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Comment by roboy | Hacker News Reader

nice! but why don't you just calculate soft shadows the way they occur physically by sampling an area light source instead of a point light source? You can basically run the same algorithm again and again with light source points in a circle from the center of the light source, then alpha-fuse them together? Am I missing something?

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The advantage of this approach is that it achieves an approximate result in a single sample.

You might require dozens of samples to get a similar result using a multisampling technique like you describe.

There are definitely more physically accurate ways to render soft shadows, but AFAIK they all involve some kind of bounce lighting where light bounces off of objects in the scene and indirectly lights other parts of it.

This looks great when combined with area light sources, like you suggest, but in my experience it's too slow for interactive demos.

I'd love to be proven wrong on this :)

The parent is saying that you can simply do the hard shadow algorithm but n times, starting from n random points uniformly sampled from a disk around the original point light source, then averaging the result. The bigger the disk the softer the shadow. This is how soft shadows are often achieved in ray tracing.

Ah, thanks for clarifying! I think someone already called this out above, but the benefit of the approach I describe is that you only need one sample per pixel. Four samples per pixel starts to feel pretty slow on my machine, and I suspect you'd need more than that to make the averaging approach look good.

One option might be to render, say, two samples per frame, and keep the previous samples when the light source doesn’t move. The shadows would be noisy while the light is moving, but quickly look smoother when it stops.

If you want to run this in real time on a phone, ray tracing is expensive.

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