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Author Topic: Perceptual rendering intent – matrix to matrix – is it possible now ?  (Read 6831 times)
Peter_DL
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« on: January 12, 2011, 12:38:03 PM »
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Hi,

This subject was repeatedly discussed in the past,
in particular with regard to ProPhoto RGB and the conversion to smaller target / matrix spaces,
however it could easily be that I missed the final conclusion.

Is it possible now to facilitate any 'smooth' perceptual gamut compression,
or are we still with RelCol ?

Appreciate your comments.
& Best regards, Peter

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FranciscoDisilvestro
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« Reply #1 on: January 12, 2011, 12:59:18 PM »
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It seems to be possible with version 4 profiles

There is a version 4 sRGB profile here
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Peter_DL
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« Reply #2 on: January 13, 2011, 02:38:39 PM »
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It seems to be possible with version 4 profiles

There is a version 4 sRGB profile here

Trying it back and forth, it does not seem to work for the conversion ProPhoto RGB to sRGB.
No merits of this version 4 profile are observed compared to ordinary sRGB IEC61966-2.1.
Thanks for the link though.

Am I doing something wrong ?


Example for reference:
> the left image was left in ProPhoto RGB
> the right image was converted RelCol to ordinary sRGB
It’s well visible on (my) screen that some image details are getting lost upon conversion.
By no means of the version 4 sRGB profile and its Perceptual RI was it possible to prevent this.

Hope it can be seen with the attached screenshot:
for the purpose of clipping it in here, the screenshot was pasted in a new file, monitor profile was assigned and then converted to ProPhoto RGB again. So the attached file is in ProPhoto RGB.

Peter

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FranciscoDisilvestro
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« Reply #3 on: January 13, 2011, 05:39:15 PM »
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In my experience there is a difference when you use the version 4 profile.

Following there is an example with a test picture

The first image is the original in ProphotoRGB. You can appreciate that there are no clipped channels in the histogram

The second image is the conversion to the standard sRGB version 2. It shows clipping especially in the red channel. It doesn΄t matter if you choose Perceptual or Relative Colorimetric, as probably everybody agrees in this matrix conversions.

The third image is the conversion to the sRGB version 4 using Relative Colorimetric intent. It is basically similar to the conversion to sRGB version 2, which was expected.

The last image is the conversion to sRGB version 4 using Perceptual Intent. In the histogram it shows a noticeable difference with the previous one (the Rel Col conversion). Even if there is still some clipping, it is less than the Rel Col conversion. That΄s why I think that perceptual intent is possible.
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digitaldog
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« Reply #4 on: January 13, 2011, 06:08:54 PM »
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For this to work as designed, both profiles have to be V4 (keep in mind that by the time the destination profile comes into play, its being handed - usually- Lab).
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Andrew Rodney
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FranciscoDisilvestro
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« Reply #5 on: January 13, 2011, 06:32:39 PM »
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Do you know if there are v4 profiles for ProPhotoRGB and AdobeRGB available?

In the www.color.org website I could find only sRGB v4
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digitaldog
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« Reply #6 on: January 13, 2011, 07:05:11 PM »
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Do you know if there are v4 profiles for ProPhotoRGB and AdobeRGB available?

Not that I’m aware of.
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Andrew Rodney
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tho_mas
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« Reply #7 on: January 14, 2011, 03:23:09 AM »
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Do you know if there are v4 profiles for ProPhotoRGB (...) available?
ISO22028-2_ROMM-RGB
http://color.org/prmg_gamutwarning.xalter
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Peter_DL
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« Reply #8 on: January 14, 2011, 07:59:41 AM »
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So how precisely would these spaces have to be used in order to 'perceptualize' the conversion from ProPhoto RGB to sRGB ?

Tried it - but failed again.

Thanks for your comment.
Peter

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tho_mas
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« Reply #9 on: January 14, 2011, 09:18:17 AM »
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So how precisely would these spaces have to be used in order to 'perceptualize' the conversion from ProPhoto RGB to sRGB ?

Tried it - but failed again.
so maybe first convert perceptual to the PRMG_RGB profile and then perceptual to sRGB V4.

