Invert x-axis of a matrix

emnullfuenf

I already asked this question in the hardware section while announcing my Oculus Rift solution:

How do i invert the x-axis of a 4x4 matrix before applying it to the sketch via applyMatrix()?

Any idea? I haven't found a simple solution in Processing yet. Next i'll have a look at javax.vecmath.Matrix4f.

GoToLoop

Dunno, but I'm gonna risk an attempt answer -> scale(-1, 0); :-SS

asimes

Assuming your 4x4 matrix is in this form:

Rx1, Rx2, Rx3, x
Ry1, Ry2, Ry3, y
Rz1, Rz2, Rz3, z
0.0, 0.0, 0.0, 1.0

You would need to multiply the top right element by -1. This would probably be element [3] if it is a single dimension array representation or element [0][3] if it is a double array.

asimes

If it is being done with matrix multiplication this would do it:

-1, 0, 0, 0
0, 1, 0, 0
0, 0, 1, 0
0, 0, 0, 1

Poersch

Like @GoToLoop wrote, using scale(-1.0, 1.0) will invert the x-axis and produce the following matrix:

-1.0  0.0  0.0  0.0
-0.0  1.0  0.0  0.0
-0.0  0.0  1.0  0.0
-0.0  0.0  0.0  1.0

You can also multiply the matrix "by hand", like @asimes mentioned, using applyMatrix():

applyMatrix(-1.0,  0.0,  0.0,  0.0,
             0.0,  1.0,  0.0,  0.0,
             0.0,  0.0,  1.0,  0.0,
             0.0,  0.0,  0.0,  1.0);

A little example - without the scale() call the arrow would be facing right:

void setup() {
    size(400, 400, P2D);
    noStroke();
    fill(#ffffff);
}

void draw() {

    background(#000000);

    translate(width / 2, height / 2);
    scale(-1.0, 1.0);

    beginShape();
    vertex(0, -20);
    vertex(60, -20);
    vertex(60, -60);
    vertex(120, 0);
    vertex(60, 60);
    vertex(60, 20);
    vertex(0, 20);
    endShape(CLOSE);

}

emnullfuenf

Thanks for the answers! I'll implement it next week and hopefully publish the Oculus Rift code.

Poersch

No problem. Would love to see your Code! And don't forget to mark the correct answer/answers. It will help others to find the solution more quickly. ;)