周围像素 $j$ 包括了自己，
Problem of Gaussian filtering:
- Also blurs the boundary
- The boundary is the high frequency that we want to keep
Bilateral Filtering 双边滤波
- The boundary → drastically changing colors
- How to keep the boundary?
- Let pixel $j$ contribute less if its color is too different to $i$
- Simply add more control to the kernel
Joint Bilateral Filtering / Cross Bilateral Filtering（意思一样）
- Gasussian filtering: 1 metric (distance)
- Bilateral filtering: 2 metrics (position distance and color distance)
- Especially good at denoising path traced rendering results!
- Use G-buffers
- G-buffers are noise-free as they are not generated by multi-bounces