RE: [Openexr-devel] OpenEXR 1.5.0, OpenEXR-Images 1.5.0 released

[Chris Cox]

Florian;

I figured it was just a terminology problem.

Normally, when describing an encoding, the baseline reference is photons.
Linear is normally equated with gamma 1.0, and the values are proportional to 
photon incidence.
Logarithmic normally means that you have taken the log of a gamma 1.0 signal, 
and that corresponds fairly well to human perception.
Gamma encoded means that you have adjusted the gamma 1.0 signal by pow( 
photons, 1/X), which also correspons well to human perception when X is about 
2.0.

If the luminance is logarithmicly encoded (or gamma ~2.0 encoded), then it is 
nearly linear with respect to human perception (L*).  But saying that is is 
"perceptually linear" is rare because of the above conventions.
Saying that it is "perceptually logarithmic" means that is is logarithmic with 
respect to human perception, and would imply log( log( photons) ) ...which is 
usually a bad thing.

Chris



-----Original Message-----
From: Florian Kainz [mailto:address@hidden
Sent: Tue 12/19/2006 7:43 PM
To: Chris Cox
Cc: address@hidden
Subject: Re: [Openexr-devel] OpenEXR 1.5.0, OpenEXR-Images 1.5.0 released
 
I don't think I have the quantization backwards.  If you want to
see for yourself, go to function insertChannels() in ImfRgbaFile.cpp,
play with the settings of the the pLinear flags, and convert files
such as WideColorGamut.exr and GrayRampsHorizontal.exr from the
OpenEXR sample image collection to luminance/chroma format with B44
compression (at this point, only B44 pays attention to the pLinear
flags).

Regarding terminology - I think calling luminance perceptually
logarithmic is appropriate since perception of luminance obeys
obeys the Weber-Fechner law, at least approximately.

Florian