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p.1 #3 · Soft proofing and profiles | |
Color management for printing starts with the output devices and works backwards to allow the display to simulate the limits of the output. The goal for soft proofing is to have the screen accurately simulate printer output, in effect forcing more saturated colors of the monitor to look as "bad" as the print will.
The management process starts by profiling the printer using a target with as many as 900 different color patches and then comparing how the printer ink/paper reproduced them with how they would look if reproduced optimally "per the numbers". The gamut of a printing device is dictated by the physical characteristics of the paper and pigments. The red, green, blue phosphor / LCD pixel on your monitor will usually be capable of reproducing much more saturated reds, blues and purples than any paper / pigment combination because the inks are actually transparent and magenta and cyan pigments are cross contaminated. The resulting printer profile, when used for soft proofing, provides a "roadmap" telling the monitor how to desaturate its color and lower its contrast on screen to simulate how the print output will look.
In the days before ICC profiles what we'd do is take a file containing the test chart RGB values and send it to the printer, then manually adjust the printer color and contrast controls until the image on the screen was as close as possible match to the print sitting in a 5000K standard viewing conditions. The result would be an appearance on screen that was far less optimal than the screen could make it, but one which provided some clue of how the colors would look when printed. You'd need one monitor for normal viewing and another for "soft proofing".
What ICC color management does is dynamically alter the output characteristics of the monitor to simulate any gamut. The Catch-22 is that what it displays is limited by its gamut. If you edit on a consumer grade LCD monitor or laptop your "window" on the color values which actually exist in the file are rather limited to a gamut similar to sRGB or even smaller. So while you have mapped the camera color values to the rather large AdobeRGB working space, what you are seeing during normal editing are any more saturated colors than your monitor can reproduce remapped to fit the gamut of the monitor in a way that makes the overall contrast and saturation of the image look good PERCEPTUALLY.
The reason a large working space is needed is because RGB and CYMK colorspaces, when mapped to the three coordinate Lab mathematical model of human color perception are much different shapes..

The comparison above shows my monitor space vs my 8/C HP printer space. The colored parts hanging out of the white wireframe are saturated colors the CcMmYKkk printer can print but the RGB monitor can't display with the same saturation. The areas where the white monitor wireframe is larger are colors the monitor can display that the inks can't reproduce.

Here's a comparison of the printer profile with AbobeRGB 1998. Digital cameras and color inkjet printers were still on the drawing boards in 1998. The Abobe 1998 colorspace was actually created to address the needs of the offset printing industry -- where I've worked since the mid-1970s. You can see there are parts of the 8/C printer gamut which fall outside of Adobe RGB but not by much. But AdobeRGB is an almost perfect fit to SWOP CYMK (SWOP = Standards for Web Offset Printing)...

the sRGB RGB space also fits inside of AbobeRGB quite nicely...

So the working space acts as a "escrow account" for the color as it gets manipulated from RGB to CYMK. Ideally the working space should be larger than any of the RGB or CYMK spaces which are the input sources and output destinations. As you can see above today's 8/C printers have a larger gamut than Abobe envisioned when designing the shape of the AdobeRGB space in 1998. Cameras now capture and high-end 8-12 color ink jets now have gamuts larger than AdobeRGB which is why many who print on high-end inkjets now use the even larger ProPhotoRGB working space for 16-bit editing.
Getting back to the soft proofing. What you don't perhaps realize is that when viewing a file normally the ICC based color management is already re-mapping any highly saturated colors your camera recorded (and the AdobeRGB colorspace preserved) to the more limited range of saturation your monitor can display so PERCEPTUALLY what you see on the monitor appears to have the same range of tone and contrast you'd experience in person looking at the same scene. When you apply the printer profile between the file values and the monitor the printer profile will cap the saturation of the screen image to that which the ink and paper can reproduce:
Normal viewing:
RAW capture > Conformed to Adobe RGB > Conformed to Monitor Profile = what you see on screen
Soft proofing:
RAW capture > Conformed to Adobe RGB > Conformed to Printer Profile > Conformed to Monitor Profile = what you see
So in effect the printer profile is acting like a frosted filter on a lens would to cut the contrast and saturation of what is in the file to what the printer can produce. As the printer vs monitor wire frames reveal there may be some saturated colors in the file the printer can reproduce but the monitor can't. So what happens when the printer profile filtered values are passed to the monitor driver for display the monitor profile will clip them to the limit of what the screen can show...
Here's a screen shot with colors in the background which a printer gamut will clip, viewed in normal editing mode...

