attribute vec4 position;
uniform mat4 supermatrix;
void main()
{
    // the value at supermatrix[3][3] is a float that is incremented by .01 every draw call
    float Q = sin(supermatrix[3][3]);
    /*
                I want to scale the vertices of a rectangle that occupies the
        top right quadrant of the screen ((0,0) -> (1,1)) vertically
        by whatever the value of Q is, so I multiply the vertices by
        a scaling matrix which should scale the vertices' y values by Q.

                As rendering progresses, the the vertices should be scaled
        by -1x to 1x in a sinusoidal shape.

                But it doesn't. As soon as Q is negative, the rectangle no longer renders.
        */
    gl_Position = position*mat4(vec4(1.0,0.0,0.0,0.0),
                    vec4(0.0, Q ,0.0,0.0),
                    vec4(0.0,0.0, Q ,0.0),
                    vec4(0.0,0.0,0.0,1.0));
}