Exponential Variance Shadow Map
Screenshots:
Docs to read:
- What is Variance Shadow Maps
- What is Exponential Shadow Maps
- A lot theory, comparison and about Exponential variance shadow maps here. You can also find some information and shader code at a presentation from Bungie: Lighting Research at Bungie.
- Integrating Realistic Soft Shadows Into Your Game Engine. PCSS - that's what we want to have but with less computation and better quality.
- NVIDIA's shadow mapping comparison: Advanced Soft Shadow Mapping Techniques
There are two similar techniques in the last document : VSM+SAT and CSM+MIPs. But VSM has too many artifacts, CSM works really bad and slow (don't like it at all).
What we do is take filter size computation from PCSS + EVSM filter + possibility change it by building MIP level.
Algorithm:
- Render a scene at a regular Shadow Map with a MultiSampling.
- Build first mip level for EVSM by take an avarage value of all samples. Convert depth value to EVSM value.
- Blur first Shadow Map mip level into the second one.
- Calculate filter size based on a current depth and several samples from the regular Shadow Map around it.
- Based on this filter size sample EVSM texture between the first and the second mip level.
As you can see, we don't need full mip maps chain. Two mips are enough. We get correct values using trilinear filtration between mip maps.
Disadvantages:
- The main is memory. You need four component float texture. And you need shadow map with multisampling also. It requires alot of memory.
- Aliasing from a far distance or acute view angle. We use SampleLevel and a standart texture filtration doesn't work. I think you can implement it by using ddx and ddy. If you know how to do it let me know.
