Perspective projection turns cube into weird tv shaped cuboid

Question

This is my perspective projection matrix code

inline m4
Projection(float WidthOverHeight, float FOV)
{
    float Near = 1.0f;
    float Far = 100.0f;

    float f = 1.0f/(float)tan(DegToRad(FOV / 2.0f));
    float fn = 1.0f / (Near - Far);


    float a = f / WidthOverHeight;
    float b = f;
    float c = Far * fn;
    float d = Near * Far * fn;

    m4 Result = 
    {
        {{a, 0, 0, 0},
         {0, b, 0, 0},
         {0, 0, c, -1},
         {0, 0, d, 0}}
    };
    return Result;
}

And here is the main code

    m4 Project = Projection(ar, 90);
    m4 Move = {};
    CreateMat4(&Move,
        1, 0, 0, 0,
        0, 1, 0, 0,
        0, 0, 1, -2,
        0, 0, 0, 1);
    m4 Rotate = Rotation(Scale);
    Scale += 0.01f;

    m4 FinalTransformation = Project * Move * Rotate;
    SetShaderUniformMat4("Project", FinalTransformation, ShaderProgram);

Here are some pictures of the cube rotating. In the shader code I just multiply the transformation by the position (with the transformation being on the left).

I am not sure if it's helpful but here is the rotation code:

float c = cos(Angle);
float s = sin(Angle);
m4 R =
{
    {{ c, 0, s, 0},
     { 0, 1, 0, 0},
     {-s, 0, c, 0},
     { 0, 0, 0, 1}}
};
return R;

I tried multiplying the matricies in the shader code instead of on the c++ side but then everything disappeared.

@Rabbid76 that seems to have fixed it. I transposed the projection and translation matrices and multiplied them on the shader side, though the rotation one seems to be correct (probably because I didn't write that one). How does OpenGL take and multiply matrices? Since it seems to behave differently than the multiplication function that I wrote. — vk123, Dec 06 '21 at 16:45
may be useful: https://stackoverflow.com/questions/17717600/confusion-between-c-and-opengl-matrix-order-row-major-vs-column-major — apple apple, Dec 06 '21 at 17:11

score 1 · Accepted Answer · answered Dec 06 '21 at 17:02

OpenGL matrixes are stored with column major order. You have to read the columns from left to right. For example the 1st column of the matrix R is { c, 0, s, 0}, the 2nd one is { 0, 1, 0, 0} the 3rd is {-s, 0, c, 0} and the 4th is { 0, 0, 0, 1}. The lines in your code are actually columns (not rows).
Therefore you need to to transpose you projection matrix (Project) and translation matrix (Move).

Perspective projection turns cube into weird tv shaped cuboid

1 Answers1