Custom matrices & OpenGL shaders.

Question

I am trying to make a simple 4x4 matrix class. The data (float) is a single dimension array, and I use this code to store numbers as if it were a grid.

const inline int ind1(short x, short y) {  // Convert coords to spot on linear array, uses SIZE
    return x * (SIZE >> 2) + y;
}

This part is in the .h file

float *data;

These are in the .cpp

Mat::Mat() {
    define();
    diagDefine(1.0f);
}

void Mat::define() {
    data = new float[SIZE];
    for (int x = 0; x < SIZE >> 2; x++) {
        for (int y = 0; y < SIZE >> 2; y++) {
            data[ind1(x, y)] = 0;
        }
    }
}
void Mat::diagDefine(float nval) {
    data[ind1(0, 0)] = nval;
    data[ind1(1, 1)] = nval;
    data[ind1(2, 2)] = nval;
    data[ind1(3, 3)] = nval;
}

The problem is that when I try to multiply the matrix to my position in the vertex shader, the triangle or whatever I am drawing disappears.

My class has orthographic, perspective, translation, rotation, and scaling.

Mat Mat::getOrthographic(float left, float right, float top, float bottom, float near, float far) {
    Mat newmat;
    newmat.data[ind1(0, 0)] = 2.0f / (right - left);
    newmat.data[ind1(1, 1)] = 2.0f / (top - bottom);
    newmat.data[ind1(2, 2)] = 2.0f / (near - far);
    newmat.data[ind1(0, 3)] = (left + right) / (left - right);
    newmat.data[ind1(1, 3)] = (bottom + top) / (bottom - top);
    newmat.data[ind1(2, 3)] = (far + near) / (far - near);
    return newmat;
}

Mat Mat::getPerspective(float fov, float aspectratio, float near, float far) {
    Mat newmat;
    newmat.data[ind1(0, 0)] = (1.0f / tan((0.5f * fov) * (3.141519 / 180.0f))) / aspectratio;
    newmat.data[ind1(1, 1)] = 1.0f / tan((0.5f * fov) * (3.141519 / 180.0f));
    newmat.data[ind1(2, 2)] = (near + far) / (near - far);
    newmat.data[ind1(3, 2)] = -1.0f;
    newmat.data[ind1(2, 3)] = (2.0f * near * far) / (near - far);
    return newmat;
}

Mat Mat::getTranslation(Vec3f &vec) {
    Mat newmat;
    newmat.data[ind1(0, 3)] = vec.x;
    newmat.data[ind1(1, 3)] = vec.y;
    newmat.data[ind1(2, 3)] = vec.z;
    return newmat;
}

Mat Mat::getRotation(double angle, Vec3f &vec) {
    Mat newmat;
    float s = sin(angle);
    float c = cos(angle);
    newmat.data[ind1(0, 0)] = vec.x * (1.0f - c) + c;
    newmat.data[ind1(1, 0)] = vec.y * vec.x * (1.0f - c) + vec.z * s;
    newmat.data[ind1(2, 0)] = vec.x * vec.z * (1.0f - c) - vec.y * s;
    newmat.data[ind1(0, 1)] = vec.x * vec.y * (1.0f - c) - vec.z * s;
    newmat.data[ind1(1, 1)] = vec.y * (1.0f - c) + c;
    newmat.data[ind1(2, 1)] = vec.y * vec.z * (1.0f - c) + vec.x * s;
    newmat.data[ind1(0, 2)] = vec.x * vec.z * (1.0f - c) + vec.y * s;
    newmat.data[ind1(1, 2)] = vec.y * vec.z * (1.0f - c) - vec.x * s;
    newmat.data[ind1(2, 2)] = vec.z * (1.0f - c) + c;
    return newmat;
}

Mat Mat::getScale(Vec3f &vec) {
    Mat newmat;
    newmat.data[ind1(0, 0)] = vec.x;
    newmat.data[ind1(1, 1)] = vec.y;
    newmat.data[ind1(2, 2)] = vec.z;
    return newmat;
}

Vertex code

#version 330                                            

layout(location = 0) in vec3 pos;                       
uniform mat4 view_mat;                                  

void main() {                                           

  gl_Position =  view_mat * vec4(pos, 1.0);             

}                                                       

Finally, here is how I send the data to the shader.

// In the matrix file
float *getRawDataAsArray() { return data; }

// In the shader.h file
void Shader::GL_SET_UNIFORM_MATRIX(const char *name, Mat matrix) {
    GLint location = glGetUniformLocation(program, name);
    if(location != -1) {
        glUniformMatrix4x2fv(location, 1, GL_FALSE, matrix.getRawDataAsArray());
    }
}

// In the main.cpp (sh is shader object that contained the GET_UNIFORM_MATRIX
sh.GL_SET_UNIFORM_MATRIX("view_mat", sod2::Mat::getRotation(3.141519 / 2, 0, 0, 1));
    sh.GL_SET_UNIFORM_4f("color", 0.0, 1.0, 0.0, 1.0);

Final note: My shaders do compile perfectly. When I run it without anything to do with matrices it works perfectly. (Dealing with color or modifying position). Thanks

Answer 1

There are 2 issues in your code:

You use the wrong glUniform* function to set the view_mat uniform in your function Shader::GL_SET_UNIFORM_MATRIX. While glUniformMatrix4fv commits 16 floats for a 4*4* matrix, glUniformMatrix4x2fv commits 8 floats for a 4*2 matrix.
See glUniform.

Further See The OpenGL Shading Language 4.6, 5.4.2 Vector and Matrix Constructors, page 101:

To initialize a matrix by specifying vectors or scalars, the components are assigned to the matrix elements in column-major order.

mat4(float, float, float, float,  // first column
     float, float, float, float,  // second column
     float, float, float, float,  // third column
     float, float, float, float); // fourth column

But your matrix is set up in row-major order:

const inline int ind1(short x, short y) { 
    return x * (SIZE >> 2) + y;
}

Either the ind1 function has to be changed to fix this issue:

const inline int ind1(short x, short y) { 
    return y * (SIZE >> 2) + x;
}

Or the matrix has to be transposed, when it is set to the uniform variable:

glUniformMatrix4fv(
    location,
    1,
    GL_TRUE, // <----------------- transpose
    matrix.getRawDataAsArray());

Or the vector has to be multiplied to the matrix from the left:

gl_Position =  vec4(pos, 1.0) * view_mat;