Toggle effects on/off in Single pass render expensive

Hi

Some random ideas in case they are helpful.

VR.WaitForGPU will probably be the main thread waiting for the render thread and GPU to finish working on the last frame. Do you get frame spikes when the post effects are on or just in the sections where you enable/disable them? The spikes from the graph look like they could be drops from 90fps to 45fps, which I think would make sense if the VR device you are running on doesn't support async timewarp/reprojection. The questions is what is causing the extra time here, it could just be a small spike which takes it over the 11ms for a 90Hz frame and then it would drop to 45Hz.

If you look at the GPU profile during this time does that also spike or if you look at the timeline view during those spikes you might be able to see if it is caused by the render thread spiking. This might give a hint as where to look further.

If the spikes occurs when the post effects are on then it could just be that the GPU time is going over 11ms and causing this. Turning off single pass VR in this case might hide your extra GPU time as the CPU cost would go up but you would still have a bad frame rate in this case.

If the spike occurs just when you enable and disable the effect then it could be caused by allocating extra RenderTextures needed to run the post effects, when running all the time they will tend to re-use buffers from the previous frames but on the first frame a new post effect is enabled it may well need to allocate some extra Render Textures. These RenderTextures would be twice the width when single pass VR is enabled which might cause the driver to have more trouble allocating them and so cause some extra time to be taken up than when non single pass vr is used. In non single pass vr mode then it's possible that temporary post effect RenderTextures will be shared between the left eye render and right eye render and so the total amount of VRAM required would be less in total as well. In this case you could just run all three post effects all the time but instead of disabling them tweak their parameters to perform no processing. You might also be able to reduce the amount of VRAM required by collapsing the three effects you use into one effect that does all the work as this might reduce the number of temporary buffers required.

Hope some of this helps.

Robin

 

For GrayScale you would probably need to add an extra shader variable which you can then use to control an extra lerp between the greyscale version and the original colour version at the end of the shader. It should add only a very small amount of extra cost (on a desktop class GPU).

For colour correction it would depend on the code for the particular shader/effect was doing, the standard assets have at least 3 different versions I think.

Hope this helps.

Robin