glm rotation in ortho space

Question

I set up my ortho projection like this :

transform = glm::ortho(0.0f, width, height, 0.0f);

this works pretty well, but when I want to use the glm::rotate function like this:

transform = glm::rotate(transform, glm::radians(45.0f), glm::vec3(0, 0, 1));

my object rotates around 0 : 0 : 0.

my vertices look like this:

GLfloat vertices[] = {
    600, 100, 0,
    612, 100, 0,
    612, 130, 0,
    600, 130, 0
};

how can I make my object rotate around its center ?

I've removed the [tag:glm] tag. [tag:glm] (generalized linear models) != [tag:glm-math] (GLM - OpenGL Mathematics) — Rabbid76, Oct 03 '21 at 10:52

score 0 · Accepted Answer · answered Oct 03 '21 at 10:40

If you want to rotate around its center you have to:

Translate the object so that the center of the object is moved to (0, 0).
Rotate the object.
Move the object so that the center point moves in its original position.

GLfloat center_x = 606.0f;
GLfloat center_y = 115.0f;

transform = glm::translate(transform, glm::vec3(center_x, center_y, 0));
transform = glm::rotate(transform, glm::radians(45.0f), glm::vec3(0, 0, 1));
transform = glm::translate(transform, glm::vec3(-center_x, -center_y, 0));

See also How to use Pivot Point in Transformations

glm rotation in ortho space

1 Answers1