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Rich black in images

Apr 27, 2012 2:27 PM

Tags: #photoshop #color #ink #limit #blacks #rich

A client has asked a question that seems strange to me regarding rich blacks in images. They want to be able to change blacks in an image that might be 80 80 80 100 to 30 30 30 100. My thought would be that they would just change the ink limit to what the printer has recommended. They could also reduce the black value in curves, but to get their limits would look strange. Before I go back and state my opinion, I would love to hear from some printers to see if they have any other opinions. They are not trying to mix colours to put them into InDesign.

 
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  • Currently Being Moderated
    Apr 27, 2012 9:06 PM   in reply to Ketrinz

    The proper way of doing this is to use the color profile of the paper stock for the printer. The CMYK numbers for most intensive blacks and ink coverage possible on particular paper and medium are set in their color profiles. This can be checked by converting a pure RGB black R=0, G=0, B=0, to different CMYK color profiles.

    So, it is a good practice to create in a RGB color space and at the end convert to the destination. If you have already created an image in a CMYK color space and have used CMYK color values that my exceed the limits of the media, you can use the destination color profile to fix that. Convert your image to a wide RGB color space like ProPhoto and then convert again to the CMYK color profile of the destination. If this is impossible the only option you have is to sample the colors of your image and correct the blacks with the various image adjustment tools like Curves, Selective Color, etc. - depending on the image this may be a lot of work if this breaks the tonal balance with the rest of the image.

     
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    Apr 27, 2012 10:09 PM   in reply to emil emil

    emil emil wrote:

     

    ...it is a good practice to create in a RGB color space and at the end convert to the destination

     

    Very sound advice.

     
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    Apr 28, 2012 9:05 AM   in reply to emil emil

    If you have already created an image in a CMYK color space and have used CMYK color values that my exceed the limits of the media, you can use the destination color profile to fix that. Convert your image to a wide RGB color space like ProPhoto and then convert again to the CMYK color profile of the destination.

    Converting to RGB as an intermediate step instead of converting CMYK-to-CMYK directly seems a useless step.

     

    Ketrinz, are you talking about graphics/illustrations or photographic images?

     
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    Apr 28, 2012 5:39 PM   in reply to Ketrinz

    Ketrinz,

    First check what is your current color space (profile) of the image from this menu at the bottom left of the document window:

     

    DocumentColorProfile.jpg

     

    If it is RGB color space like sRGB IEC1966-2.1 , Adobe RGB, etc., all you have to do is convert to the CMYK color profile of the printer. To do that choose Edit > Convert to Profile and from the CMYK menu of the dialog that opens, choose the color profile of the printer. If you get the color profile file of the printer from the print shop you have to put it in the color profiles folder of your system in order to appear in this menu. For Windows the location is C:\Windows\System32\spool\drivers\color.

     

    If it is CMYK color space like U.S. Web Coated (SWAP) v2 then using Convert to Profile, convert once to ProPhoto RGB and then again to the original CMYK color profile.

     
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    Apr 28, 2012 3:32 PM   in reply to c.pfaffenbichler

    c.pfaffenbichler wrote:

    ...

    Converting to RGB as an intermediate step instead of converting CMYK-to-CMYK directly seems a useless step. ..

     

     

    Intermediate color space will ensure achieving the most intense blacks possible without exceeding the allowable ink coverage set in the color profile. The problem is an image with blacks that exceed the allowable ink coverage. For example C=100, M=100, Y=100, K=100 will exceed the ink coverage set in any paper color profile. To fix this using color management conversion, the currently used CMYK color space has to be converted to an intermediate color space and then converted again back to the originally used CMYK color space. I prefer wider RGB color space to ensure less rounding errors.

     

    Here's a very simple example. Create a document with U.S. Web Coated (SWAP) v2 color profile and fill it with flat black color C=100, M=100, Y=100, K=100.

    Convert the document to ProPhoto RGB profile.

    Convert back to U.S. Web Coated (SWAP) v2 and you will get the most intensive black possible with the maximum CMYK values for the allowed ink coverage.

     

    You will still get similar result if you use another CMYK color space for the intermediate conversion but you still need an intermediate conversion anyway because you can't convert to the same color space.

     
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    Apr 29, 2012 3:28 AM   in reply to emil emil

    Sorry, I had misunderstood, converting to the same profile indeed needs the intermediate step.

     

    I prefer wider RGB color space to ensure less rounding errors.

    I think you may be confusing clipping (of out-of-gamut colors) and rounding in this example; the rounding of the new values will have to occur regardless of the width of the space according to the bit depth, even if the result may be imperceptible.

    Edit: Or I could be misunderstanding you again …

     
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    Apr 29, 2012 4:33 PM   in reply to c.pfaffenbichler

    I did mean rounding errors when replying to your remark, although in my initial response to the OP I suggested wider color space having in mind mostly the out of gamut colors. So, it is for both reasons. Yes, the rounding errors will occur anyway with any color space conversion but I believe their range will be less.

    I thought that you may be mentioning using another CMYK space because matching colors between different CMYK color spaces is less of a problem.

    For example experimenting with the same case from my previous post with the flat black color or even better, a grayscale ramp from black to white with 10 shades of gray, each gray being 10% different in all inks, and converting this in two different ways, first U.S. Web Coated (SWAP) v2 > ProPhoto RGB > U.S. Web Coated (SWAP) v2 and second Web Coated (SWAP) v2 >U.S Newsprint (SNAP 2007) > U.S. Web Coated (SWAP) v2. Although the newsprint profile has narrower  color space when converted back to the original CMYK using the Relative Colorimetric rendering intent, it will result in very similar color values to the ProPhoto RGB conversion so, out of gamut colors will not be an issue and the only difference between the two conversion will be in the range of the rounding errors.

