Color Correction Advice

Discussion in 'Digital Darkroom' started by dfperrault, Jul 17, 2021.

  1. Attached is an image of an art print that has a color patch to left. Is it possible to do color correction with all of the color patches in Photoshop or another program?

    The problem is the color values of the original color patches are unknown. I am under the impression that a color checker software with a known calibrated target is required eg. Xrite.

    I figured the best I can do is set the white balance and white and dark levels. I have 100 of these files to adjust. I suspect the corrections may all be the same if they were photographed at the same time and lighting conditons..

    I have a book that has high quality images of the same prints, so I can visually adjust each one by one.

    I plan to publish these photos in a book.

    The other idea I had is that I have access to a color corrected file of the same print different edition. I could write a program in Matlab to look at the color difference between the two prints and create a normalization file to match the colors of the two prints. The color corrected file version copyrighted so I can't use it directly. Getting the two files aligned and registered will take a lot of work and might required some machine learning algorithm to do it successfully. It might be best to do it my eye.

    Last edited: Jul 17, 2021
  2. I think you could color balance the photo using the photographed grey scale. Next (if you plan to print it) you could set the values of darks and lights in the print itself in CMYK. It depends on paper used, machine type etc. so it is best to ask the printing house about details.
  3. digitaldog

    digitaldog Andrew Rodney

    That isn't a MacBeth ColorChecker but there should be published Lab values of this target and easier still, in an RGB Working Space, all gray patches should be equal RGB (neutral) or as close as you can get depending on the data (encoding) and where the WB is applied.
    I think I have that target around the color museum somewhere and can measure the individual color patches and produce LAB.
  4. I think this may be more difficult than you think.

    I assume that what you have posted is a photo of color patches sitting next to a page of a book (or in any case, next to a print). Changing color balance using the color patches would take care of errors in capturing and displaying the image. (using the gray scale, I got a shift of +11 toward magenta and +1 toward yellow in Adobe Camera Raw.) However, if the paper itself has yellowed with age--which looks to be the case here--using the color patches won't address that problem.

    I've been mulling over exactly this issue with some artwork that was done on paper that has deteriorated and yellowed quite badly. One could guess what white the margins should be and adjust accordingly, but that would distort the colors, which show the underlying medium to varying degrees. I haven't figure out how best to do this, but perhaps someone else here has more experience.
    Jochen likes this.

  5. Rodney, Thank you. I wrote haiku for all one hundred prints in the series, that I am trying to publish.

  6. Yes I agree, I would like a good starting point by eliminating the camera/illumination effects. This will likely turn into a color restoration project. I have a book with excellent color version of these prints. So I will manually adjust them to match. I wish I had the bandwidth to play around with machine learning image processing.

  7. After I correct for the camera and source, is it worth while trying to create a compensation matrix from manually selected points across the color space from a higher quality reference image to restore faded colors, etc.? Or is color restoration by eye the easiest approach?
  8. digitaldog

    digitaldog Andrew Rodney

    By eye, especially if all colors fall within display color gamut.
  9. For publishing in a book it would be risky. I've spent a lot of time cmyk correcting photos for magazines, books etc and in my experience you can trust the numbers only.
  10. Do you actually want or need exact reproductions of the prints as they now appear?

    Surely the artist's intention was that the lightest parts of the print appear white? That doesn't happen with paper, which starts to yellow almost as soon as it's made.

    Vegetable ink dyes will also fade over time. So there's a high chance that those prints look nothing like they did when freshly made.

    Sometimes, just making the yellowed paper of a copied print properly white gives a better impression of the artist's intention than a colour-true reproduction - great for a museum archive, but not so great for enjoying the artwork as the artist intended decades ago.

    BTW, that looks like a Kodak Q13 reprographic colour swatch. You'd need to know which issue it was to get a colour reference from it, since they weren't consistent over time.
    Last edited: Jul 19, 2021
    dfperrault likes this.
  11. digitaldog

    digitaldog Andrew Rodney

    What are the exact CMYK numbers necessary for the OP for say just the color patches; you are sure they have an ICC profile for the output that will be used to convert from RGB Working Space that is?
  12. digitaldog

    digitaldog Andrew Rodney

    The original target is a Q13 or Q14 but there were others produced by Kodak over many decades for differing uses.
    As mentioned, just that kind of target was iffy as outlined on the Colorsync list many years ago:

    On May 5, 2008, at 17:58 , Roger Breton wrote:

    That's a Q13. That target was popular with films but has fallen by the
    wayside with digital. The problem with this chart is that it wasn't produced
    to close tolerances like the Munsell charts to be able to speak of useful
    "averages" -- not that I know of. That chart is printed with CMYK inks so
    its colors "values" are only specified with screen percentages like 100%
    cyan and so forth. The best you can make is to have access to "typical"
    CIELab values for each patches, and convert them to RGB if you need,
    thereafter. So that you could use them in your workflow.
    I guess the Q13 can still prove useful in color reproduction.

    I do have Lab values that were published as well.
    dfperrault likes this.
  13. I would set white, gray and black patches in rgb to standard values. Next I would move to cmyk and work on shadows/midrange cmy globally and k for contrast. I would make 4 different files and hardproof it in the printing house. Here in Europe profile is just fogra. I would also make cmyk to add more k to create colors in this case. Pretty standard procedure.
    Last edited: Jul 21, 2021
    dfperrault likes this.
  14. digitaldog

    digitaldog Andrew Rodney

    What are standard values for CMYK output to a device undefined as yet? Please provide specific numeric formula for CMYK for white gray and black, without having any notion of Black Generation for the device.
  15. digitaldog

    digitaldog Andrew Rodney

  16. digitaldog

    digitaldog Andrew Rodney

  17. Just what I said earlier - best to contact a printing house dtp guy.
  18. I said rgb standard values, it is much easier to make it in rgb. Next CMYK files and hard proofs.
  19. digitaldog

    digitaldog Andrew Rodney

    What are 'standard' RGB values for what Working Space from those target patches shown in the fist post?
    Only rule really: When RGB values are equal, in an RGB Working Space, it is neutral. There are indeed absolute values for White or Black: zero is a black hole, 255 is as white as white gets. But that doesn't mean the target seen from the OP should be anywhere near those absolutes! So much for 'standard' RGB values.

    Actually having the Lab values of all those color patches makes converting this to an RGB value pretty easy....

    Yes, considering all capture devices (for eventual CMYK output) are RGB, it's easier to correct RGB values if in an RGB Working Space. Is it in an RGB Working Space?
    Move to CMYK how? For CMYK, gotta have a specific ICC output profile to get there. Stating just "FOGRA" when there are dozens of flavors of FOGRA while further, we have zero idea what, where and how the book will be printed, FOGRA is an assumed and thus moot recommendation.
  20. Thanks for the swatch link. I would like (not need) the colors to be adjusted to how they appear when they were created. Which is more difficult than how they appear when they were documented. Since they are wood block prints from the 1800's. I have a book that has these prints with beautiful colors. I will try your swatch out, as a rough guide to correct the source and camera. I have to give more thought to doing it by eye vs the numbers.

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