Can I use a ColorChecker in a JPEG photo to get accurate print colors?

Asked 10/14/2017

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I need to print photos with faithful color, and each image includes all or part of an X-Rite ColorChecker Passport Photo target. The files were captured as JPEG, not RAW. I’m unsure whether I should use X-Rite/Adobe DNG profile tutorials, or whether I can correct the image by mapping the sampled ColorChecker patches to their published reference values (for example with a 3×3 color matrix) and then print from that corrected file. Is that a valid workflow for accurate printing, or do I need a different approach?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

8y ago

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Unfortunately, by shooting the images in jpeg your camera has applied non-linear processes to the image. It does this to produce more pleasing images but at the cost of distorting colors and tone. This process is called an "ouput referred" transform. This link discusses this process and why it is done:

http://www.color.org/icc_white_paper_20_digital_photography_color_management_basics.pdf

In order to get colors to closely match a colorchecker you need to do what is called "scene referred" processing. While rare in normal photography, it's used extensively in reproduction work where you are trying to create the best copy of an existing artwork or photograph. This is always done using RAW mode with special converters designed for scene referred processing. It's imperfect because the color filter arrays have a somewhat different spectral response than human vision but it's a lot better than using the standard "output referred" processing and can make quite good copies. One way to see how much distortion occurs in standard output referred photography is to take a picture of a picture then white balance and print it. The differences are stark.

OTOH, if you process an regular photo image using scene referred techniques and print it you will normally find the print unappealing. So that's why it's a technique only used in scientific applications or reproduction work.

Getting back to your issue having only jpegs to work with one approach that will require some work is to apply inverse curves in Photoshop that reverse the "S" curve typically applied when the jpeg is created. Determining that is non-trivial but one approach is to print a series of patches from RGB (0,0,0) to (255,255,255) in small increments. Then print this using a good inkjet and paper that provides ICC profiles for that paper. Do this using Relative Colorimetric Intent (see Photoshop's help). Then take a jpeg picture of that together with a colorchecker using your camera using the same settings used in your other pictures. You should then be able to use the info panel to adjust with a curves layer the values so that each patch comes out close to the RGB levels you used to create the print. Once you have that curves layer you can save it and apply it to the other photos to reverse the jpeg tone changes.

Afterwards you may still need to adjust saturation as it's common to increase saturation somewhat in output referred photos. But first fix the tone curves.

Sorry, but this is a tedious process but I'm not aware of any simpler approach. Further, do not expect a print of your image to look very good. They normally appear more washed out that what you are used to which is why photographers don't use scene referred processing outside of specialized fields.

There is a considerable amount of detailed information on color and how it is handled by printers and cameras at www.color.org should you go down this path and need more info.

Originally by user58107. Source · Licensed CC BY-SA 4.0

user58107

8y ago

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A ColorChecker is most useful when you start from RAW. With JPEG, the camera has already applied non-linear processing (tone curve, contrast, saturation, white balance, color rendering, etc.) to make the image look pleasing. That means the file is already "output-referred," so a simple correction from the chart patches to their nominal values usually won’t fully recover faithful scene color.

So: using DNG camera profiles is generally a RAW workflow, not the right starting point for JPEG captures.

Your 3×3 matrix idea may improve the JPEG somewhat, but it is only an approximation because JPEG processing has already altered color and tone in ways a simple linear transform cannot fully undo.

For the best color accuracy, especially in reproduction work, capture RAW and use a scene-referred workflow with proper profiling. For printing, you also need printer/paper color management; correcting the camera image alone does not guarantee the print will match.

In short: with JPEG you can make limited improvements, but if your goal is truly faithful color, re-shoot in RAW and use a fully color-managed workflow from capture through print.

UniqueBot

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8y ago

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