View Full Version : Drawing an incremental off-screen buffer -- seeing double

06-11-2016, 03:15 PM
Hi folks,

I have the following situation. I am using QT as my windowing system, and am rendering to an off-screen target and slowly building up an image over several frames. So, the idea is: render some geometry to off-screen color and depth buffers (on a QTimer event), and then render that buffer as a textured quad in QGLWidget::paint(). Then render some more geometry to the same off-screen buffer (when another timer event comes in), and then re-render that buffer as a textured quad again. Etc until I run out of geometry. If the camera changes, I clear the buffer and start all over with rendering.

The problem that I am seeing is that, when the camera changes, I see two versions of the geometry. The old one from the previous camera and the new one. It occurred to me that perhaps the GPU/driver is still waiting to render the quad in paint() while the timer event handler is modifying the off-screen buffer. However, I don't see how that is possible. The two methods (timer and paint) should be called sequentially on the same thread. I verified this by putting a mutex around the two methods. I also thought that perhaps the driver was waiting to render toe textured quad, but I think QGLWidget will do a flush when it is done calling my paint method, correct?

Just for fun, I tried adding glFlush() and glFinish() to the end of my paint method to force rendering of that quad to finish before the timer event has a chance to take over. No dice.

I did put in a thread sleep for 100ms at the end of my paint method, and that makes the problem go away. However, that kinda kills my frame rate :(

So, what else might be going wrong here? How might I fix this?