A single GPU_Flip() isn't updating the screen

[ramirez7]

I've been writing a little tool to visualize hitbox collisions. It's a function that takes two hitboxes, draws them + their collision manifold (if any), and then waits for Quit/Esc.

The code is basically

• GPU_Init() with default flags
• Draw shapes
• Visualize manifold with circles and lines
• GPU_Flip()
• Loop over SDL events until Quit/Esc
• GPU_Quit()

For some reason, the screen isn't being updated on that GPU_Flip() for me (it's just showing the pixels that were there if that makes sense - the way it behaves when you just spawn a window).

When I put GPU_Flip() in my wait loop and call it a bunch of times, everything does work. But I'm surprised a single one isn't working.

There is some indirection here: I am calling sdl-gpu from Haskell in its repl using my bindings. That said, I think I've done everything I need to for that to work (RTS is threaded, I'm using more than one native thread to back the green threads, the loop is running in a bound thread and not being shifted around OS thread in the b/g, etc). It is possible this is something on my end of the Haskell/C divide, but I figured I'd ask (and I'd like to fix it on my side anyways ofc).

[ramirez7]

I've noticed that if I do a certain amount of GPU_Flips in a row before my wait loop, it works. On my machine, it's about 20. Which makes it feel like some sort of race condition to me.

[ramirez7]

I've refactored the code to be more like my game loops:

• GPU_Init() with default flags
• Loop:
  • GPU_Clear()
  • Draw shapes and manifolds
  • SDL_PollEvent() and see if any Quit/Esc events are in there
    • If no, GPU_Flip() and SDL_Delay(100)
    • If so, GPU_Quit()

And it works fine. So maybe my understanding of how sdl-gpu works is off.

[grimfang4]

I am not at a computer to test anything, but you could try GPU_FlushBlitBuffer() before the flip. SDL_gpu does hold on to data in order to render it all in one go and double-buffering could also be involved (so front and back buffers might appear out of sync if you aren't clearing and flipping them every frame).

[ramirez7]

GPU_FlushBlitBuffer() didn't make a difference either.

[ramirez7]

I'll confirm I can make a minimal repro in C.

[albertvaka]

Can you share it with us?

[ramirez7]

Here's a C repro:

https://gitlab.com/macaroni.dev/sdl-gpu-hs/-/merge_requests/11/diffs