Model matrix in GLM
c++, glm-math, opengl
Solution
so we first translate then rotate and at last scale.
... Well, quite the opposite.
Your "complete" matrix looks like this (leaving the rotate aside for simplicity) :
proj * view * translate * scale
, right ? Well this is correct: this means that your point X will be transformed this way :
proj * view * translate * scale * X
, which means that you will first apply scale, then translation, then positioning relative to the camera, then projection. Which is perfect.
Your problem seems to be the "scale-before-translate" stuff. Imagine a translation of 10 on the X axis, a scale of 2. You apply your matrix on, say, a ship.
- You scale your ship. You have now a big ship, but still at the origin
- You translate your ship. It's now still as big, but at 10 units of the origin.
If you do the opposite :
- You translate your ship. Its center is now at 10 units of the origin
- You scale your ship. Every coordinate is multiplied by 2 relative to the origin, which is far away... So you end up with a big ship, but centered at 2*10 = 20. Which you don't want.
Does that make sense ?
(Source: opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/)
Problem
I migrate from my own matrix/vector operations to GLM and i don't understand one thing. A model matrix in OpenGL - model_matrix = scale_matrix * rotate_matrix * translate_matrix, so we first translate then rotate and at last scale. But then i try to do so in GLM it shows quad at right place only if i use inverse order of multiplication (translate * rotate * scale), but works as it should for MVP matrix (projection * view * model). Example code ``` using namespace glm; mat4 projection = ortho(0.0f, 1.0f, 0.0f, 1.0f); mat4 translate = translate(mat4(1.0f), vec3(0.5f, 0.5f, 0.0f)); mat4 rotate = rotate(mat4(1.0f), 90.0f, vec3(0.0f, 0.0f, 1.0f)); mat4 scale = scale(mat4(1.0f), vec3(0.5f, 0.5f, 1.0f)); mat4 m = translate * scale * rotate;// must be scale * rotate * translate mat4 mvp = projection * mat4(1.0f)/*view matrix*/ * m; glUseProgram(shader->prog); glUniformMatrix4fv(shader->uniforms[0]/*um_mvp*/, 1, GL_FALSE, value_ptr(mvp)); ... ``` Vertex shader ``` attribute vec3 av_pos; attribute vec2 av_tex; uniform mat4 um_mvp; varying vec2 vv_tex; void main() { vv_tex = av_tex; gl_Position = um_mvp * vec4(av_pos, 1.0); } ```