     
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    Apr 29, 2012 11:06 PM   in reply to emil emil

    But will using a wider color space not actually cause more (not absolutely sure if the term is the correct one here) quantization because the originally present discrete steps will have to be bunched into a smaller section of available steps?

    I am aware that this is pretty much academic and the possible effect will in many cases be practically imperceptible anyway.

     

    The screenshots show (undithered) gradients in sRGB and the result of a round-trip to Lab.

    srgbGradientScr.jpgsrgbGradientScr2.jpg

     
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    Apr 30, 2012 12:25 AM   in reply to c.pfaffenbichler

    c.pfaffenbichler wrote:

     

    But will using a wider color space not actually cause more (...) quantization

     

    That's my thinking too. I would think sRGB actually gives the least rounding errors, provided the image can be squeezed in there.

     

    ProPhoto, for example, is very compressed in the shadows, which is why you are guaranteed to see shadow banding in 8 bits.

     

    Again, perhaps academic.

     
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    Apr 30, 2012 7:39 AM   in reply to c.pfaffenbichler

    You are both correct. However I was talking about different rounding errors (not banding) and may be that's not the correct term. I was talking about achieving the most intense blacks possible with the allowed ink coverage when fixing an image that exceeds the ink coverage.

     

    edit: OK, never mind what I said above. and I removed the example illustrating what I mean because I was making  wrong conclusions.

    So, yes the rounding errors will be greater with round trips if using wider color space as a trade off of avoiding color clipping.

     
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    Apr 30, 2012 12:23 PM   in reply to emil emil

    Don't feel bad. We've been told for so long to use large color spaces that it has become a sort of dogma that no one questions ("ProPhoto is the only color space real men use", that sort of thing...)

     

    But there are very real downsides too. Color spaces can be too big, and personally I tend to avoid ProPhoto. Adjustments in the shadow values become very sensitive and the histogram is so compressed that there is no real help there. I prefer Adobe RGB whenever possible.

     
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    Apr 30, 2012 12:31 PM   in reply to twenty_one

    ProPhoto is fine - as long as you use 16 bit/channel.

    At 8 bit/channel the quantization in ProPhoto is too visible.

     
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    Apr 30, 2012 9:35 PM   in reply to twenty_one

    D Fosse wrote:

     

    Don't feel bad. We've been told for so long to use large color spaces that it has become a sort of dogma that no one questions ("ProPhoto is the only color space real men use", that sort of thing...)

     

    But there are very real downsides too. Color spaces can be too big, and personally I tend to avoid ProPhoto. Adjustments in the shadow values become very sensitive and the histogram is so compressed that there is no real help there. I prefer Adobe RGB whenever possible.

    I don't feel bad I'm well aware and understand perfectly the quantization issues with wider color spaces,  What made me mentioned the "rounding error" was having in mind something else. I don't make round trips color conversions to fix ink coverage because I do it right in the first place by creating or working in RGB first and then converting to CMYK at the end. My advice was offered as a last resort for fixing ink over coverage. So, my mentioning of the "rounding" was purely based on experiments when replying in this thread and not on some regular practice. The problem was that I wasn't completing the experiments properly because I believed that the color  panel works in a certain way which turns out not to be so. I made certain CMYK grays with all channels being the same ink coverage like 80, 70, 60, 50, and etc. and converted the CMYK image to different color spaces like sRGB and also ProPhoto for comparison. But I didn't bother converting these grays back to the same CMYK space and instead tested the conversion by sampling the RGB images and checking their eventual CMYK values using the color panel in CMYK color mode. I always thought that the color panel shows CMYK values of RGB images by converting from the current RGB color space to the working CMYK color space set in the color settings. When checked like this, there was a differences between the CMYK values of the sRGB and ProPhoto image conversion and the ProPhoto looked slightly better because it was producing more ink coverage across the channels. Later I decided to actually test converting back to the CMYK space and was surprised to see that both sRGB and ProPhoto ended up with identical values and this made me remove my test results from my previous post and also change my opinion.

     
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    May 1, 2012 4:12 AM   in reply to emil emil

    Understood.

     

    emil emil wrote:

     

    I always thought that the color panel shows CMYK values of RGB images by converting from the current RGB color space to the working CMYK color space set in the color settings. When checked like this, there was a differences between the CMYK values of the sRGB and ProPhoto image conversion and the ProPhoto looked slightly better because it was producing more ink coverage across the channels

    Maybe this is where the rounding errors show up - the panel readout in 8 bits (difference), and the final conversion in 16 (ending up equal).

     
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    May 1, 2012 6:42 AM   in reply to twenty_one

    It wasn't that, the test file (conversion) was in 8 bit not 16 and I just realized what a silly mistake I've been making when testing. Look at the image below and notice the difference between the CMYK values in the Color panel and the Info panel. I was checking only the Color panel while the Info panel is more informative about what is going on. The problem was that ProPhoto document was not the active document when clicking with the eyedropper on it LOL. When clicking with the eyedropper on a non active document the Color panel samples the RGB value in that document's color space but assumes that this is the color space of the active document, sRGB in my case, and converts to the CMYK sliders as such. The difference is to big to be quantization errors and I wrongly interpreted it as better conversion with ProPhoto when it comes to ink coverage.

     

    Mistake.jpg

     

    Live and learn

     
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    May 1, 2012 7:47 AM   in reply to emil emil

    Ahh...

     
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