Or try to convert first perceptual to PhotogamutRGB and then relcol to sRGB (regular sRGB).
http://photogamut.org/E_ICC_profile.html

The latter should be a better workflow anyway as the V4 sRGB profile often introduces a color shift.
« Last Edit: January 14, 2011, 09:22:09 AM by tho_mas » Logged
Peter_DL
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« Reply #10 on: January 14, 2011, 02:04:23 PM »
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Or try to convert first perceptual to PhotogamutRGB and then relcol to sRGB (regular sRGB).
http://photogamut.org/E_ICC_profile.html

There is indeed a slight but perceivable competitive edge with PhotoGamut RGB as the intermediate space.
Some more image details seem to be maintained.

Peter

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left image:  ProPhoto RGB --> relcol to regular sRGB
right image:  ProPhoto RGB --> perceptual to PhotoGamut RGB --> relcol to regular sRGB

« Last Edit: January 14, 2011, 02:54:47 PM by Peter_DL » Logged
tho_mas
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« Reply #11 on: January 15, 2011, 04:05:21 AM »
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There is indeed a slight but perceivable competitive edge with PhotoGamut RGB as the intermediate space.
Some more image details seem to be maintained.
the question here is whether or not the colours of this image are clipping in your monitor profile.
ProPhotoRGB is assigned to the file attached but obviously it's a screenshot and therefore the colours are limited to the gamut of your monitor profile anyway...
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Peter_DL
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« Reply #12 on: January 15, 2011, 05:03:04 AM »
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the question here is whether or not the colours of this image are clipping in your monitor profile.
ProPhotoRGB is assigned to the file attached but obviously it's a screenshot and therefore the colours are limited to the gamut of your monitor profile anyway...

Sure - however, seeing the loss of color saturation and image details on my screen (particularly with plain RelCol from ProPhoto RGB to sRGB) implies that the monitor gamut stretches beyond sRGB for the relevant colors here; which can be shown separately by a Customized Proof setup and out-of-gamut marks. Alternatively such visual comparison can be done while having the "Desaturate Monitor Colors" option enabled in the Color Settings. Problem is that such image degradation and loss of details is hard to quantify otherwise than by a visual comparison.

Viewing the above image in a non-color-managed web browser, there should be a slight difference visible regarding the image details of the upper left petal (left vs. right image). Viewing the above image in a color-managed environment such as Photoshop with ProPhoto RGB assigned will require that your monitor gamut stretches beyond sRGB as well for the relevant colors.
However, I agree that this is all a 'construct' to share what I see more clearly on my screen:

PhotoGamut RGB provides some sort of perceptual gamut compression,
whereas it didn’t work for me with the version 4 profiles mentioned initially.

Anyway thanks for your comment.
Peter

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« Last Edit: January 15, 2011, 05:17:21 AM by Peter_DL » Logged
bjanes
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« Reply #13 on: January 15, 2011, 10:37:21 AM »
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So how precisely would these spaces have to be used in order to 'perceptualize' the conversion from ProPhoto RGB to sRGB ?

Tried it - but failed again.

Thanks for your comment.
Peter

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The first step of using a Ver 4 profile similar to ProPhotoRGB would be to get the image into that profile. ACR does not render into such a profile, but does render into ProPhoto RGB. To get the image into the Ver 4 profile create a scene referred image using Photoshop and ACR as explained in this ICC paper. To obtain scene referred data one must set the tone curve to linear, but some adjustments may be used as explained here. One can then convert to a wide gamut v4 profile such as ISO22028-3_RIMM-RGB-exCR.icc or linear_RIMM-RGB_v4.icc. From there, one could convert to a Ver 4 output profile using the methods outlined in paragraph 11 of the paper, using either option A or option B. Current perceptual rendering is not very smart, since it compresses the gamut of an image even if the image contains no out of gamut colors in the destination space.

However, rendering from scene referred to output referred is not a simple process as explained by an excellent paper by Karl Lang. Visual editing of wide gamut images is limited by the gamut of the monitor. The better monitors cover AdobeRGB and this is sufficient for many images. The newer ink jets have a gamut that exceeds aRGB for some colors (mainly saturated colors at a relatively low luminance), whereas, due to a gamut mismatch, aRGB has a wider gamut at higher luminances. The human perceptual system is not that sensitive to saturation at low luminances and I am not sure how much difference this expanded gamut at low luminances is perceptible in actual prints. Perhaps Jeff Schewe or Nick Raines can comment.

