CODEWITHSUNDEEP
c#unity-game-enginemathvectorgame-development
daddylongleggs
daddylongleggs

Reputation: 43

How to calculate the 3D vector perpendicular to a polygon surface in a specific direction

I'm creating an asteroid mining game in Unity 3D, where "down" is the center of the planet you are on (So I am mostly using local rotations when dealing with vectors). I have a working physics-based character controller, but it has problems dealing with slopes. This is because when I add a force to the character, currently it pushes along the player's forward vector (see picture). What I want to do is calculate the vector that is parallel to this terrain surface, but in the direction that the player is facing (so that the player can move up the slope).

ms paint drawing

I originally thought that i could just find the vector perpendicular to the normal, but then how do I know which direction it will be in. Also complicating matters is the fact that the player could be oriented in any way in relation to the global x, y, and z.

Either way, I have the surface normals of the terrain, I have all of the player's directional vectors, but I just can't figure out how to put them all together. I can upload code or screenshots of the editor if necessary. Thanks.

Upvotes: 3

Views: 1030

Answers (1)

Nico Schertler
Nico Schertler

Reputation: 32587

The usual way is to factor out the component of the forward vector that is parallel to the surface normal:

Illustration

fp = f - dot(f, n) * n

See vector rejection.

This formulation will also make fp shorter the steeper the slope is. If you don't want that, you can re-scale fp afterwards to have the same length as f.

Upvotes: 4

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