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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. |
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For the tensor tree representation, reciprocity can be used to |
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derive all needed information for 4-D distributions, so only one |
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tree is stored to save file space and memory. However, isotropic |
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(3-D) data does not use reciprocity for sampling, even though it |
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can use it to answer BTDF queries, because it would require too |
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much work to build up a 4-D structure then derive the 3-D information |
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from it. So, this is a caveat, and genBSDF will create reverse-flow |
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isotropic BTDF data if run with +forward and +backward in this case. |
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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 data 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. |
