You probably need to know a little about vector calculus for this one, which is something that's rarely learned outside a maths or physics degree. The first thing is that dNdv and dNdu are 3 dimensional vectors representing the amount the normal vector changes as you travel accross the surface of the model in the v and u directions respectively (those are the same u and v you get in a UV map). In order to figure out how strongly curved the surface is at any point, we want to know how long those two vectors are. A 3D vector is usually expressed as a set of 3 values like this: "(x,y,z)" or "(component0, component1, component2)". To find out how long the vector is, we get the square root of the sum of the square of each of the three components: Sqrt(comp0^2 + comp1^2 + comp2^2) That's what iikuuk is doing in those two top right columns of nodes. The component funtions extract those three values, the power functions square each of them, then they are added together. The top two nodes over on the left get the square roots (two, since there are two vectors). At this point we have a measure of the curvature of the surface, but we really want a measure that's always going to be between 0 and 1, rather than a measure that could conceivably be any positive value, so we have to do a little trick to convert our measure to a measure between 0 and 1. What iikuuk does with the next couple of nodes is this: newmeasure=oldmeasure/(oldmeasure+50) This is always smaller than 1 and could be as small as zero but no smaller. The rest is really just about choosing a suitable cutt off point - how big does your measure need to be before you decide to chip the paint off? A few general observations: * This relies on having a good UV mapping. It needs to be a low distortion mapping to work well. * This won't work on models where the vetices have been split to create hard edges. You need a model with continuous, unbroken mesh across those sharp edges. * You can cut down the number of nodes used by adding dNdv and dNdu together (use a color math node rather than a math node) and calculating the magnitude of the result rather than adding the magnitudes of the two separately. It'll give slightly different results but does essentially the same thing and is arguably a more "correct" approach.

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