I've been standardising the turnarounds for the different train carriages (making sure the camera is the same in each one, and the light, etc), and as I was doing this I made a chance discovery. The composite process I was using involved 2 shadow passes - a hard, black/white raytraced shadow pass; and a smooth, graduated light-angle ramp shader pass. However, when setting this up for one of the carriages, I accidentally left the raytracing on in the ramp-shader pass; when I did a test render this revealed an error that was built into my compositing process.
The old approach, because of it's use of 2 multiplied layers with 50% opacity, created odd effects when the hard shadow was overlaid on the soft shade - for example, giving darker shades when a shadow was cast on a moderately shadowed face than an unshadowed (but supposedly darker) face adjacent.
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| Old Shadow Layer |
By contrast, combining the raytrace shadow and the light-angle shade in one pass gives a much more realistic shading, without odd results like the left-hand carriage end face being lighter than the shadowed center section
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| New Shadow Layer |
To show the effect of this change, I've done full composite tests
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| Old Method Composite |
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| New Method Composite |
As a side effect of this change, the number of render-layers involved in the composite is reduced by 1/5th, which should cut down on render time
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