[argyllcms] Re: [argyllcms] Re: Xorg video card lookup tables, tone response curves, etc


-----Original Message-----
From: Samer Abdallah <samer.abdallah@xxxxxxxxxxxxxxx>
To: argyllcms@xxxxxxxxxxxxx
Date: Wed, 16 Apr 2008 06:10:19 +0100
Subject: [argyllcms] Re: Xorg video card lookup tables, tone response curves, 
etc

> ...
> So one option would be to force 8 bit-per-channel log-encoded data
> into a
> restricted contrast range of 1000:1 or less, with some way to
> control where in
> the 10000:1 range to place it (ie an overall brightness factor).
>
> ** having just written this, I looked up specs for some LCD tv's and
> some of them claim 5000:1, even 8000:1 in some cases. Well, I
> don't know how they measure this and whether or not it is accurate,
> but even if it were the case, if you're going to make sensible use of
> the
> entire range, eg for movies with bright highlights, explosions,
> images of the sun, caving expeditions etc you're going to need
> more than 8 bits per channel.
>
> Anyway, logarithmic quantization of an 800:1 range in 256 steps gives
> a ratio of 1.026 per step, and the JND for human vision is apparently
> 1 or 2%, so that seems reasonable to me. And you only have to go to
> 10 bits
> per channel to get a ratio of 1.009 across the whole of a 10000:1
> contrast range with a uniform logarithmic encoding.
>
> On a more general note, if resources are so tight that we are worrying
> about the precise details of the the human perceptual response, eg the
> difference between the L* curve and a logarithmic one, then really we
> should be doing things adaptively for particular images, or at least
> types of images on the basis of their statistical structure, because
> that's
> what perceptual systems really do - things like the L* curve are only
> average
> responses and are the result of adaptation at many levels, but the
> variance
> is at least as important as the mean. To give a not very exciting
> example,
> with the old 256 colour displays, any attempt to choose a fixed set
> of 256
> colours for displaying images generally gave poor results no matter
> how evenly
> they were spaced in whatever average perceptual space you choose, so the
> per-image colour map was a necessity.
> ...

Yes. We are approaching to High Dynamic Range Images. I have a great confidence 
that it's a general trend in our life. Of course, it's advantageous to store 
most of images in a logarithmic scale color space, especially with HDR. But 
untill we have extremely high dynamic range displays, we have to adapt our 
image data to particular viewing conditions. There is no matter in which scale 
(e.g. gamma or log) the output device, TRCs or profile are. There is the matter 
general obsolescent 8bit per chanel "bottle neck" between profile LUT and 
display in the display interface. The special TRC (not a simple gamma) have 
advantage in the case when the display hardvare tone resolution is greater then 
8bit input TRC resolution. In that case the special adapted (logarithmic or 
more complex) TRCs are needed. Moreover, may be own TRCs for each image are 
needed. At this stage we can introduce the "image TRC mapping". But the best 
way is to eliminate legacy 8 bit at all.

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