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marks60810224
Known Participant
October 9, 2017
Answered

gamut warning changes pixels

  • October 9, 2017
  • 2 replies
  • 1964 views

I use View>Proof Setup>Custom>Device To Simulate, and then I choose a profile for whatever photo paper I'm printing to.

Then when I check View>Gamut Warning, some of the pixels (that are NOT out of range) change color, especially in the dark areas of the image. This is not the same as the pixels that out of gamut, which, of course, would change to whatever color I have chosen in my preferences for displaying out of gamut pixels.

Basically, the darkest parts of the image lack contrast and tend to "posterize" when Gamut Warning is not checked, and checking Gamut Warning makes these dark parts of the image look better, with better detail.

In other words, turning on Gamut Warning is "correcting" the shadows. Without Gamut Warning on, some of the very darkest parts of the image (particularly blues/purples) don't display correctly and they don't look like my actual print.

This happens only when I choose a profile for a manufacturer's paper (and for whatever paper I choose including, Hahnemuehle, Harman, or Canon); it doesn't happen when I choose one of the pre-set profiles such as Monitor RGB or sRGB, et. al. And it happens on three different monitors I have, including a finely calibrated NEC.

By the way, I turn off View>Extras when I look at this, because the Pixel Grid interferes with the perception of the color of the pixels (turning Gamut Warning off and on changes the brightness of the Pixel Grid itself, which I am told is normal, and I don't have an issue with that).

To try to see whether this happens for yourself, use Custom view of a paper profile, zoom in to 3200%, then look at a dark part of the image (but not pure black; try a part of the image with very dark blues), turn of Pixel Grid, then check and uncheck Gamut Warning. See whether the pixels change.

(1) Does this happen for you? (2) Why would it be happening for me?

Thanks.

    This topic has been closed for replies.
    Correct answer davescm

    You're right, this secondary discussion about the general usefulness of gamut warning does confuse the issue. So let's leave that for now.

    I certainly don't see any banding or other artifacts in the shadows here, when turning soft proof on. It looks as I would expect it to look, corresponding to the print, IOW showing only the effects of the gamut clipping that actually happens. BTW I'm using an Eizo wide gamut monitor that mercilessly exposes any shortcomings that might be there.

    The difference with gamut warning on is apparently wrong here. It causes a general lifting of deep shadows that neither matches the original or the print. It's a third variety coming out of nowhere.

    I'll look more closely into it later today, and post some screenshots once I find a suitable original (showing the effect well). I'm on my way out and don't have time to dig into it right now.


    I had this problem years ago with an Nvidia driver. It was an error seen when using the GPU code.

    Noel Carboni had already raised the issue and found that switching the GPU mode to Basic (which forced the CPU to do the colour shading) resolved the issue in the short term. It may be worth setting the  GPU to Basic or even turning it off temporarily and see if you get the same.

    See this:

    Strange behaviour softproof and Gamut warning

    Edit to add - I just checked and I don't see the issue on my current AMD card & driver with Advanced GPU mode checked

    Dave

    2 replies

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    October 10, 2017

    Yes, the GPU setting matters. In "Basic" mode there is no difference with gamut warning on or off. In "Normal" or "Advanced" mode something weird happens.

    And yes, gamut warning on is the correct rendering. I didn't look closely enough before.

    So this is obviously OpenGL inaccuracies, just like the ProPhoto bug we know from before. BTW that's OpenGL, not OpenCL which is something else. The OpenCL checkbox is irrelevant here. OpenGL is the engine that runs display color management in Normal/Advanced modes, among many other things.

    Here's a simple gradient, divided into three horizontal strips. The file is Adobe RGB, video card NVidia Quadro K420, GPU Advanced:

    I've always kept GPU at Basic on my work system because of the ProPhoto bug. Here I need absolute accuracy. All the more reason to continue that practice.

    M
    marks60810224Author
    Known Participant
    October 10, 2017

    I don't see Open GL in the Performance tab or in Advanced Settings in the Performance tab. Not that it's grayed out; rather just not there. Using latest Photoshop update.

    davescm
    Community Expert
    davescmCommunity Expert
    Community Expert
    October 10, 2017

    Unlike Open CL, Open GL is not a separate option in Preferences. It is enabled when you enable the GPU. The functions it is then used for vary by the drawing mode Basic, Normal or Advanced.

    Dave

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    October 9, 2017

    Yes, I do see a very slight difference in near-black. Windows 10, CC2017, Epson papers.

    I would never have noticed this, as I never touch gamut warning. It's a completely useless tool IMO. It has a certain threshold before it kicks in, and when it does it says nothing about how much. So it tells you absolutely nothing.

    Still, there shouldn't be a difference. No idea what this is.

    M
    marks60810224Author
    Known Participant
    October 9, 2017

    Thank you very much for your reply. Especially thanks for confirming that this is not an issue that only I am having.

    What's disturbing is that, at least for me, the image looks correct only when Gamut Warning is on. With Gamut Warning off, the very dark blue parts look "posterized", lacking sufficient "gradation" to show detail (for example, the subtle creases in a dark blue sweater someone is wearing). That should not be the case; the image should look right even with Gamut Warning off.

    /

    About Gamut Warning and its ordinary function. I do find it useful. I look at the out of gamut areas of the image, then use the Sponge brush (or, even better, a selection and Image>Adjustments>Hue/Saturation) to desaturate right to the point where it pops into gamut.; or whatever other says, such as cloning or painting.

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    October 9, 2017

    Well, you will see some differences when soft proof is turned on, long before gamut warning shows any clipping. That in itself is normal - it starts to clip, but not enough for gamut warning to show it.

    If there is clipping, and the visual soft proof indicates I should do something about it, I take a more targeted approach. I look at the histogram to determine which channel is clipping, high or low end, and then examine that channel to see specifically what needs to be done. A wholesale desaturation always takes too much across the board.

    Clipping in itself need not be a problem. But if it kills texture and general "air", it can often be fixed without any perceived loss of saturation.

    Anyway, as far as I can see without running extensive tests, the non-gamut warning version appears to be the correct one. Even more reason to avoid it if you ask me.

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