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Author Topic: really understanding clipping  (Read 10609 times)
FranciscoDisilvestro
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« Reply #60 on: July 05, 2013, 07:18:22 AM »
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Yeah, LDO….! That is the entire point of the whole RAW histogram discussion. What's the use of stating the obvious?

I was making a coment to Bill (bjanes) post about sources of clipping when processing raw files. I agree that this does not apply to a pure RAW histogram discussion.

You should also be aware that the output colorspace rendition on the camera is usually a perceptual rendition, not the usual matrix conversion, therefore clipping indication may still be accurate, regardless of colorspace.

I was not aware of this. I'm wondering how do you perform a perceptual conversion to sRGB or AdobeRGB if they are matrix colorspaces?
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bjanes
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« Reply #61 on: July 05, 2013, 09:04:48 AM »
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This is an interesting comment, worth spending some time on imo.  It entails understanding and visualizing color spaces (camera, colorimetric and human) in 3D, something most people (including myself) haven't done much of.  Can you elaborate Francisco?

Here is an example of clipping due to white balance and color space limitations and the effect of exposure. The camera is the Nikon D3 set to AdobeRGB, but using the default picture control with normal contrast. The camera is set to 12 bit NEF. A saturated yellow flower causes clipping of the green and red channels as shown on the RGB histogram(yellow = red + green). The exposure time for 3 shots is shown. Reducing exposure eliminates the green clipping, but the red clipping can not be removed with exposure reduction. The raw files are not clipped. The camera luminance histogram does not show clipping and was not helpful in this case.



Looking at the 1/200 s exposure with ACR and rendering into ProPhotoRGB shows no saturation clipping. Exposure is - 0.5 EV due to the baseline offset of +0.5 EV that ACR uses for the D3.



In AdobeRGB there is considerable clipping of the reds.



Neg exposure compensation eliminates the clipping, but the image is quite dark.



Looking at the image and color spaces with Colorthink demonstrates the underlying principles. The yellow is out of gamut of Adobe RGB with normal exposures resulting in high luminance yellows. The gamut of RGB color spaces decreases with luminance and lowering the luminance can bring the yellow into gamut at a lower luminance.



Unfortunately, the yellow is out of gamut of my Epson printer as shown.  For printing one could edit the yellow to give less saturation or allow clipping to occur as long as the highlight yellow detail is not completely lost. The latter approach works best for me.

Regards,

Bill
« Last Edit: July 05, 2013, 09:06:31 AM by bjanes » Logged
Jim Kasson
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« Reply #62 on: July 05, 2013, 10:55:36 AM »
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Nice job, Bill. Theory and practice combined in one easy-to-follow set of screen grabs.

Jim
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Jack Hogan
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« Reply #63 on: July 05, 2013, 11:51:34 AM »
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Fascinating examples and tools Bill.  Depending on the coordinates of the color and the shape of the color space it looks like in some cases reducing exposure might bring a color into gamut, in others it may not, and in others again one may need to increase exposure in order to bring the color into gamut Smiley  Two questions:

1) Should we be talking about brightness and/or exposure?
2) If a captured color falls outside of, say, aRGB will it necessarily result in one of the three channels saturating (clipping) when rendered neutrally into aRGB's cube?

Thanks again,
Jack
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Jim Kasson
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« Reply #64 on: July 05, 2013, 12:07:44 PM »
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2) If a captured color falls outside of, say, aRGB will it necessarily result in one of the three channels saturating (clipping) when rendered neutrally into aRGB's cube?

Jack,

The short answer is: probably. It depends on the method used to translate the raw values to aRGB. It is likely that the raw processor uses a three-by-three matrix multiply of the demosaiced raw RGB values followed by truncation of values above full scale (255 or 1, or something else entirely, depending on the normalization) and below 0. If that's the case, out-of-gamut values will be clipped, and you are right to be suspicious of any pixel where R=0 or 255, G=0 or 255, or B=0 or 255.

More sophisticated color space conversion algorithms might compress the out-of-gamut colors so that they fall within the aRGB gamut. I don't know of raw processors that use that kind of perceptual rendering, but that doesn't  mean they're not out there.

Jim
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BartvanderWolf
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« Reply #65 on: July 05, 2013, 12:27:54 PM »
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More sophisticated color space conversion algorithms might compress the out-of-gamut colors so that they fall within the aRGB gamut. I don't know of raw processors that use that kind of perceptual rendering, but that doesn't  mean they're not out there.

Hi Jim,

While not exactly compressing OOG colors, RawTherapee allows to scale the Linear gamma data before demosaicing. They warn that it might cause issues further down the line, but it does allow to address some of the highlight (or shadow) and OOG clipping issues. It's up to the user to decide what trade-off, if any, is more important. Power to the user, who can also use it in a dual conversion strategy (by blending normal + OOG conversions, compressed or not).

Cheers,
Bart
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Jim Kasson
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« Reply #66 on: July 05, 2013, 04:52:54 PM »
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It's up to the user to decide what trade-off, if any, is more important. Power to the user, who can also use it in a dual conversion strategy (by blending normal + OOG conversions, compressed or not).

Thanks, Bart. I like that philosophy.

Jim
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Jack Hogan
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« Reply #67 on: July 06, 2013, 02:40:59 PM »
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Thanks Jim, makes sense.

Jack
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