CODEWITHSUNDEEP
c++openglglslglm-mathvertex-shader
olegst
olegst

Reputation: 1267

Strange MVP behavior during translation

I'm learning the basics of OpenGL and trying to translate a triangle with ortho projection. I don't see what I expect, what am I doing wrong?

My Vertex buffer:

Vertices[0] = Vector3f(-1.0f, -1.0f, 0.0f);
Vertices[1] = Vector3f(1.0f, -1.0f, 0.0f);
Vertices[2] = Vector3f(-1.0f, 1.0f, 0.0f);

glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);

Vertex shader:

#version 330

layout (location = 0) in vec3 Position;

uniform mat4 gMVP;

void main()
{
    gl_Position = gMVP * vec4(Position, 1.0);
}

MVP calculation and rendering:

    glm::mat4 p = glm::ortho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f);
    glm::mat4 v = glm::lookAt(glm::vec3(0,0,1), glm::vec3(0,0,0), glm::vec3(0,1,0)); 
    glm::mat4 m = glm::translate(glm::mat4(1.0f), glm::vec3(0.5f, 0.0f, 0.0f));
    glm::mat4 MVP = p * v * m; 

    glUniformMatrix4fv(gMVPLocation, 1, GL_TRUE, &MVP[0][0]);

    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    glDisableVertexAttribArray(0);

    glutSwapBuffers();

the result:

enter image description here

Shouldn't it just move to the right along the X axis?

Upvotes: 2

Views: 102

Answers (1)

Rabbid76
Rabbid76

Reputation: 211258

The 3rd parameter of glUniformMatrix4fv causes that the matrix will be transposed when it is set to the uniform. OpenGL Mathematics (GLM) constructs the matrices in the same way, as GLSL constructs variables of type mat4.
The matrix has not to be transposed when it is set to the uniform:

glUniformMatrix4fv(gMVPLocation, 1, GL_TRUE, &MVP[0][0]); 

glUniformMatrix4fv(gMVPLocation, 1, GL_FALSE, &MVP[0][0]);

In GLSL the vector has to be multiplied to the matrix from the right. See GLSL Programming/Vector and Matrix Operations

Upvotes: 4

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