Does anyone know how the matrix for a projected planar shadow is arrived at? I know what the OpenGl matrix looks like, but I don't how it's derived.

The matrix is:

[D- LxA -LyA -LzA -LwA ]
[-LxB D-LyB -LzB -LwB ]
[-LxC -LyC D-LzC -LwC ]
[-LxD -LyD -LzD D-LwD ]

where the P = xA + yB + zC + wD and L is a vector representing the light source. D is the dot product of N.L

Thanks.