Passing uniform 4x4 matrix to vertex shader program

Question

I am trying to learn OpenGL and following this: http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/

Up until the point where they started passing matrices to the vertex shader to translate the triangle they where drawing I was following along.

This is the shader program where it starts to go wrong:

#version 330 core

layout(location = 0) in vec3 vertexPosition_modelspace;
uniform mat4 MVP;

void main(){
    vec4 v = vec4(vertexPosition_modelspace,1); // Transform an homogeneous 4D vector

    gl_Position = MVP * v;

    //mat4 M = mat4(
    // vec4(1.0, 0.0, 0.0, 0.0),
    // vec4(0.0, 1.0, 0.0, 0.0),
    // vec4(0.0, 0.0, 1.0, 0.0),
    // vec4(0.0, 0.0, 0.0, 1.0)
    //);
    //gl_Position = M * v;
}

If I use the commented out code instead of the line gl_Position = MVP * v;, everything works. A triangle is drawn to the screen. I can also change to M matrix to move the triangle around and scale it.

In order to keep things as simple as possible I am just passing the vertex shader an identity matrix in the MVP field. The code looks like this:

mat4x4 MVP;
mat4x4_identity(MVP);

int i,j;
for(i=0; i<4; ++i) {
    for(j=0; j<4; ++j)
        printf("%f, ", MVP[i][j]);
    printf("\n");
}

GLuint MatrixID = glGetUniformLocation(programID, "MVP");

// Send our transformation to the currently bound shader,
// in the "MVP" uniform
// For each model you render, since the MVP will be different (at least the M part)
glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);

I am using linmath.h instead of glm (https://github.com/datenwolf/linmath.h). The loop prints:

1.000000, 0.000000, 0.000000, 0.000000, 
0.000000, 1.000000, 0.000000, 0.000000, 
0.000000, 0.000000, 1.000000, 0.000000, 
0.000000, 0.000000, 0.000000, 1.000000,

And confirms that MVP is indeed an identity matrix. However when I run the program, I see no triangle on the screen. Only the background (which is dark blue btw).

You can see slightly more of the main C program here: https://gist.github.com/avwhite/68580376ddf9a7ec9cb7

If needed I can also provide the whole source code for the main program, the vertex-, and the fragment shader.

I am thinking this has something to do with how the MVP matrix is passed to the shader program, but I am completely new to OpenGL, so I really have no idea what is going on.

By doing "printf("%u\n", MatrixID);" Right after the call to glGetUniformLocation. It prints 0 every time. — Andreas Vinter-Hviid
– Andreas Vinter-Hviid, Commented Aug 16, 2014 at 18:37

Reto Koradi · Accepted Answer · 2014-08-16 19:03:07Z

27

glUniform*() calls set values for the current program. You need to call glUseProgram() before the glUniform*() call. In this example:

glUseProgram(programID);
glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);

answered Aug 16, 2014 at 19:03

Reto Koradi

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4 Comments

Andreas Vinter-Hviid Over a year ago

This seem to be working! Thank you very much! I should have looked at the full source code provided with the tutorial instead of just the snippets. Somehow it didn't occur to me that I might have put something in the wrong place.

user007 Over a year ago

Does this mean you have to call glUseProgram even if you are using the same shaders in the next draw call but with different uniform data?

Lupus Ossorum Over a year ago

@user007 you shouldn't.

Michael Sinclair Over a year ago

matrixID should be glGetUniformLocation(shaderProgram, uniformStr)

kupihleba · Accepted Answer · 2018-05-26 20:12:25Z

9

Consider using value_ptr rather then &MVP[0][0]

glUniformMatrix4fv(MatrixID, 1, GL_FALSE, glm::value_ptr(MVP));

genType::value_type const* glm::value_ptr (genType const & vec)

Return the constant address to the data of the input parameter

answered May 26, 2018 at 20:12

kupihleba

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