issues rotating a matrix (openGL/glm) [closed] - c++

Question

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 2 years ago.
Improve this question
i've been creating a basic gui recently, and i'm trying to give the user the ability to rotate a shape/image/sprite. however, when i use glm::rotate to attempt to change the model matrix i get some unexpected results. the transformed shape changes dimensions, rotates way too far, and also rotates about a seemingly random point.
shader code:
#version 330 core
layout(location = 0) in vec2 pos;
uniform mat4 model;
uniform mat4 projection;
uniform mat4 view;
void main()
{
gl_Position = projection * view * model * vec4(pos.x, pos.y, 1.0f, 1.0f);
}
rotation code:
m_modelMatrix = glm::mat4(1.0f);
m_modelMatrix = glm::rotate(m_modelMatrix, 45.f, glm::vec3(0, 0, 1);
view and projection setup
glm::mat4 proj = glm::mat4(1.0f);
glm::mat4 view = glm::mat4(1.0f);
proj = glm::perspective(glm::radians(53.f), 1.f, 0.1f, 100.f);
view = glm::translate(view, glm::vec3(0.0f, 0.0f, -3.0f));
everything is in 2d.

Answer 1

Welcome to Stack Overflow! :)
I assume that you already have a setup with a projection and view matrix, like you pointed out in the comments. It should look like this,
#version 330 core
layout(location = 0) in vec2 pos;
uniform mat4 projection;
uniform mat4 view;
uniform mat4 model;
void main()
{
gl_Position = projection * view * model * vec4(pos.x, pos.y, 1.0f, 1.0f);
}
the projection matrix should have been created with
glm::ortho( ... );
or
glm::perspective( ... );
and the view matrix should have been created with
glm::lookAt( ... );
as described in https://glm.g-truc.net/0.9.2/api/a00245.html.
If further explanation is required, I advise you to look at https://learnopengl.com/Getting-started/Coordinate-Systems as a reference.
I also assume that the problem originates from previous transformations and how this rotation is used to modify an existing model matrix. E.g. the problem only occurs when you have specified a model matrix other than the identity matrix.
When you create the model matrix, you have to make sure that you use the correct order of multiplication:
glm::mat4 translation = ... ;
glm::mat4 rotation = ... ;
glm::mat4 scale = ... ;
glm::mat4 model = translation * rotation * scale;
Since you will multiply a vector from the right hand side to the model matrix, this order guarantees you that no distortions will occur. The vector will be scaled first, then rotated and finally translated.
When you try to rotate the model matrix with,
glm::rotate(model, angle, axis);
you essentially multiply the rotation matrix from the right hand side and break the multiplication order of the model matrix.
model * rotation_from_user_input
= translation * rotation * scale * rotation_from_user_input
You would have to change the rotation of the model matrix like this
rotation *= rotation_from_user_input;
and then update the model matrix itself to fix the distortions.
model = translation * rotation * scale;