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Vector 2D normals

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Having a 2D Vector (x,y), it has two normals, (-y,x) and (y,-x). If you make a function that returns the normal of a 2d vector, which of both is usually more useful? i mean for return that and not the other.

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Generally, it will make no difference as long as you are consistent. The only times I can think in which it would make a difference is for instance computing the edge normals for a polygon; depending on the polygon winding (clockwise or anticlockwise) one normal or the other would point into the polygon or out of the polygon.

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As long as you are consistent you should be ok with either conventions. I think it's more common to use (-y,x) since it represents a “positive” rotation of 90° and { (x,y), (-y,x) } is a positively oriented basis (it has the same orientation of the current basis). It also corresponds to the multiplication by the imaginary unit when using complex numbers.

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