[Radiance-general] Re: measuring color

[Greg Ward]

This is an interesting thread.  I just wanted to add a couple of notes.

First, you should check out the products available from X-rite for 
spectral measurement:

	http://www.xrite.com/product_overview.aspx?Line=24

I have used their spectrophotometers in the past, to good effect.  I 
think they're prices are competitive, but not what I'd call "cheap."

Second, a quick explanation of what macbethcal does.  Macbethcal takes 
a captured image (scanned, photographed, etc.) of a Macbeth 
ColorChecker chart and attempts to derive a transformation that takes 
this capture to the correct values known for the Macbeth chart.  It 
first computes an overall response function for each channel using the 
6 gray patches at the bottom of the chart.  Then, it computes a 3x3 
color matrix based on a least squares minimization of error on the 
unsaturated colors.  Saturated colors are allowed to go "out of gamut" 
as needed to find a reasonable solution.

In my experience, this process works well for flatbed scanners and some 
capture methods, but sometimes does poorly with certain digital 
cameras.  I do not know why.  As Alexa and Jack pointed out, macbethcal 
does not account for changes in the illumination source -- this is 
assumed to be part of the capture process (i.e., a scanner with a 
builtin source), and since the purpose is to derive a transform that 
recovers colors under this souce, this is appropriate.

If you wish to simulate general reflectance spectra under different 
illuminants, I suggest you go with a full spectral measurement and 
follow the procedure outlined in the following paper:

	http://www.anyhere.com/gward/papers/egwr02/

Third, I wanted to point Alexa and other interested readers to the 
following reference, which explains the parts of human vision pcond 
attempts to simulate and the algorithms behind it:

	http://radsite.lbl.gov/radiance/papers/lbnl39882/tonemap.pdf

I'll be the first to admit that it is not a perfect simulation of human 
vision, and this is an ongoing area of research.  (Try a websearch on 
"color appearance models" to get an idea.)  However, I think others 
will agree that pcond can be useful, and is an improvement over the 
standard "linear with clamping" method for image display.

-Greg