can somebody enlighten me what the 'amplitude' (I think it should be
'length') of the vector for the spotlight-definition does?
Thanks,
Alexa
···
Subject: spotlight definition
Date: Fri, 12 Dec 2003 17:46:11 +0000
From: Alexa I. Ruppertsberg <[email protected]>
Organization: University of Bradford
To: [email protected]
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Alexa I. Ruppertsberg
Department of Optometry
University of Bradford
Bradford
BD7 1DP
UK
I know it's a bit strange, but since a spotlight sometimes acts like it originates behind its real position due to some magnification in its optics, this is a quick way to simulate the effect of a circular porthole on a distant source in the virtual focus position of the spotlight. Since it's always preferable to apply an actual photometric distribution, this approximation is probably of limited utility. Just use a short vector if you don't know what it means, or avoid the spotlight type altogether.
-Greg
···
From: M BLOJ <[email protected]>
Date: December 15, 2003 3:09:52 AM PST
Subject: spotlight definition
Date: Fri, 12 Dec 2003 17:46:11 +0000
From: Alexa I. Ruppertsberg <[email protected]>
Organization: University of Bradford
To: [email protected]
Hi,
can somebody enlighten me what the 'amplitude' (I think it should be
'length') of the vector for the spotlight-definition does?
Thanks,
Alexa
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Alexa I. Ruppertsberg
Department of Optometry
University of Bradford
Bradford
BD7 1DP
UK
The weights correspond to the conversion matrix between Radiance's RGB color space and CIE XYZ (the Y channel, specifically). This is not very well correlated to the V-lambda curve, because the conversion depends so strongly on the exact choice of red, green, and blue primaries. In the case of Radiance and most standard RGB systems, the red primary is much closer to the peak of V-lambda than is the blue primary. Thus, the contribution of the blue channel to luminance is much reduced.
I hope this explains things for you.
-Greg
···
From: M BLOJ <[email protected]>
Date: December 16, 2003 7:22:23 AM PST
Hello,
we are trying to get our heads around the weights in the equation for
computing luminance: L=179*(0.265R+0.67G+0.065B). We do understand
why one needs different weights for different channels, however we are
surprised that the red channel has a higher weight than the blue
channel. This does not correspond to what is known about human vision
(V-lambda, the luminous efficiency function).
We welcome any helpful comments about this matter.