[HOpenGL] clear [ColorBuffer]

[Sven Panne]

On Friday 16 February 2007 19:35, h. wrote:
>  [...] and the problem:
> The ColorBuffer is not cleared, the window shows what was there before it
> was created. [...]

Just a small addition: As was already pointed out, rendering in OpenGL is not 
synchronous, which is a very good design decision given the capabilities and 
architectures of today's graphic cards. If you use single-buffering (as in 
your example), a 'flush' or 'finish' is needed, see:

http://www.haskell.org/ghc/docs/latest/html/libraries/OpenGL/Graphics-Rendering-OpenGL-GL-FlushFinish.html

If double-buffering is used, GLUT's 'swapBuffers' is your friend:

http://www.haskell.org/ghc/docs/latest/html/libraries/GLUT/Graphics-UI-GLUT-Window.html#v%3AswapBuffers

Example:

--------------------------------------------------------------------------
import Graphics.UI.GLUT

main :: IO ()
main = do
   _ <- getArgsAndInitialize
   initialDisplayMode $= [ DoubleBuffered ]
   createWindow "Hello World"
   displayCallback $= do clear [ ColorBuffer ]; swapBuffers
   mainLoop
--------------------------------------------------------------------------

Other UI toolkits with an OpenGL canvas will have something similar 
to 'swapBuffers', e.g. 'glDrawableSwapBuffers' in Gtk2Hs, IIRC.

Cheers,
   S.