[Radiance-general] measuring color

[Alexa I. Ruppertsberg]

Hi Jack,

I have not (yet) embarked on the use of a digital camera for calibration 
purposes, but I imagine that you also have to figure out the sensitivity 
of your sensors in the camera, as they will determine your 'signal', 
i.e. picture,  let alone that exposure etc. will also affect it. 
Actually, it sounds quite complicated in contrast to using a spectro.

I agree that as long as you measure a macbethcolour checker card and 
your data sample under the same illumination, you could go ahead.The 
routine described in the book relies heavily on macbethcal (about 
neither you nor me have an idea how it works) and also points out its 
limitation (the word 'approximation' is used). Also, nobody could 
explain to me what magic things 'pcond' does, so I rather do not use it 
(the word 'mimick' appears rather often in the man pages). Human visual 
perception cannot be accounted for by a magic RADIANCE function. If it 
would, then why are several hundred scientists world-wide still studying 
it? I agree, that the results of RADIANCE are visually appealing and 
that the physics are simulated quite well, but only if you input 
accurate data your results will be accurate.

Cheers,
Alexa


Jack de Valpine wrote:

> Hi Alexa,
> 
> Just a minor comment here. I have no doubt that the using a more 
> sophisticated measuring device will yield more accurate results. 
> However, I believe the method of using the machbethcolor checker card as 
> described in Rendering with Radiance is designed to calibrate to the 
> current lighting conditions, that is, assuming the use of a digital 
> camera, if we want to sample a particular material in a give lighting 
> condition, then we also sample the color checker chart under the same 
> conditions with the same sampling device (digital camera) and then 
> perform the calibration with macbethcal and pcomb. I do not know the 
> underlying method that Greg put together to do the calibration, so I am 
> not sure how he has accounted for varying light conditions vis a vis the 
> internal dataset that Radiance uses as part of the calibration mechanism.
> 
> -Jack de Valpine
> 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Alexa I. Ruppertsberg
Department of Optometry
University of Bradford
Bradford
BD7 1DP
UK
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~