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

[Chris Cox]

Florian;

if the data is digitized or encoded in any way - then any description of it 
must include the encodiing.
For example: log compresssion of sound/pressure values can be done in analog 
electronics, but you still have to describe it as log encoded or you'll get 
lousy results when you try to playback or process that signal later.

You really should use light as the baseline (truth) and reference the encoding 
to that.
Saying that something is "perceptually linear" tends to confuse things.

Stating that human perception of light is approximately logarithmic is fine.
But describing an encoding (as you did) as "perceptually logarithmic" is quite 
misleading (as I described previously).

Chris



-----Original Message-----
From: Florian Kainz [mailto:address@hidden
Sent: Tue 12/19/2006 9:27 PM
To: Chris Cox
Cc: address@hidden
Subject: Re: [Openexr-devel] OpenEXR 1.5.0, OpenEXR-Images 1.5.0 released
 
Statements to the effect that human perception of physical quantities
such as luminance, force or sound pressure is logarithmic do not refer
to data encoding.


Chris Cox wrote:
> 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
> 
>