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Topic: Hsphere subdivs vs. samples taken

posted by Torgeir Holm
archived on 16.8.2003

Someone asked if you can multiply HSphere subdivs and Interp. samples to determine the total number of rays in each sample. You can't.

Intepolation samples are only used while rendering, not while calculating the irradiance map. There is a separate interpolation setting in the advanced settings rollout that lets you set the value used while rendering. The interpolation samples value determines how many GI samples will be used to calculate the final illumination at each pixel in the rendered image.

Basically HSphere subdivs works like this:
When VRay decides it wants a GI sample at a particular location it imagines a virtual hemisphere aligned to the surface normal around that location. It then shoots out a number of rays in quasi random (based on QMC algorithms) directions inside this hemisphere. These rays are then averaged to get the average illumination of that particular location. The location is added to the irradiance map, and VRay moves on to the next sample location.

To get the numbers of rays for each sample you can square the HSphere subdivs value. So for 1 subdiv, VRay only shoots a single ray at that point, in a random direction. This gets you really blotchy results since there is no averaging going on at all. I usually start at 20 or so and increase it to get a smooth solution. The list below shows how many rays will be in a GI sample with different HSphere subdivs. It might also help in estimating rendertimes. For instance going from 25 to 50 will make VRay trace 4 times as many rays per GI sample, and will probably make your GI calculation take 4 times as long. Going to 100 in the same case would give you 16x the calculation time.

HSphere subdivs: 1 = 1 ray
HSphere subdivs: 2 = 4 rays
HSphere subdivs: 3 = 9 rays
HSphere subdivs: 4 = 16 rays
HSphere subdivs: 5 = 25 rays
HSphere subdivs: 6 = 36 rays
HSphere subdivs: 7 = 49 rays
HSphere subdivs: 8 = 64 rays
HSphere subdivs: 9 = 81 rays
HSphere subdivs: 10 = 100 rays
HSphere subdivs: 15 = 225 rays
HSphere subdivs: 20 = 400 rays
HSphere subdivs: 25 = 625 rays
HSphere subdivs: 50 = 2500 rays
HSphere subdivs: 100 = 10000 rays

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