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Bug 4806

Summary: Question: Can I draw something with software render over OpenGL surface?
Product: SDL Reporter: Vitaly Novichkov <admin>
Component: videoAssignee: Sam Lantinga <slouken>
Status: RESOLVED INVALID QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: icculus
Version: HG 2.1   
Hardware: All   
OS: All   

Description Vitaly Novichkov 2019-09-19 13:16:37 UTC 
I have a problem:
- I using OpenGL directly by raw calls (not via SDL's render API which I use as a fallback only).
- I want to draw some sort of "loading" animation.
- At the same time, while loading process, I loading new textures into OpenGL.
- OpenGL is known that it can't be used in multiple threads safely.
- Because of that, I can't load new textures in background and at the same time draw anything on OpenGL context. Therefore, while loading, I draw nothing, just black screen and the stuck of the window until the loading process will complete.

The question: How I can draw anything on the window by using software render to don't disturb OpenGL and let it receive all necessary textures?
Comment 1 Ryan C. Gordon 2020-03-23 03:34:52 UTC 
So some options for this:

- Load the textures you need for the loading screen first and then use them while loading everything else. Load some amount of the total textures per frame, perhaps drawing the loading screen and swapping buffers every ~33 milliseconds ( about 30fps) if you're animating something (less if not), regardless of how much you managed to load that frame.

- Create a second GL context on your loading thread, and mark it as "shared" with the main context, so they both use the same textures. After loading is done, optionally delete the second context (or keep it around for future loading). All actual drawing happens in the main context.

You can't do any software rendering, though. This isn't an SDL restriction, it's an OpenGL one. The closest you could get is uploading pixels you generated yourself in memory to a texture...and then you're in the same position.

--ryan.