[RESOLVED] Corona Bump Converter - Theory?

[cjwidd]

What exactly is the Corona Bump Converter doing under the hood? I'm curious if there are ways to arbitrage the CBC for other purposes, maybe even gloss / roughness adjustment.

[romullus]

I don't know what do you expect from bump converter, but i doubt it would be useful for anything else than for bump mapping. Thing is that bump maps in 3ds max are treated differently than most other maps and they has some serious limitations. For example, you can't modify bump maps with standard tools, you mostly can't combine several bitmaps and procedurals, also procedural maps tend to give incorrect results when using them as bump maps. Unfortunatelly autodesk doesn't give a damn about this problem, so Corona had to introduce special map which makes bump mapping much more useable.

[cjwidd]

So does it apply a LUT or do some sort of contrast enhance, or what is it doing?

[Njen]

It essentially converts the input colour to the native Max bump delta. If you are doing anything more complicated that using the Output node to affect the scale of the bump, you need the bump converter, otherwise your input maps will have no effect in the bump slot. The bump converter will slow down the render just a tiny bit because it is performing a shader evaluation, but there is really no other way to get around it, so it is necessary.

The issue is with Max itself, not Corona, for treating the bump data differently to every other slot.

[maru]

The not-so-technical explanation is that 3ds Max treats bump differently than other mappable material properties. You can easily observe that by plugging a procedural Cellular map into diffuse and bump and observing how it works fine in the diffuse channel, but bump does not react to things like color changes in the Cellular map. Corona "fixes" that by turning the input map into standard black-and-white information. This also relies on UVW mapping, so if your object has incorrect UVWs, it might glitch.

[cjwidd]

I might be asking an overly technical question, but it was borne out of the fact that after using the bump converter with a variety of grayscale maps, it seems like the bump converter enhances the bump effect. I naively thought maybe there was some sort of S-curve, or LUT, or something happening under the hood that would explain that.

[maru]

Here is the answer from the dev team:
The bump vector is computed from the input map derivatives/local difference. It evaluates the map in several places and computes the difference of the values. Larger difference in given direction (e.g. along X-axis) means stronger bump in the same direction.
It is a standard approach when computing bump from texture/height map.

[cjwidd]

Very good, thank you!