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New ICC v4 sRGB Color Space May Prevent Clipping Converting From ProPhoto RGB

Sep 20, 2008 11:02 PM

Just discovered and tried out this new sRGB color space downloaded from this page:

http://www.color.org/srgbprofiles.xalter

Here's a cropped demo with histograms of a raw image I've been working on in ProPhotoRGB in ACR 3.7 and CS2:

http://www.flickr.com/photos/26078880@N02/2874068887/sizes/o/

Please disregard the PRMG name in the demo. I got confused as to which was which on that site and thought that this version of sRGB was a PRMG=(Perceptual Reference Medium Gamut) profile. It's called sRGB_v4_ICC_preference.icc.

Note the different color shifts using the Perceptual intent and unchecking Black Point Compensation=(BPC). Neat little features embedded in this profile, but not sure about how it renders certain colors close to clipping in the shadows. That site doesn't recommend mixed use of ICC v4 with v2 color spaces and output device profiles but I did it anyway just to see what it does.

Who knows this may be the equivalent of handing scissors to children, but I'm just one of those curious children and thought I'ld share anyway.
 
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  • Currently Being Moderated
    Sep 21, 2008 4:15 AM   in reply to Tim Lookingbill
    Tim,

    An interesting post, but upon reading the ICC White Paper 26, which is referenced in the link you posted, I'm not sure you are using the profile correctly. If you look at A1-3 in the annex, you will see that the original image is in sRGB and the first step is to assign the v4 sRGB profile to the sRGB image and one goes from there.

    In A4 the task is to start with an image originally rendered to a print referred state, and the first step is to assign an appropriate v4 color space or printer profile. If you are starting out with a ProPhotoRGB image, you would have to assign a v4 ProPhotoRGB profile and go from there. Normally, the ProPhoto source would not be print referred.

    In the additional example in A4, starting with an image in aRGB, the image was converted to sRGB with colorimetric rendering (with clipping) and then the v4 sRGB profile is assigned. It would seem that the optimal work flow would be to assign the ProPhotoRGB image to a v4 ProPhoto space and then go from there.

    Hopefully Eric Chan or some other expert will correct and clarify any mistakes we have made in our posts.

    By the way, your histograms are from cached data as indicated by the yellow triangle at the top right of the images, and it would be better to use the entire data by clicking on the triangle.
     
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    Sep 21, 2008 4:19 AM   in reply to Tim Lookingbill
    Duplicate post deleted.
     
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  • Currently Being Moderated
    Sep 21, 2008 1:43 PM   in reply to Tim Lookingbill
    On Sun, 21 Sep 2008 08:58:15 -0700, Tim_Lookingbill@adobeforums.com wrote:

    > When did making a picture turn into brain surgery?

    When we started using histograms, and stopped using our eyes.
     
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  • Currently Being Moderated
    Sep 25, 2008 4:18 AM   in reply to Tim Lookingbill
    There is no Pro Photo RGB spec, but its profile is similar to ROMM RGB which is standardized in ISO 22028-2.

    The ROMM RGB reference medium is similar to the ICC v4 perceptual reference medium (PRM), which is a reflection print.

    The bottom line is, good Pro Photo RGB images are more or less print-referred to the ICC PRM.

    A primary design goal in developing the v4 sRGB profile was conversion from the PRM to sRGB with minimal clipping.
     
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    Sep 26, 2008 5:56 AM   in reply to Tim Lookingbill
    >There is no Pro Photo RGB spec, but its profile is similar to ROMM RGB which is standardized in ISO 22028-2.

    >The ROMM RGB reference medium is similar to the ICC v4 perceptual reference medium (PRM), which is a reflection print.

    >The bottom line is, good Pro Photo RGB images are more or less print-referred to the ICC PRM.

    >A primary design goal in developing the v4 sRGB profile was conversion from the PRM to sRGB with minimal clipping.

    Thanks, Jack. This information is very useful.

    The ICC White Paper 26 describes the tone mapping for a reflection print but does not go much into the color gamut of the PRM. The wire frame plots show that the color gamut is larger than sRGB, but how does it compare to the ROMM RGB gamut? Would there be clipping if one uses the PRM with ROMM RGB?

    http://www.color.org/ICC_White_Paper_26_Using_the_V4_sRGB_ICC_profile. pdf

    The differentiation of Scene Referred vs. Output Referred spaces is a bit confusing. As Karl Lang discusses in his excellent white paper, output referred images are tone mapped to the lower dynamic range of a photographic print. ProPhotoRGB is output referred, but ACR uses ProPhotoRGB primaries with linear tone mapping for scene referred images. Other than a gamma of 1, what is the difference?

    http://wwwimages.adobe.com/www.adobe.com/products/photoshop/family/pro photographer/pdfs/pscs3_renderprint.pdf
     
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  • Ramón G Castañeda
    11,247 posts
    Jul 27, 2006
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    Sep 26, 2008 11:43 AM   in reply to Tim Lookingbill
    >converting from ACR ProPhoto to any version of sRGB has no affect.

    Color conversions, like inanimate objects such as a rock, have no affect. They do have an effect on images, though. ;)
     
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    Sep 26, 2008 12:40 PM   in reply to Tim Lookingbill
    I'll try to respond to a few items above.

