Why glFinish for Some Drivers or Vendors

Because there's basically no information as to why certain drivers / vendors require calling glFinish between frames, the best I could do was gather information from Chat-GPT -

be me - Certain drivers seem to require glFinish before swap. Any documented insight into this?

be chat-gpt - The requirement for calling glFinish() before swapping buffers on certain drivers is an issue related to how different GPUs and drivers handle the rendering pipeline and buffer swapping. While it's not explicitly mandated in the OpenGL specification, some drivers have behavior that benefits from calling glFinish() to ensure proper synchronization between the GPU and the display system. Here's why this might happen:

1. Driver-Specific Behavior

Some OpenGL drivers (particularly on older or less optimized systems) may not fully synchronize the pipeline without a manual intervention like glFinish(). This can lead to incomplete frames being swapped to the display, or the pipeline being flushed too late. By calling glFinish(), you force the GPU to complete all previously issued commands before swapping buffers, which can help ensure that the rendered frame is complete.

• NVIDIA: On some NVIDIA drivers, using glFinish() may help with synchronization issues where the GPU hasn’t completed rendering when the swap occurs.
• Intel: Integrated GPUs (like Intel's) sometimes have issues where they are slower to synchronize between CPU and GPU without glFinish().
• AMD: Older AMD drivers also occasionally show issues that require glFinish() before glSwapBuffers() to ensure proper display.

2. VSync and Buffer Management

When VSync is enabled, buffer swaps should ideally occur at the vertical blanking interval (VBI) to avoid tearing. However, some drivers may not respect VSync properly unless you ensure that all GPU commands are completed before the swap. glFinish() forces the driver to ensure that the frame is fully rendered and ready to be presented to the screen.

3. Double and Triple Buffering

If you're using double buffering, OpenGL typically uses an implicit synchronization between rendering and swapping buffers. However, with triple buffering, there can be more frames in flight, and glFinish() might be needed to avoid over-rendering frames ahead of the display refresh. This is because triple buffering allows the GPU to continue rendering frames even if a buffer is waiting to be swapped, which can sometimes cause tearing or incomplete frames being displayed unless forced to finish rendering.

4. Swap Interval and Timing Issues

In cases where swap interval is set (e.g., for VSync), some drivers may have timing issues, especially when the system is under heavy load. If glSwapBuffers() is called while the GPU is still processing previous frames, artifacts or incomplete frames might appear. Calling glFinish() forces a flush of the GPU pipeline, ensuring that all previous commands are complete before the swap.

Documented Insights:

While the OpenGL specification does not explicitly require glFinish() before glSwapBuffers(), several sources and community forums discuss situations where it's needed:

• OpenGL Wiki - Synchronization: The OpenGL Wiki highlights that implicit synchronization generally happens with buffer swaps but doesn’t rule out cases where explicit flushing (glFinish() or glFlush()) might be needed due to driver issues. OpenGL Wiki - Synchronization
• NVIDIA Developer Documentation: NVIDIA provides guidance on best practices, sometimes recommending glFinish() in specific performance-related debugging cases, particularly when dealing with multi-threading or complex GPU pipelines.
• Community Observations (Stack Overflow, Forums): Various community discussions across platforms like Stack Overflow and Khronos forums have observed the need for glFinish() before glSwapBuffers() on specific hardware configurations, often to resolve visual artifacts or performance issues.

Performance Impact:

Using glFinish() forces a full synchronization between CPU and GPU, which can reduce performance, as it introduces a stall while waiting for the GPU to finish all operations. Therefore, it should be used only if necessary (e.g., debugging or when a driver shows specific problems).

Conclusion:

The need for glFinish() before glSwapBuffers() is typically driver-specific and arises due to differences in how synchronization between the rendering pipeline and the display system is handled. While it's not part of the OpenGL specification, calling glFinish() can help ensure proper synchronization, especially on problematic drivers or when using VSync. If your application runs without visual issues or performance degradation without glFinish(), it's usually best to avoid it for the sake of performance.