Add screen capture example

Author: Dan Rosén

examples/ScreenCapture.hs

@@ -0,0 +1,32 @@
+-- Screen capture (screenshot)
+-- Captures 1600x900 and writes a 24-bit ascii PPM image on stdout.
+module Main where
+
+import qualified Graphics.X11.Types as X
+import qualified Graphics.X11.Xlib.Image as X
+import qualified Graphics.X11.Xlib.Display as X
+import qualified Graphics.X11.Xlib.Screen as X
+
+import Foreign.C.Types
+import Data.Bits
+
+main :: IO ()
+main = do
+    let (w,h) = (1600,900)
+    disp <- X.openDisplay ":0"
+    let scr = X.defaultScreenOfDisplay disp
+    root <- X.rootWindow disp (X.screenNumberOfScreen scr)
+    img <- X.getImage disp root 0 0 w h (-1) X.xyPixmap
+
+    let int :: CUInt -> CInt; int = fromIntegral . toInteger
+
+    let rgb :: CULong -> String
+        rgb v = unwords [ show (0xff .&. (v `shiftR` s)) | s <- [16,8,0] ]
+
+    let ppm = unlines [ unwords [ rgb (X.getPixel img x y)
+                                | x <- [0..int (w-1)] ]
+                      | y <- [0..int (h-1)] ]
+
+    putStrLn (unlines ["P3",show w,show h,"255",ppm])
+    X.destroyImage img
+