These days I often look at the real shadows of trees or any objects. Strangely they don't look natural or real for me. It makes me thinking of what kind of soft shadow I want to see.
The softness of shadows is from frequency of light. Around edges the shadow looks sharp, because it is resulted from high frequency. Some of soft shadows are from low light frequency.When we render a shadow map, it is analogous to the HIGH frequency light. It is proper to the shadows close to the object but improper to the shadow far from the object.
We need LOW frequency shadow map for distance shadows. Low frequency shadow map can be retrieved by Gaussian blur. The blurred shadow map must have the same resolution to the original shadow map.
One problem here is again how to blur the shadow buffer which contains encoded values.
Simplest way is to sample nine times and calculate Gaussian blur, but this way will be too slow.
I have already described how to get blurred depth buffer with linear filter before: http://wrice.blogspot.com/2010/04/blurring-depth-buffer-with-linear.html
Today I found that I can also do this with the Gaussian filter on PS3. The trick is very similar to the one with linear filter, but it needs more buffer bits on each channel. And this is not for down sampling but only for blur.
The Gaussian filter on PS3 does "[ 1, 2, 1 ] [ 2, 4, 2 ] [ 1, 2, 1 ] / 16".
Since it divides by 16 ( =2^4 ), we need to have 4 bits as buffer on each color channel.
It means that we can use 4bits on Red channel, 4 bits on Blue channel, 4 bits on Green channel, and 8 bits on Alpha channel. Therefore, we can use total 20bits, which is large enough.
The way to use those shadow maps is to sample on the non-blurred shadow map first and sample one more time on the most blurred shadow map. The difference value between those two sampled values will represent how much the shadow needs to be blurred.