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Bug 512

Summary: opengl multisample setup
Product: SDL Reporter: Robert Pointon <rpointon>
Component: videoAssignee: Ryan C. Gordon <icculus>
Status: RESOLVED WONTFIX QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2    
Version: 1.2.10   
Hardware: PowerPC   
OS: Mac OS X (All)   

Description Robert Pointon 2007-10-30 04:07:20 UTC 
Once multisample is enabled on the buffer then upon buffer swap the hidden buffer always has its color data reset to the the clearColor - it is if a glClear(GL_COLOR_BUFFER_BIT) had implicitly been called.

Was reported Nov 2006 to Apple as Bug ID# 4805200, they have finally(!!) responded to say:
You can get the behavior he wants by simply adding kCGLPFABackingStore to your pixel format. That will retain the back buffer in a multisample context on ATI.
We have added Testing/Tests/Extensions/ARB_multisample/flush to verify this works.

Note: in the game engine sauerbraten <http://www.cubeengine.com/index.php4> that uses SDL, we rely on the fact that the backbuffer is NOT cleared so as to minimise artifacts in texture T junctions.
Comment 1 Sam Lantinga 2007-12-29 13:47:07 UTC 
Technically this is not a bug.  The contents of the buffers after a swap is not defined.

We could potentially add an environment variable to unofficially support this, but my gut feeling is that you're setting yourself up for lots of trouble if you assume this when you move to non-Apple platforms.