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A SHORT EXPLANATION OF BSDF MATRIX DIRECTIONS
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Radiance uses whichever transmission matrix is appropriate and
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available for the direction of ray travel. If you are rendering
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from the eye to the interior surface of a window, this typically
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corresponds to the "backwards" light direction, which uses the
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"Transmission Back" matrix from the XML file if present. This
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naming is due to the unfortunate precedent WINDOW 6 uses of considering
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the exterior of the window the "front" even though in Radiance
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models, it corresponds to the opposite side from the window surface
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normal.
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If there is no "Transmission Back" matrix but there is a "Transmission
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Front" (as might be generated by WINDOW or 'genBSDF +forward
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-backward'), Radiance will interrogate the "Front" matrix in the
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reverse direction, relying on reciprocity working properly. The
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reverse logic applies if the eye starts from outside (looking at
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the back side of the window surface), in which case the preference
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is for the "Transmission Front", using reciprocity on the "Back"
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matrix if it's not available.
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Something similar happens using dctimestep or rmtxop, except that
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in this case, the "Transmission Front" is *always* favored, and
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reciprocity is used on "Transmission Back" if it isn't available.
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The reason for this is two-fold. First, the "Transmission Front"
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matrix is the right way around for the 3-phase method already.
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Second, WINDOW 6 generally produces correct data for "Transmission
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Front," but in some cases just has dummy values as a placeholder
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for the "Transmission Back" matrix. (Someone correct me on this
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if I am mistaken, but that's what I understood.)
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The situation is a bit simpler in the case of reflection, since one
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cannot construct the front reflection matrix from the rear or vice
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versa. If one of the reflectance matrices is missing during
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rendering, then the surface gets no reflected contributions from
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that side. The 3-phase method does not make use of reflectance,
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so it has no bearing on dctimestep or rmtxop.
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