Ice is something that I assume to have ~0 friction, but you are right, the explanation was not really clear on how handle the whole thing mathematically as the point of where it will become larger is not at 0.
For game usage of physics, I think average would be a good solution I think.
An alternative idea could be weight based combination. With weight based I mean that you define a "friction" which is assumed to be the "0 impact" friction, so if one of the 2 materials has that friction, the combined friction would be the friction of the other material. If it is below then the combined is lower than the other and if the materials friction is higher than that value, the combined would be larger than the other materials friction.
Most likely a not that understandable explanation so here a mathematical version :)
combinedFric = maxFric + (minFric - zeroImpactFric) * maxFric = maxFric * (1 + minFric - zeroImpactFric)
As an example (pure fantasy)
Tire: 0.8
Polished Metal: 0.5 (zeroImpact)
Ice: 0.1
Now if the tire is on ice this would mean:
combinedFric = 0.8 * (1 + 0.1 - 0.5) = 0.8 * 0.6 = 0.48
Average in this case would be 0.45
The difference isn't really present.
So for regular cases, using that assumption would work.
Did you have a look at the NewtonPlayGround on how they handle this kind of stuff?
PS: Main problem behind this whole ideas of mine and perhaps others is that friction as such is defined as a force factor that specifies the brake force between 2 different materials. It is not an attribute of a single material.
For that reason, in above weight-based idea I better should have called it surfaceFactor and the "combinedFric" is actually the friction between those 2 materials.
For game usage of physics, I think average would be a good solution I think.
An alternative idea could be weight based combination. With weight based I mean that you define a "friction" which is assumed to be the "0 impact" friction, so if one of the 2 materials has that friction, the combined friction would be the friction of the other material. If it is below then the combined is lower than the other and if the materials friction is higher than that value, the combined would be larger than the other materials friction.
Most likely a not that understandable explanation so here a mathematical version :)
combinedFric = maxFric + (minFric - zeroImpactFric) * maxFric = maxFric * (1 + minFric - zeroImpactFric)
As an example (pure fantasy)
Tire: 0.8
Polished Metal: 0.5 (zeroImpact)
Ice: 0.1
Now if the tire is on ice this would mean:
combinedFric = 0.8 * (1 + 0.1 - 0.5) = 0.8 * 0.6 = 0.48
Average in this case would be 0.45
The difference isn't really present.
So for regular cases, using that assumption would work.
Did you have a look at the NewtonPlayGround on how they handle this kind of stuff?
PS: Main problem behind this whole ideas of mine and perhaps others is that friction as such is defined as a force factor that specifies the brake force between 2 different materials. It is not an attribute of a single material.
For that reason, in above weight-based idea I better should have called it surfaceFactor and the "combinedFric" is actually the friction between those 2 materials.