The projected point can be expressed as  v' = Mv
where  M = (P · l) I - lpT is the planar shadow matrix.

The projection shadow matrix gives a simple way to project any vertex v from a light l onto a plane p.

To use, simply form it, and then load it into the matrix stack:

    glm::mat4 planarShadowMatrix = glm::dot(plane, lightPos) * glm::mat4(1) - glm::outerProduct(lightPos, plane);

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