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Author Topic: Perceptual colour profile conversion issue  (Read 20125 times)
bjanes
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« Reply #40 on: May 05, 2008, 08:55:09 AM »
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Bill, I was looking into DCRAW's source code but I feel a bit lost to answer this: DCRAW uses Camera -> XYZ matrices (provided to Coffin by Adobe), and then another transformation XYZ -> output colour space used. In fact both matrices are multiplied in advance to do Camera -> output colour space in just one step.

The question is: is it possible that the exposure level of a given pixel has an influence to determina whether it will fall inside or outside the output colour profile space?

Think of a pixel in a scene captured with RGB levels=(10,22,45), if it was shot 1 f-stop more exposed, for sensor linearity it would be coded as (20,44,90).
Is it possible that (10,22,45) when converted to the output colour space falls into sRGB gamut, but (20,44,90) gets clipped in sRGB?

I want to believe then that if (20,44,90) clips in the output colour profile but (10,22,45) doesn't clip, is just for saturation (luminosity) reasons, not because the Hue determined by the pixel of the scene can alternatively belong to or not belong to the output profile gamut. Right?

If the answer to all that is yes, it means the degree of exposure of the RAW data will affect the possibility of converting levels on a pixel to a given output colour profile, so the Exposure slider in ACR is a good tool, not only to cancel the effect of the white balance, but also to avoid clipping due to excesive input exposure.

Am I right?

BR
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Guillermo,

Yes, the exposure can affect saturation clipping as shown in this example. This composition was given two exposures differing by one f/stop. With the greater exposure, the red channel exhibits saturation clipping in sRGB and this is eliminated by giving one stop less exposure. However, the picture is too dark. One can achieve the same effect by using -1 EV exposure compensation in ACR (not shown). In this case, it is better to use ProPhotoRGB in the properly exposed image.

Clipping in red channel
[attachment=6419:attachment]

Clipping display in ACR
[attachment=6420:attachment]

Image with 1 EV less exposure in camera
[attachment=6421:attachment]

ProPhotoRGB, proper exposure
[attachment=6422:attachment]

Examination of the raw file of the properly exposed image shows that none of the channels are clipped in the raw file. The RGB multipliers for white balance are RGB 2.648, 1.0, 1.356 as shown by DCraw.

Raw histogram:
[attachment=6423:attachment]
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Guillermo Luijk
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« Reply #41 on: May 05, 2008, 11:21:47 AM »
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Good example of the use (and need) of large work spaces for certain pictures. This makes me think that all those endless discusions about sRGB vs AdobeRGB vs ProPhoto RGB simply have no general answer since the appropiate work space will depend on input scene and output application gamuts.
And we have to add to that tradeoff the fact that there is currently not a proper Perceptual conversion implementation.

Thanks Bill.
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madmanchan
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« Reply #42 on: May 05, 2008, 12:53:18 PM »
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GLuijk, in principle you would want to use the smallest working space that holds all the colors you need for the given image. So in that sense you are correct, it's image-dependent. In practice it's easier in a photographer's workflow to pick one, so which one? We all know the drawback of a small space is tendency to clip colors, and the drawback of a large space is potentially insufficient precision to get smooth tones. So my general recommendation is to decide for yourself which of these two is the lesser evil. In a final print, can you tell the difference between images processed identically but using different working spaces?
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digitaldog
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« Reply #43 on: May 05, 2008, 01:07:56 PM »
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In a final print, can you tell the difference between images processed identically but using different working spaces?
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The final print made on a printer of 3 years past, today or one we'll use in a year? We've seen how the gamut capabilities of just Epson inks has changed over the course of a very few years, most users want to revisit an image they worked on and maybe reprint it later. That said, I want a big honking working space!
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Andrew Rodney
Author “Color Management for Photographers”
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bjanes
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« Reply #44 on: May 05, 2008, 01:56:40 PM »
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Good example of the use (and need) of large work spaces for certain pictures. This makes me think that all those endless discusions about sRGB vs AdobeRGB vs ProPhoto RGB simply have no general answer since the appropiate work space will depend on input scene and output application gamuts.
And we have to add to that tradeoff the fact that there is currently not a proper Perceptual conversion implementation.

Thanks Bill.
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This by [a href=\"http://www.colorwiki.com/index.php?title=Vistas_New_Color_Management_System_-_WCS&printable=yes]article[/url] by Steve Upton implies that the new color management system in Windows Vista may alleviate some of the problems with perceptual rendering. Unfortunately few graphic artists use Windows and few Windows users run Vista. Perhaps someone can comment.

Bill
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