    The digital values in a Pro Photo RGB file should generally be the same as for a ROMM RGB file. The exception is that all possible Pro Photo RGB values are "legal" but not all ROMM RGB values are "legal." ISO 22028-2 restricts the legal ROMM values to those with PCS LAB values between 0 and 100 L* and -128 to +128 a* and b*.

    There is no enforcement if you use the illegal values in a ROMM file but as some of these values do not represent possible colors this is not advisable. I think it is good practice to try to stay mostly inside the ICC v4 PRM gamut with Pro Photo/ROMM images. You can check this using the gamut warning profile on the ICC site.

    If you have a Pro Photo RGB image you should just assign the ROMM profile to it. Converting to ROMM RGB should not change anything in the ideal sense but there is the possibility to introduce rounding errors and mismatch black points.

    The main difference between the Pro Photo RGB profile and the ROMM RGB profile is the former includes black scaling to zero (as is common with v2 color space and display profiles) and the latter does not (as is required with v4 profiles).

    In the duck picture, the reason for the difference is the ROMM profile black is at L*=3 so this is where the lowest blacks land when converting MRC to v2 sRGB (which has the sRGB blacks scaled to L*=0). In this case leaving BPC off is analogous to turning on "Simulate Black Ink" in Photoshop Proof Setup.

    All the ACR color space choices are v2 profiles with black scaling. If you want a v4 profile embedded you have to assign it (if you have a corresponding v4 profile) or convert to it.

    Both the sRGB and PRM gamuts fit within the ROMM RGB legal encoding range, but if you use a v2 profile with black scaling you should always turn on BPC when combining with a v4 profile. Otherwise the v4 profile will think the v2 profile represents a device with an infinite dynamic range. When BPC is on the Color Engine scales the black point of the source profile to the black point of the destination profile.

    The sRGB gamut extends outside the PRM gamut in some places, and the PRM gamut extends outside the sRGB gamut in other places. If you convert using a colorimetric intent in either direction some of the gamut will be clipped. The purpose of the sRGB v4 perceptual transforms is to minimize clipping in both directions.

    I can't do this justice here but basically scene-referred images are encodings of the scene colors and output-referred images are encodings of the picture colors on some medium for which the picture colors have been optimized. You can make a scene-referred image by setting the ACR sliders (except the white balance sliders) to zero and the curves tab to linear. You make an output-referred image when you adjust the sliders to non-zero values to make a nice picture as viewed on some medium. For example you might adjust the sliders differently to get the best results printing colorimetrically on glossy photo paper vs. on plain office paper.

    Often an important part of the transform to output-referred includes a midtone contrast and saturation boost. While there will likely be some highlight and may be some shadow compression, it is misleading to think of this transform only as a compression to some output medium dynamic range. In some cases the output medium dynamic range is larger than that of the scene.

    Usually the transform to output-referred is more complicated than a simple gamma function.

    Regardless of the image state (scene-referred or output-referred) the sRGB, Adobe RGB, or Pro Photo RGB nonlinearities will be applied to create the image data that you open into Photoshop.

    We are working in the ICC to prepare more information for posting on this topic.
     
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  • Currently Being Moderated
    Sep 26, 2008 3:10 PM   in reply to Tim Lookingbill
    >The sRGB gamut extends outside the PRM gamut in some places, and the PRM gamut extends outside the sRGB gamut in other places. If you convert using a colorimetric intent in either direction some of the gamut will be clipped. The purpose of the sRGB v4 perceptual transforms is to minimize clipping in both directions.

    Yes, I went back and looked at the ICC white paper 26. In some areas, the sRGB gamut does extend beyond the PRM space. Since sRGB is criticized as being too limited, I don't know if the PRM space is really that good if you are using a really wide gamut printer such as the new Epson ink jets. Better stick with ProPhotoRGB and take care of out of gamut colors yourself.
     
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    Sep 28, 2008 7:13 AM   in reply to Tim Lookingbill
    >I don't know how anyone would know whether ACR's ProPhotoRGB would be scene or output referred when it's all histogram driven which is how I based my edits according to how I remembered the scene. What type of referring are these edits based on? How would a profile know? Are they going by the histogram data in the final edited image?

    I should have clarified that I was referring to the internal working space that ACR uses. Since photo receptors are linear devices, their output is scene referred by definition. If you render into ProPhotoRGB from ACR, then you get an print referred image with a gamma of 1.8

    >Is the dynamic range (excluding gamut) of ProPhoto RGB much wider than all the versions of sRGB profiles both ICC v2 and v4?

    As far as I know, if bit depth and gamma are held constant, then all RGB working spaces using a gamma curve have the same DR. With linear encoding such as in raw files, the DR is limited by the bit depth: a bit depth of 12 yields 12 f/stops of potential DR. However, the practical DR is limited by noise and posterization in the shadows. If you use a gamma curve (power function), you need fewer levels in the shadows. Log encodings are even better. Since the slope of a power curve is infinite at a luminance of zero, many spaces such as sRGB and L*a*b use a linear ramp for low luminances. This link gives some details.
     
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