glm::cross should support 2d vector.

[zhou13]

So I don't need write my own:

double cross(dvec2 const& a, dvec2 const& b)
{
    return a.x * b.y - b.x * a.y;
}

[bcrist]

What does that function have to do with the cross product? That looks suspiciously like glm::determinant to me.

[zhou13]

This is often used in computational geometry. You can understand it as the the length of vector a x b if we treat a and b as a 3d vector in z=0 plane..

[bcrist]

Sure, length(cross(dvec3(a,0), dvec3(b,0))) and determinant(dmat2(a,b)) are equivalent, and both represent the area of a 2D parallelogram with sides a and b, which is a useful thing. But that doesn't mean it should be called the cross product.

[zhou13]

Well, naming is just a convention. For example, numpy supports such 2d operator in cross product: https://docs.scipy.org/doc/numpy/reference/generated/numpy.cross.html, which may be not a precise name. I could write return glm::determinant(glm::dmat2(a, b)), but that is a little bit awkward.

[bcrist]

I think this is a good addition to GLM, but I still think it shouldn't be called glm::cross, or at least should be put in an extension header so that people who don't want this overload don't get it. For one thing, GLSL, HLSL, and Cg all define the cross function specifically as a function that both returns and takes 3-component vectors as parameters. Mathematically speaking, the cross product, scalar triple product, and the function described here are all just types of exterior products. Perhaps a new GTX_exterior_product extension would be useful?

[Groovounet]

I added a vec2 implementation of cross in within an GTX_exterior_product extension. :D

Thanks for suggesting and discussing the topic!
Christophe