Using VSync with SDL_UpdateWindowSurface()

What is VSync?

VSync stands for Vertical Synchronization. It's a display technology feature that synchronizes your application's frame presentation rate with the monitor's refresh rate.

Monitors refresh their image row by row, from top to bottom. If your application updates the frame buffer (swaps the front and back buffers) while the monitor is in the middle of drawing a frame, the monitor might display the top part of the old frame and the bottom part of the new frame simultaneously. This visual artifact is known as screen tearing.

VSync prevents screen tearing by ensuring that the buffer swap only happens during the vertical blanking interval - the brief period when the monitor has finished drawing one frame and is preparing to draw the next one. This guarantees that the monitor always displays a single, complete frame.

Enabling VSync with SDL_UpdateWindowSurface()

Here's the important part: You cannot directly enable or disable VSync for the SDL_UpdateWindowSurface() function through the SDL API itself.

The SDL_Surface API operates primarily on the CPU. SDL_UpdateWindowSurface() copies your CPU surface data to the window. While the underlying window system might apply some form of VSync implicitly when updating the actual screen content, SDL doesn't provide a switch for you to control this behavior at the SDL_UpdateWindowSurface() level.

VSync control in SDL is primarily associated with the hardware-accelerated SDL_Renderer API. When creating a renderer, you can explicitly request VSync:

1SDL_Renderer* renderer{SDL_CreateRenderer(
2  window,
3  -1, // Use the first available rendering driver
4  SDL_RENDERER_ACCELERATED 
5  | SDL_RENDERER_PRESENTVSYNC 
6)};
7
8if (!renderer) {
9  // Handle error: VSync might not be supported
10  std::cerr << "Renderer creation failed: "
11            << SDL_GetError() << std::endl;
12}

When using a renderer created with SDL_RENDERER_PRESENTVSYNC, calls to SDL_RenderPresent() will automatically wait for the vertical blanking interval before swapping the buffers.

Should You (Try to) Use VSync with Surfaces?

• Pros: If VSync were active (perhaps via graphics driver settings, see below), it would prevent screen tearing, leading to a smoother visual experience.
• Cons: VSync limits your application's frame rate to the monitor's refresh rate (e.g., 60 FPS on a 60Hz monitor). If your application takes longer than one refresh interval to render a frame, VSync can cause stuttering. It can also potentially increase input latency, as the application might have to wait before presenting a completed frame.

Achieving VSync Indirectly

While SDL doesn't offer a direct function, you might be able to force VSync for all applications (including your SDL surface-based one) through your graphics card's control panel (e.g., NVIDIA Control Panel, AMD Radeon Software).

This forces the graphics driver to handle synchronization, but it's an external setting, not controlled via your code.

Conclusion

You cannot enable VSync via SDL functions when using SDL_Surface and SDL_UpdateWindowSurface(). VSync control is a feature of the SDL_Renderer API (SDL_RENDERER_PRESENTVSYNC flag and SDL_RenderPresent()).

If screen tearing is a problem with your surface-based application, consider switching to the SDL_Renderer API for better performance and direct VSync control, or investigate forcing VSync through your graphics driver settings.