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Re: [css3-color] ICC implementation

From: Chris Murphy <lists@colorremedies.com>
Date: Sun, 13 Jul 2008 00:12:31 -0400
Cc: www-style@w3.org
Message-Id: <A1C33D15-B023-4413-9E3F-FCE6EA465767@colorremedies.com>
To: David Woolley <forums@david-woolley.me.uk>

On Jul 12, 2008, at 7:12 PM, David Woolley wrote:
> Example of images that are clearly gamma 1.0, and have been for  
> several years, in spite of my pointing out the problem, and their  
> being obviously wrong, supplied off list.

I think you're confused about how tone reproduction curves work. A  
digital raw capture from a camera is linear encoded. You can think of  
each sensor in the chip as a photon counter. Its response is linear  
which means it captures a scene with double the luminance, the sensors  
will report double the values; if the scene triples in luminance, the  
sensors report triple the values, etc. It's not logarithmic like human  
vision, or any of our displays.

Absolute colorimetry of any scene rapidly gets very bright, so most  
properly exposed captures to a camera do the same thing, and will be  
represented substantially more by the bits above 50% than below it.  
There are many more bits dedicated to the top/lighter 1/4 than the  
bottom/darker 3/4's because of this. Human vision clamps down on this  
to keep us from having sensory overload. Turn on a second light with  
exact equivalence to the first and the room does not get twice as  
bright, as far as human vision is concerned. To the camera it would be.

If you were to take this raw linear capture, not tone map it, and put  
it up on a display, it will look terribly bright and washed out.  
That's true if it is a correctly exposed shot.

This is why no manufacturer does what you are claiming they do, which  
is they have images that have not been tone mapped, and are still  
linear. You have it exactly backwards.

The images in your example are too dark. If lightened in Photoshop,  
shadow detail becomes obviously noisy, which is a hallmark indication  
of underexposure with today's digital capture technology.

So I think you're confused on the difference of an image that is a  
linear encoded, and an underexposed image.

Chris Murphy
Color Remedies (TM)
New York, NY
Co-author "Real World Color Management, 2nd Ed"
Received on Sunday, 13 July 2008 05:11:02 UTC

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