![]() See the synchronization discussion below. You may want to consider calling glFinish. "My rendered objects lag behind the mouse cursor, but only when Vsync is enabled!" You probably have several frames queued up in the GPU.For example, forcing Vsync Off in a driver's control panel will prevent Vsync, even if swap interval is set to 1 in your application. Your application's use of swap interval may be overridden by external, driver-specific configuration.Some ATI GLX drivers may report WGL_EXT_swap_control yet actually export glXSwapIntervalSGI.Disabling the synchronisation on low frame rates prevents the common problem where the frame rate syncs to a integer fraction of the screen's refresh rate in a complex scene.Īdaptive vsync can be enabled with wglSwapIntervalEXT(-1) or glXSwapIntervalEXT(-1). This extension brings "adaptive vsync" as featured in modern gaming consoles to the PC.Īdaptive vsync enables v-blank synchronisation when the frame rate is higher than the sync rate, but disables synchronisation when the frame rate drops below the sync rate. Recent GL drivers implement a new WGL/GLX extension called EXT_swap_control_tear. glXSwapIntervalEXT(1) is used to enable vsync glXSwapIntervalEXT(0) to disable vsync. The extension provides glXSwapIntervalEXT(), which also directly specifies the swap interval. Check the GLX-specific extensions string via glXQueryExtensionsString() to verify that the extension is actually present. Use the GLX_EXT_swap_control extension to control swap interval. wglSwapIntervalEXT(1) is used to enable vsync wglSwapIntervalEXT(0) to disable vsync. The extension provides the wglSwapIntervalEXT() function, which directly specifies the swap interval. Check both the standard extensions string via glGetString(GL_EXTENSIONS) and the WGL-specific extensions string via wglGetExtensionsStringARB() to verify that WGL_EXT_swap_control is actually present. Use the WGL_EXT_swap_control extension to control swap interval. This is commonly called "triple buffering". Thus, one can ease the CPU burden on waiting for v-blanks by rendering to a third buffer, then blitting that to the back buffer, and then doing a swap. Assuming that there are no other issues that would prevent such execution (trying to render to a buffer that is being read from, for example). If they do not affect the back buffer, either by rendering to a framebuffer object, another form of off-screen buffer, or something else that isn't the back or front buffers, then these rendering commands can be scheduled as normal. Note that the problem of OpenGL commands backing up due to a waiting v-blank happen because these commands are trying to affect the back buffer. Rendering calls made in that time will back up, eventually forcing a stall to wait for the actual swap. It will also induce stalls, for the same reason as above: the GPU has to wait 15.2ms every other v-blank interval for a buffer swap. It will effectively take two full v-blank intervals to display an image to the user, turning a 60fps program into a 30fps program. Therefore, the CPU driver will stall the rendering thread in an OpenGL command (it doesn't have to be in a buffer swapping command) if there are too many commands waiting for the v-blank.Īlternatively, if the renderer takes slightly longer than the v-blank intervals to render, say 18ms, then a different problem can result. For example, if the v-blank intervals come at 16.6ms intervals (60fps refresh), but the rendering of a frame only takes 4ms, then buffer swaps can back up. Exactly when that happens is subject to the swap interval setting.Ī swap interval greater than 0 means that the GPU may force the CPU to wait due to previously issued buffer swaps. Video drivers can override these values, forcing a swap interval of 1 or 0 depending on settings the user provided in the video card's control panel.Ī call to the platform-specific buffer swapping function (such as SwapBuffers in Windows) means that, once all previously issued drawing commands have completed, the contents of the back buffer should be swapped into the front buffer. A swap interval of 0 specifies that the GPU should never wait for v-blanks, thus performing buffer swaps as soon as possible when rendering for a frame is finished. ![]() A swap interval of 1 tells the GPU to wait for one v-blank before swapping the front and back buffers. The term "swap interval" itself refers to the number of v-blanks that must occur before the front and back frame buffers are swapped.
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