Why does my vector not rotate correctly OpenGL/GLM?

Question

I am trying to learn how to do some transformations on 3d points in OpenGL. Using this cheat sheet I believe that I have the correct matrix to multiply to my vector which I want to rotate. However, when I multiply and print the new coord, I believe that it is incorrect. (Rotating 1,0,0 90deg cc should result in 0,1,0 correct?) Why is this not working?

My code:

glm::vec4 vec(1.0f, 0.0f, 0.0f, 1.0f); 
glm::mat4 trans = {
    1.0f, 0.0f, 0.0f, 0.0f,
    0.0f, cos(glm::radians(90.0f)), -sin(glm::radians(90.0f)), 0.0f,
    0.0f, sin(glm::radians(90.0f)), cos(glm::radians(90.0f)), 0,
    0.0f, 0.0f, 0.0f, 1.0f 
};
vec = trans * vec; //I should get 0.0, 1.0, 0.0 right?
std::cout << vec.x << ", " << vec.y << ", " << vec.z << std::endl;

The above prints 1.0, 0.0, 0.0 indicating that there was no change at all?

I also tried using the rotate function in GLM to generate my matrix rather then manually specifying but I still did not get what I think should be correct (I got a different wrong answer).

glm::mat4 trans = glm::rotate(trans, 90.0f, glm::vec3(0.0, 0.0, 1.0));  //EDIT: my bad, should've been z axis not x, still not working

The above prints: -2.14..e+08, -2.14..e+08, -2.14..e+08

(PS: I just took Geometry in the previous school year, my apologies if the math is incorrect. I have a basic understanding of matrices and matrix multiplication that I picked up today to learn OpenGL transformations but other then that I'm sort of a noob at this)

If the Input is (1,0,0,1) and the Output should be (0,1,0,1), you rotation should be around the Z Axis. — Hannes Hauptmann
@HannesHauptmann ok, I tried making that change, but it still doesn't return the correct answer glm::mat4 trans = glm::rotate(trans, 90.0f, glm::vec3(0.0, 0.0, 1.0)); — Ashwin Gupta
On the left side you see how OpenGL handles it. I am 100% this is a rotation around the Z axis :) — Hannes Hauptmann
@HannesHauptmann you are right I'm sure. I just am getting mixed up probably. But I tried the Z axis and it still isn't working? — Ashwin Gupta
I am currently checking the glm specifications, I never used GLM before ^^ — Hannes Hauptmann

Jean Catanho Jean Catanho · Accepted Answer · 2016-06-22T08:20:23

In your code, you're rotating a unit vector on the x-axis around the x-axis and that doesn't change the vector (imagine rotating a pencil around itself, the direction doesn't change at all).

To achieve what you previously wanted, you should rotate the vector around the z-axis using the matrix like this:

glm::mat4 trans = {
    cos(glm::radians(90.0f)), -sin(glm::radians(90.0f)), 0.0f, 0.0f,
    sin(glm::radians(90.0f)), cos(glm::radians(90.0f)), 0.0f, 0.0f,
    0.0f, 0.0f, 1.0f, 0.0f,
    0.0f, 0.0f, 0.0f, 1.0f
};

Besides that, glm::mat4 trans = glm::rotate(trans, 90.0f, glm::vec3(0.0, 0.0, 1.0)); isn't returning the desired result because trans need to be initialized before passing it to glm::rotate. Try writing it like this:

glm::mat4 trans; // Default constructor called, trans is an identity matrix
trans = glm::rotate(trans, glm::radians(90.0f), glm::vec3(0.0, 0.0, 1.0));

Still, you might not get the expected vector (0.0f, 1.0f, 0.0f) due to precision loss when using glm::radians and cos/sen. In my test, I got the vector (-4.37113883e-008f, 1.0f, 0.0f) and -4.37113883e-008 is really -0.0000000437113883 which is a number very close to 0, the expected result.

Why does my vector not rotate correctly OpenGL/GLM?

2 Answers