Am 03.04.2017 um 11:26 schrieb muyu:

> 2) The calibration factor may be wrong. I simulated the reference panel and set
> the diffuse as 1.0. The value is the same over different viewing angles. But I
> found that when I increased the height of the light source, the value may change
> from 239 to 240 (why?). Then the reflectance in the scene is caculated by diving
> this calibration factor. Is this the right way?

I'm not sure I really understand what you are doing there.


If you are multiplying the `diffuse` parameters by 240/255, then you are
doing it wrong: The parameters should be set to the actual reflectance
and transmittance, /not/ "calibrated" somehow.


Also, if you place a light source of colour `rgb <1,1,1>` with no light
fading at a reasonably far distance directly overhead a planar surface
with `pigment { rgb <1,1,1> }` and `finish { diffuse 1 specular 0
ambient 0 }`, you should get a result brightness of 100% (corresponding
to an encoded pixel value of 255), /not/ something around 95%
(corresponding to an encoded pixel value of about 240).

As for the distance-dependency of the brightness, you might be measuring
off-center, and seeing a dip in brightness due to the light impinging
non-vertically; this dip is smaller the further away you place your
light source.

An alternative to a far-away light source would be a parallel light source.

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