I have a question regarding the the shdw and draw settings.
I created a particle flow that I cached to disk. It contains 10 million particles. I placed lights in the scene and created a cool look using absorbtion shdw and draw.
Now I want to create a higher resolution render but want to contain the same look. So I cached more particles to disk. Lets say I use now 20 million particles. The problem is that the look is now different. If I changed the Shdw and Draw than I can come close but it is still not the same.
Is there a way to calculate the new setting if I use for example twice more particles?
The general rule is that adding 2x more particles requires you to reduce the Density multipliers for shadows and final pass drawing by the same factor. Each particle contributes a per-particle Density value. If you have 2x more particles, the Density per cubic unit of space will also increase about 2x, so you have to counteract by reducing the global density scale 2x.
For example, if the Shadow multiplier was 5.0 Exponent -1, you would have to change it to 2.5 Exponent -1.
If you added 10x more particles (10 partitions of 10 million particles each), then you would need to reduce the multipliers 10x, so you could just enter 5.0 Exponent -2 (one order of magnitude less Density by changing the exponent down by 1)…
However, the results might not be perfectly identical, because lower density particle clouds might have too much empty space between the particles to block the light well. When you increase the particle count via Partitioning, you get a lot of that empty space covered, and the particle cloud becomes generally more solid from the point of view of the light and camera. So it might appear to be blocking more light even after adjusting the multipliers. For that reason, rendering at a fraction of the image resolution and reducing the Shadow Map Size when rendering the lower particle count could be an useful approach to produce comparable results to higher count rendering at higher image resolutions with larger shadow maps.