Houdini math note

dot product:Determine the general direction of velocity.(acute angle or obtuse angle)

In order to determine that the Angle between @v and the X-axis is acute or obtuse.

x=(1,0,0)

if (@v dot x) > 0 = +x, else ＝ opposite direction

dot product Find the angle between two vectors.

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vector a = set(0,1,0);
vector b = set(1,0,0);
f@angle=degrees(acos(dot(a, b)/length(a)*length(b)));

@angle=90

Also, it can be written as:

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vector a = normalize(set(0,1,0));
vector b = normalize(set(1,0,0));
f@angle=degrees(acos(dot(a, b)/a*b));

Similarly, when cos = 0,1, and -1, they are going in the same direction, perpendicular, and opposite.