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Bruno

Rendering a quad, facing a normal

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Hi, This is to be used in bullet decals. So, lets imagine i have the point of impact, and the normal of the face at where the impact was made. With this two values, is there a way to render a quad that faces that normal ? thanks

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Yes. You need two vectors perpendicular to the normal, let's call them Up and Right. These can be obtained for example using cross product. Given a unit-length normal vector N:

// epsilon is something like 1e-3
// we do this to prevent floating-point inaccuracies
// also, cross(v,v) is a null vector which we don't want
if(N.y < 1-epsilon)
Right = cross(N, (0,1,0))
else
Right = cross(N, (1,0,0))

Up = cross(Right, N)

Now, the vertices for the quad are (if you want the quad centered around the point of impact):

V1 = Point - Up/2 - Right/2
V2 = Point - Up/2 + Right/2
V3 = Point + Up/2 + Right/2
V4 = Point + Up/2 - Right/2

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