Regards,

Bill
« Last Edit: January 15, 2011, 07:55:29 PM by bjanes » Logged
tho_mas
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« Reply #14 on: January 15, 2011, 03:23:07 PM »
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One way to get a ver 4 profile similar to ProPhotoRGB would be to create a scene referred image using Photoshop and ACR as explained in this ICC paper.
another way is to simply download the V4 version of ROMM-RGB: http://color.org/prmg_gamutwarning.xalter / http://color.org/profiles/ISO22028-2_ROMM-RGB.icc
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bjanes
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« Reply #15 on: January 15, 2011, 07:58:16 PM »
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another way is to simply download the V4 version of ROMM-RGB: http://color.org/prmg_gamutwarning.xalter / http://color.org/profiles/ISO22028-2_ROMM-RGB.icc


That is obvious, but one must get the image from ProPhotoRGB into the ISO22028-2_ROMM-RGB.icc. Are you merely suggesting assigning the ProPhotoRGB image to the Ver 4 ISO22028-2_ROMM-RGB.icc? Does that work? I have revised my post to clear up the misunderstanding.

Regards,

Bill
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tho_mas
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« Reply #16 on: January 16, 2011, 06:18:30 AM »
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Are you merely suggesting assigning the ProPhotoRGB image to the Ver 4 ISO22028-2_ROMM-RGB.icc? Does that work?
IMO, yes, it does work (there is a slight difference in highest saturated blues and magenta, but negligible... IMO). You could also convert relcol + BPC (literally "lossless" in 16bit).
Of course, with a decent RAW software you could also render directly into ISO22028-2_ROMM-RGB.

The OP referred to a perceptual conversion to sRGB... so for this particular purpose I think converting relcol + BPC from ProPhoto to ISO22028-2_ROMM-RGB as an "intermediate" color space should be fine in order to utilize the V4 sRGB profile.
Then again, me personally I would use the said PhotogamutRGB profile (or maybe the PRMG_RGB profile) as intermediate color space for such a purpose anyway as the V4 sRGB profile produces color shifts (warm tones turn blueish/greenish).

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Peter_DL
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« Reply #17 on: January 16, 2011, 07:24:39 AM »
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The OP referred to a perceptual conversion to sRGB... so for this particular purpose I think converting relcol + BPC from ProPhoto to ISO22028-2_ROMM-RGB as an "intermediate" color space should be fine in order to utilize the V4 sRGB profile.

Starting with ProPhoto RGB
-> RelCol + BPC to ISO_22028-2_ROMM-RGB
-> Perceptual + BPC to the PRMG_RGB profile (or sRGB_v4)
-> RelCol or Perceptual + BPC to regular sRGB

… the problematic step seems to be the final conversion to regular sRGB.
At the end the results seem to be undistinguishable from ProPhoto RGB -> RelCol to sRGB.

Peter

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tho_mas
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« Reply #18 on: January 16, 2011, 07:57:36 AM »
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Quote
-> RelCol or Perceptual + BPC to regular sRGB
perceptual to regular sRGB does not exist. The regular sRGB profile does not contain a perceptual table so if you set perceptual in the conversion settings it's actually relcol. Also BPC is actually "included" in perceptual (but not needed with matrix profiles on both ends going from black L*0 to white L*100 anyway).
Just a note...

Quote
… the problematic step seems to be the final conversion to regular sRGB.
At the end the results seem to be undistinguishable from ProPhoto RGB -> RelCol to sRGB.
this is why I would suggest to use PhotogamutRGB as intermediate color space (which should be fine for most purposes). One of the design goals of this colour space was to achieve a visual similarity to sRGB (while actually being much larger than sRGB) so that you could use the profile also in non colour managed workflows.

Basically: avoid bright high saturated colours ;-)

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Peter_DL
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« Reply #19 on: January 16, 2011, 09:20:55 AM »
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perceptual to regular sRGB does not exist. The regular sRGB profile does not contain a perceptual table so if you set perceptual in the conversion settings it's actually relcol. ...Just a note...

That's commonly understood, however, with these profiles i.e. PRMG_RGB (or sRGB_v4) being used as the source space, we just did not want to exclude 'anything' in the course of testing.  If only the target space needs to include a perceptual table, which is what conventional wisdom suggests, I'm wondering why we ever started to convert to ISO22028-2_ROMM-RGB first, before targeting the smaller PRMG_RGB (or sRGB_v4) spaces. Anyway.

Peter

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« Last Edit: January 16, 2011, 09:22:53 AM by Peter_DL » Logged
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