What is wrong with my perspective projection matrix?

Question

I'm using WebGL 2. I set up my projection matrix as follows:

function ProjMatrix(width, height, near, far, fov)
{
    var ar = width / height;
    var fac = 1.0 / Math.tan(fov/2.0);

    return [
        fac/ar, 0.0, 0.0, 0.0,
        0.0, fac, 0.0, 0.0,
        0.0, 0.0, -(far+near)/(far-near), -1.0,
        0.0, 0.0, -(2*far*near) / (far-near), 0.0
    ];
}

I initialise it and pass it to the shader as follows:

// set up the matrices
var matProj = ProjMatrix(canvas.width * 1.0, canvas.height * 1.0, 0.1, 100.0, 60.0*Math.PI/180.0);
dumpMatrix(matProj);
gl.uniformMatrix4fv(program.uProj, false, new Float32Array(matProj));
gl.uniformMatrix4fv(program.uView, false, new Float32Array(IdentityMatrix()));
gl.uniformMatrix4fv(program.uModel, false, new Float32Array(IdentityMatrix()));

The model is as follows:

var modelData = [
    -1.0, -1.0, 1.0, 1.0,   1.0, 0.0, 0.0, 1.0,
    1.0, -1.0, 1.0, 1.0,    0.0, 1.0, 0.0, 1.0,
    0.0, 1.0, 1.0, 1.0, 0.0, 0.0, 1.0, 1.0,
];

Where the first 4 numbers on each row are the position, and the other 4 are the color.

When using this model and this matrix, nothing at all is drawn. HOWEVER, if I use an identity matrix, and the coordinates are z=0 and x and y between 0.5 and -0.5 instead, it DOES get drawn, so the problem must be with the matrix.

Can anyone suggest what could be wrong here?

Rabbid76 Rabbid76 · Accepted Answer · 2018-03-16T17:50:07

The projection matrix transforms from view space to clipspace. You have specified a right handed projection matrix.
See Understanding OpenGL’s Matrices

This means, if the X-axis points to the left, and the Y-axis points up, then the Z-axis points out of the viewport (Note the Z-Axis is the cross product of the X-Axis and the Y-Axis).

Since you haven't specified an view matrix which transforms from world space to view space, your have to specify your coordinates in view space.

Invert the z component of your coordinates to solve the issue:

var modelData = [
    -1.0, -1.0, -1.0, 1.0,    1.0, 0.0, 0.0, 1.0,
     1.0, -1.0, -1.0, 1.0,    0.0, 1.0, 0.0, 1.0,
     0.0,  1.0, -1.0, 1.0,    0.0, 0.0, 1.0, 1.0,
];

What is wrong with my perspective projection matrix?

1 Answers