GLM: How to transpose a vector?

Question

Maybe I'm just missing something in the docs, but it seem it's not possible with GLM to take the transpose of a vector. I also see no mat3x1 or mat1x3 types. Also glm::transpose doesn't work for vectors. Am I missing something or is this just a feature lacking in GLM?

Solution 1

GLM is based on GLSL, where there's simply no need to transpose a vector. If you do vector/matrix multiplication, it will multiply the vector in the way that works for the size of the matrix (unless it would have to change the order of the multiplication). So if you have a mat4 and do mat4*vec4, your vec4 is considered a column vector. If you do vec4*mat4, it is considered a row vector. If you do mat2x4*vec4, you get an error, while vec4*mat2x4 works (as a row vector).

So in general, there's no reason to need to "transpose" a vector. The system simply does whatever works.

Solution 2

As a reference for people looking for how to transpose a vector (primarily for calculating outer products - u v^T) in GLSL/GLM; its:

glm::core::function::matrix::outerProduct(u, v)

Nicol's GLM link now 404s as their API links have changed format from:

• OLD Link: http://glm.g-truc.net/api-0.9.4/a00133.html#ga5d896e8651512fc098a677dbe403eeac

• New Link: http://glm.g-truc.net/0.9.4/api/a00133.html#ga5d896e8651512fc098a677dbe403eeac

Comments

[aeskreis]

I see, I think that is what I wasn't understanding. Thank you.

[Chuck]

What if you want to calculate something like v * transpose(v) where v is a vec4. What I want is a mat4 as a result. I.e. multiplying a 4x1 and a 1x4 to get a 4x4. I don't see how to do that in GLM. Maybe you can't?

[Nicol Bolas]

@Chuck: Well, that's probably because it's not a very useful operation. Besides, you can do the math for it yourself easily enough; it's just pairwise multiplying each element of the vector.

[Chuck]

Of course I can do it myself. However, it is a very useful general purpose operation. For instance, it is a standard piece of things like the matrix formulation of the Rodrigues' rotation formula. It would be nice to have merely as syntactic sugar. Languages like APL, A+, k, Matlab, etc... provide this out of the box.

[Nicol Bolas]

@Chuck: The operation you're talking about is the Outer Product. Which both GLSL and GLM can do.