Progress Report ~4

Met with Matt on Wednesday to update him on my progress, and discuss what I plan to do next.

I have optimised the SSAO performance by using  downscaled SSAO and Blur textures. Both texture are half the screen size, and the final term is gained by upsampling the final blur texture using bilinear interpolation. This resulted in a considerable performance gain - the framerate(fps) is almost doubled, but the quality of the SSAO has suffered somewhat. However, I consider this to be an acceptable tradeoff, considering how much lower the frametime (ms) is now.

There are a few main areas that can be optimised to increase performance:

1. Texture Lookups: This is probably the most performance consuming aspect of the SSAO shader. Downsampling the SSAO and blur textures reduces the number of lookups and increases performance.
2. Shader Organisation: Combining shader operations and reducing the amount of code can help to optimise the shaders. I will probably leave this optimisation till the end, as it will reduce the readability of the code. 
3. No. of Samples: Reducing the number of samples does result in faster performance, however the quality dip is not acceptable. Currently the sample no. stands at 16, which provides a reasonable balance between quality and speed. 
4. Blurring: The type and range of blur used will also be a factor in the performance of the program. Right now, a very simple gaussian blur has been implemented which uses two passes - vertical and horizental. Other types of blur such as bilateral(edge detecting) have yet to be tested. 

In regards to the upsampling, I am currently looking into bilateral upsampling - which is essentially bilinear upsampling, which uses the normals and depths as additional weights in order to respect the edges. Thus, the edges of objects will not be blurred. I am concerned that this may decrease performance, as it requires more texture lookups, however I am going to try and implement it for comparison.

In regards to the blur pass, I am looking into implementing a smart blur which respects the edges - using normals/depth as a weight. As with the bilateral upsampling, this may impact the performance, but, depending on the quality increase, may be worthwhile.  

I hope to get the majority of this done before next week, and then I will start on the global illumination aspect of my shader.

Also - In my previous post, I stated that I had changed the way I calculated the depthDifference from my original implementation: ie. from OccluderSample.a - linearDepth to sample.z - OccluderSample.a, because the original calculation produced odd results. I have now realised, that this because I had them in the wrong order. It should have been: linearDepth - OccluderSample.a. This produces correct looking results - which to my eyes seem slightly more defined than the sample.z - OccluderSample.a results. For now, I have implemented the ability to switch between the two within the program.