Substrate implementation proposal

[Leonardo Giordani]

> I'm not sure that would be a big problem. Leonardo -- do you think a tile
> limitation for substrates of 64x64 would be problem? Otherwise -- we've
> always wanted to make the tilesize configurable, right :-)

I think that _if possible_ it would surely be a better solution to let the 
tilesize be configurable. The tilesize is a sort of measure of the repeat 
degree of the substrate: if giving total freedom to the user about the tile 
size is a problem we could give him a set of pre-selected sizes (64x64, 
128x128, ...).

- Trasmittance
About the "trasmittance" field of the subtrate: trasmittance is the measure of 
the amount of light passes through the material (it is also related to the 
thickness, but by now forget this). What we _see_ is always only reflectance 
(the amount of light that returns to our eyes), but trasmittance is involved 
in the computation of the total reflectactance of two or more superimposed 
layers. This is the _only_use of trasmittance in a paint application, where 
you look at the medium always from the front side.

Substrate is the "last" material on our virtual desktop, i.e. it is stacked 
over nothing, its trasmittance should be set to 0 and not changed, that is 
there should be no "trasmittance" field in the substrate.

- More on thickness
Reflectance and trasmittance are not property of the pure material, but depend 
on the thickness of the medium: the material has two different parameters (in 
the Kubelka-Munk model, at least) from which reflectance and trasmittance are 
computed, given the thickness.
We can give directly the reflectance of the substrate without definig it 
through KM or other models, but it should be a reflectance/thickness measure, 
which must be multiplied by the current height to obtain reflectance. Once 
multiplied it becomes a real reflectance (between 0 and 1) and normalized 
between 0 and 255 can be used as the color of the substrate.

(Sorry for the academic explanation, was just to be clear)

Leo

-- 
Leonardo Giordani

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