LayoutTests/css3/filters/resources/grayscale-color-matrix.fs

void main()
{
    css_ColorMatrix = mat4(
        0.333, 0.333, 0.333, 0.000,
        0.333, 0.333, 0.333, 0.000,
        0.333, 0.333, 0.333, 0.000,
        0.000, 0.000, 0.000, 1.000
    );
}