ 
      \imgPtr -> do
        res <- gdImageStringFTCircle imgPtr
                   (int ctrX) (int ctrY) (double rad) (double textRad)
                   (double textFill) cFontName (double fontSize)
                   cTopTxt cBottomTxt  color                    
        unless (res == F.nullPtr) (C.peekCAString res >>= ioError . userError)

saveAlpha :: Bool -> Image -> IO ()
saveAlpha b i = withImagePtr i $ \p -> gdImageSaveAlpha p $ if b then 1 else 0

alphaBlending :: Bool -> Image -> IO ()
alphaBlending b i = withImagePtr i $ \p -> gdImageAlphaBlending p $ if b then 1 else 0

--
-- * Colors
--

rgb :: Int -- ^ Red (0-255)
         -> Int -- ^ Green (0-255)
         -> Int -- ^ Blue (0-255)
         -> Color
rgb r g b = rgba r g b 0

rgba :: Int -- ^ Red (0-255)
          -> Int -- ^ Green (0-255)
          -> Int -- ^ Blue (0-255)
          -> Int -- ^ Alpha (0-127), 0 is opaque, 127 is transparent
          -> Color
rgba r g b a = 
    (int a `F.shiftL` 24) .|.
    (int r `F.shiftL` 16) .|.
    (int g `F.shiftL` 8)  .|.
    int b


--
-- * Utilities
--

int :: (Integral a, Num b) => a -> b
int = fromIntegral

double :: (Real a, Fractional b) => a -> b
double = realToFrac