Here I've selected soft proofing using my HP 8/C printer profile and enabled the out of gamut warning...

The colors shift and all the colors which are out of the gamut of the printer get grayed out, telling me exactly where the clipping is occurring.
What was interesting about this image was that the soft proofing didn't change the subject in the foreground much, in part because most of the colors she is made up of fall in the core area of the file / monitor / printer gamuts and thus weren't remapped by the colorspace remapping. That's a key point to realize about ICC color management. The majority of the colors a camera captures in an average scene will fall inside the center where all the gamuts overlap, so there isn't much change. The change is mainly the remapping of any highly saturated colors like the purple background in this shot.

Knowing from the soft proofing that colors would change I could do one of three things:
1) Don't change the flie, just print it: If I don't alter the file in any why it will wind up looking similar to the soft-proofed image (with the OOG warning turned off): less saturated and with less contrast because of the fact that's the best the ink and paper can do with the file values. When the RGB file is sent to the printer the color management engine inside the printer will remap the maximum RGB values in the file into the maximum saturation of M+Y, C+Y, C+M combinations the printer can produce. The printer color management will clip the file values.
2) Convert to Printer Profile: If I convert the file from the AdobeRGB working space using the printer profile used for soft proofing Photoshop will clip the colors in the working space to the limits the profile says the printer can reproduce. When going that route you take on faith that the profile actually does represent the limits of what the printer can print. When the file is send to the printer I'd tell it to let Photoshop manage the color. There's still an RGB > CYMK translation process involved, but the printer will not manipulate the values per the printer profile since its already been done in photoshop. Often when people encounter wonky results when printing its due to double profiling: the printer profile is applied first by Photoshop then again by the internal conversion engine.
The inherent risk in converting the profile in Photoshop vs letting the Costco printer just manage the conversion (as in option 1) is that an incorrect profile might clip some colors the printer paper could reproduce with more saturation. A simple way to test this would be to shoot a photo with deep saturated reds and purples and print the file both ways: 1) letting the printer manage the color, or 2) converting to the printer profile.
3) Manually adjust color to make the OOG warning disappear: What I did with the photo above is open Hue /Saturation and manually reduce the saturation of the file values until the Out Of Gamut warning disappeared. That did manually what the Convert to Profile process would do automatically: reduce the gamut of the file values to match the gamut the printer can actually reproduce.
All three are different means to the same end. As I said in the beginning the actual appearance of the print is determined by the limits of the gamut of the printer. All the printer profile is doing in the workflow is forcing the monitor to simulate the more limited gamut of the printer to the extent its phosphors/LCD can simulate the much different CYM pigment characteristics.
So if you find you get good results soft proofing with some printer profiles and not with others the root cause of the problem is the printer profile not accurately reflecting the output of the printer. In the case of an ink jet it might be due to mechanical issues such as clogged ink jets, variation in inks, etc. In the case of the Costco printers a deviation from what the profile predicts may be due to the exposure level of the photographic process, batch variation in the printing paper, or photo processor variables.
While I understand the process I don't obsess over it. Quite to the contrary. The most important thing I've learned working in color reproduction is the weakest link is human perception. Absent a side-by-side "pixel peeping" comparison a wide range of reproduction will look fine. But whenever you compare two images side-by-side there will be a difference and one will be perceived to look better. Show a client two proofs side-by-side and 90% of the time they will ask for one that's in the middle. What you do then is wait until the next day, so them either one individually, and they'll think it looks fine.
In essence what color management is about is managing perceptual expectations so every step of the reproduction / display process makes the images look "normal" perceptually. Perception is based on anchor tones and memory colors. If the shadow values are too light or highlights too dark a print will look flat. If a skin tone is slightly cool or warm whether or not it looks "normal" will depend on the context clues the rest of the image provides, but in almost every case a skin tone which is biased on the green / magenta axis will be perceived.
Chuck
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