Change vertex colours based on steepness

Questions & Answers
1

I’m messing around with generated terrain using some triangles and Perlin Noise, and I was wondering how I would change the colour of the vertices based on the steepness.

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My best guess would be to get the angle between the triangle and Vector3.up and then just evaluate that on a gradient for every triangle in the terrain, but I’ve been having a hard time figuring out how I would actually do that.
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Thanks!

2

With a triangle, you have 3 points by default – conveniently enough for a plane, which has a normal vector:

float CalculateSteepnessAngle(Vector3 a, Vector3 b, Vector3 c)
{
    // Get 2 vectors from these 3 points, it doesn't matter 
    // which ones we use as long as they're from the same point
    var ab = b - a;
    var ac = c - a;

    // The cross of those 2 vectors will be the planar normal
    var n = Vector3.Cross(ab, ac);

    // The angular difference between this normal and the
    // world's up vector can be used for a steepness value
    return Vector3.Angle(n, Vector3.up);
}

So with that, you should be able to determine “steepness” for a given triangle on your terrain.