问题:

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.

回答1:

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]);

回答2:

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

转载请标明出处:Passing uniform 4x4 matrix to vertex shader program

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