VrayAlSurfaceMTL sss density scale weird behavior

[LudvikKoutny]

Hi,

I've got into problems again due to Vray shaders using potato units.

I have a scene that has meters as system units. If I create a new VrayAlSurface shader in that scene, it does not work out of the box since radius values in AlSurfaceMTL are not correctly defined as distance units, therefore if I create a new material, Max does not recognize value as the distance value to be converted to meter system units.

So I have to resort to using sss density scale potato multiplier, but that's where another problem comes in: If I have meters as system units, and radius values are in undefined units, then the common sense would suggest one radius unit = one meter, but that seems to be wrong. For example if sss1 radius is 1.5, and I assume it is meters, then I should set sss density scale to 0.01 to convert those meters to centimeters. Yet I actually have to do opposite (!) to get the scale right. To set sss density scale to 100 to convert meters to centimeters? O_o

1.5 meters * 100 = 1.5 centimeters? What is the logic behind that? :O

It really should not take 15 minutes of guesswork to set up AlSurface in scenes with non-default system units...

Thanks

[vlado]

I have followed the code of the original alSurface shader, so that materials can be ported 1:1. I don't intend to change that for V-Ray 3.x

Best regards,
Vlado

[LudvikKoutny]

Well material porting certainly is a valid reason, but regardless, don't you find it confusing? In general, if someone tells you he will "scale" the number 5 ten times, do you expect the result to be 50 or 0.5? If anything, it would at least be good to re-name it. But then again, naming it "inverse scale" would probably not add much to the usability

[vlado]

Well material porting certainly is a valid reason, but regardless, don't you find it confusing?

Yes, I did find it confusing, and I was itching to change it, but portability was more important in this case. In a few months, when the original alSurface material for Arnold is dead, we will probably change it to be more logical...

Best regards,
Vlado

[LudvikKoutny]

Okay, I guess I will have to wait.

I think making radii in AlSurface defined as distance units would solve it in the meanwhile. Then, if I would drop AlSurface to scene with CM as system units, radius would show as 1.5cm, and if I did the same in scene with M as system units, It should still be 1.5cm. I've noticed that every material and map that has any value defined as distance units gets correctly converted between different system unit scales.