Today I've added cheap camera motion blur to the Brigade 2 path tracer using the OpenGL accumulation buffer. The technique blends the pixels of one or more previous frames with the current frame and works very well for real-time path traced dynamic scenes, provided the framerate is sufficiently high (10+ fps). The difference in image quality is huge: path tracing noise is drastically reduced and since all calculations pertaining to the accumulation buffer are hardware accelerated on the GPU, there is zero impact on the rendering performance. Depending on the accumulation value, you essentially get two to ten times the amount of samples per pixel for free at the expense of slight blurring caused by the frame averaging (the blurring is actually not so bad because it adds a nice cinematic effect).
Below is a comparison image of a stress test of an indoor scene without and with motion blur applied (rendered on 8600M GT). The images were rendered with multiple importance sampling using only 1 sample per pixel to better show the differences (converging is off):
Comparison of the scene with the rotating ogre from a previous demo (see http://raytracey.blogspot.com/2011/12/videos-and-executable-demo-of-brigade-2.html) rendered at 1 sample per pixel (on a 8600M GT), with (right) and without motion blur:
The following image has nothing to do with frame averaging but it is an image from an earlier test of the MIS indoor scene with a highly detailed Alyx character (40k triangles) with a Blinn shader applied. It rendered very fast:
I'll upload some videos of the above tests soon.