Name NV_delay_before_swap Name Strings GLX_NV_delay_before_swap Contributors Miguel Angel Vico, NVIDIA Jeannot Breton, NVIDIA Eric Werness, NVIDIA Andy Ritger, NVIDIA James Jones, NVIDIA Cass Everitt, NVIDIA John Carmack, id Software Contact Miguel Angel Vico, NVIDIA Corporation (mvicomoya 'at' nvidia.com) Status Complete. Version Last Modified Date: 11/08/2013 Revision: 1 Number OpenGL Extension #445 Dependencies Requires GLX 1.1 Requires EXT_swap_control Interacts with EXT_swap_control_tear This specification is written against the wording of the GLX 1.4 and is based on the WGL_NV_delay_before_swap specifications Overview For most interactive applications, the standard rendering loop responding to input events on a frame granularity is sufficient. Some more demanding applications may want to exchange performance for the ability to sample input closer to the final frame swap and adjust rendering accordingly. This extension adds functionality to allow the application to wait until a specified time before a swapbuffers command would be able to execute. New Procedures and Functions Bool glXDelayBeforeSwapNV(Display *dpy, GLXDrawable drawable, GLfloat seconds) New Tokens None Additions to the GLX 1.4 Specification [Add the following to Section 3.3.10 of the GLX specification (Double Buffering)] glXDelayBeforeSwapNV blocks the CPU until seconds before a synchronized swap would occur on a particular GLX window drawable. It also returns a boolean value equal to True when the implementation had to wait for the synchronized swap and False otherwise. The parameter accepts positive floating point values not larger than the length in seconds of the swap period on the associated drawable. When buffer swaps are synchronized, the swap period is composed of one or multiple video frame periods. A video frame period is the time required by the monitor to display a full frame of video data. A swap interval set to a value of 2 means that the color buffers will be swapped at most every other video frame. If is smaller than 0, glXDelayBeforeSwapNV will return False and will not wait for the end of the swap period. If is greater than a swap period, glXDelayBeforeSwapNV will return immediately without generating any error and the return value will be False. The application should use a delay large enough to have time to complete its work before the end of the swap period. If is close to 0.0, the application may miss the end of the swap period and it will have to wait an additional swap period before it can swap. If glXDelayBeforeSwapNV detects that there is less than seconds before the end of the swap period, it will return immediately and the return value will be False. The implementation will not wait an additional video frame period to have an exact delay of seconds. If buffer swaps are unsynchronized, glXDelayBeforeSwapNV will return immediately and the return value will be False. It could happen for multiple reasons, for example if the swap interval is equal to 0, if the window is in a mode switch or if no monitors are active. GLX Protocol One new GLX protocol command is added. DelayBeforeSwapNV 1 CARD8 opcode (X assigned) 1 17 GLX opcode (glXVendorPrivateWithReply) 2 4 request length 4 1341 vendor specific opcode 4 GLX_DRAWABLE drawable 4 FLOAT32 seconds => 1 CARD8 reply 1 unused 2 CARD16 sequence number 4 0 reply length 1 BOOL waited 23 unused Errors glXDelayBeforeSwapNV generates BadValue if parameter is less than zero. glXDelayBeforeSwapNV generates GLXBadWindow if parameter is not a GLXWindow or Window XID. Usage Examples Here is a simple example that shows how an application can use GLX_NV_delay_before_swap to lower input latency when rendering its frames. void DrawFrame(Display *dpy, GLXDrawable drawable) { // Render the slowest part of the frame DrawScene(); // Make sure there's no remaining work on the GPU glFinish(); // Wait for the end of the swap period glXDelayBeforeSwapNV(dpy, drawable, 0.0015); // Sample inputs and adjust the image before the SwapBuffer glXSwapBuffers(dpy, drawable); } Issues (1) What happens if glXDelayBeforeSwapNV is called and the delay is larger than the time left until the end of the swap period? RESOLVED. The function returns immediately. We also added a return value to glXDelayBeforeSwapNV to help the application detects this situation. When glXDelayBeforeSwapNV returns False, but didn't generate any error, it means that it didn't have to wait because it got called less than seconds before the end of the swap period. (2) Should we add a function that return the amount of time until the end of the swap period? RESOLVED. It would be nice to know exactly when the current swap period is going to end, but in some configurations it's not possible to return a value that we can guarantee will always be accurate. (3) How does glXDelayBeforeSwapNV interact with GLX_EXT_swap_control_tear? RESOLVED. glXDelayBeforeSwapNV always attempts to stall until the specified time before the SwapBuffers command could complete. With swap_control_tear, the swap will wait until a fixed swap period if possible, but perform an unsynchronized swap otherwise. If the swapbuffers would wait, then glXDelayBeforeSwapNV will wait similarly if required, but if the swap period is already past and the swapbuffers would execute unsynchronized, then glXDelayBeforeSwapNV would return immediately. (4) Why does this extension delay before execution of SwapBuffers rather than on a potential swap period? RESOLVED. Given that the expected use case is to wait until before the SwapBuffers would execute to sample input, having any cases where the behavior of the delay mismatches the behavior of the swap (such as swap_control_tear or swap_interval!=1) can cause significant issues in when the input is sampled. Revision History Rev. Date Author Changes ---- -------- -------- ----------------------------------------- 1 10/22/13 mvicomoya Internal revisions.