Losing mid-tone detail after black removal

[Schnitzel]

Hello everyone,

I'm trying to remove the black channel from an image, for a special project which requires printing only in CMY.
In Photoshop, under "Convert to Profile", I used the "Custom CMYK" profile: I measured with a spectro my HP Indigo's primaries and overprints, set the dot gain curves as simulated by the press, 300% ink limit, and black generation set to "None".

Indeed, the black channel is removed completely, but aside from a small hue shift, the picture loses some detail in the mid-tone area (see attached scan). Can someone tell me why that happens and how to fix it? Am I approaching it the right way?

For original image was converted from Adobe RGB to my press ICC profile, then converted again to remove the black channel using perceptual intent (Relative Colorimetric didn't produce better results).

Thanks for any help.

 

[Stephen Marsh]

What channel data is it that is missing? When you mouse over the original CMYK image, what are the channels where this data exists (info palette)? My guess is that this is probably CMK...

I would perhaps try the following:

* Have the original CMYK image open and the new CMY image open
* Dupe the layer of the CMY image so that you have a new layer to work with, which will give you more options for compositing before flattening
* Target/click on the cyan channel name in the CMY image in the channels palette, then turn on the preview/eye icon of the CMYK composite channels. Double check the title bar to ensure that only the Cyan channel is targeted/active while the full colour preview is active
* Use the Image/Apply Image command, select the cyan channel of the CMYK image as the source, and set the CMY image's second duped layer as the destination. Blending mode should probably be darken or perhaps normal blend mode. This should "stamp" the single channel data from the CMYK image into the single channel of the CMY image.
* Repeat the last two steps for the MY channels.
* When it comes to the data of the original image K channel, apply this K data to the CMY channels as desired/required. After you are done with the Apply Image command, click back on the name of the CMYK composite channel to activate all the CMYK channels.
* Adjust the blending mode, opacity, layer option blend if sliders and or layer mask as desired to blend in this new layer into the base CMY image.

One could also try using this approach by "stamping" the RGB channels of the original into the CMY channels with various blending modes or opacities.

Or perhaps one could layer the RGB image over the CMY image in luminosity or other blend modes.

Or one could try to use the channel mixer, to move the K data into the CMY channels.

There are many options!

As to "why" - the legacy Custom CMYK engine of Photoshop pre-dates ICC profiles, it is a very flexible tool, however it is not of the same quality/results of modern profile creation software. Your loss of detail could be due to using an incorrect dot gain value (experiment with zero or other values), or it could be that you edited the primary and secondary colours instead of leaving them at the defaults. It is hard to say without access to the original image files.


Hope this helps,

Stephen Marsh

 

[Lukas Engqvist]

is detail lost or is your monitor reaching the limits what it can display? It is very tricky to try and know what is going on here. We are talking about bright saturated yellows, and they are the weak spot of most RGB profiles (and monitors). I would check the original image with gamut warning with the proof profile set to the monitor profile... if you find that there are gamut warning areas it means your monitor is not suited to judge subtle variations in colour. (Note that the yellow of the indigo is cold, or towards the green, and using perceptual will give colour shifts. Also Indigo colours are more opaque than offset which means you do no not have the same range of saturated warm yellows as in offset)

You may do best to go from CMYK to LAB to Custom CMYK using relative conversion. If you start in CMYK and end in CMYK you can even use "fill with history" and set it to luminosity to bring back the lost info.

Or as Stephen says the channel mixer may be the best way to go. (unless you can have someone make you a device link profile)

 

[Schnitzel]

@Stephan, thank you for your post. I will try to apply some of the methods you suggested.
The reason I use the color management way instead of manual work is mainly because I need control over ink limit. For this test I was able to get away with 300% but perhaps in the future I'll have to go below that. The custom profile allows me to set the GCR parameters that I need.

@Lukas, I'm sorry I wasn't clear about the attached file - what you see is a scan of the printed image, not a capture of the display on screen. Perhaps the image is not ideal, I'll have to use a better test image to see if there are problems in other colors.
By the way, I was able to replicate this problem by using standard profiles such as SWOP, instead of the custom measurements and values I used.

I'll look into this further. Thank you both for helping!

 

[Stephen Marsh]

@Schnitzel, if you would like to crop out the detail section of the original RGB before conversion to CMY and post it to the forum or offlist I will kick this around in Photoshop.

Stephen Marsh

 

[Lukas Engqvist]

If you need profiles with other ink limit try the ones you find here colormanagement

 

[Schnitzel]

Thanks, I'll check that out. Some good test images there.

 

[meddington]

So you have an ICC profile for your HP press, but are using Photoshop's Custom CMYK to generate a "CMY" profile, is that right? Do you have the means to generate a "CMY" profile from your actual press data (the measurement data that created your HP press profile) using say, ProfileMaker?

Photoshop's "Custom CMYK" is rather archaic, though you may have to work in some retouching to save detail anyway.

 

[Schnitzel]

I'm using Argyll CMS (http://www.argyllcms.com) to generate my profiles. I tried to generate a CMY profile, but Adobe's software didn't list it as a CMYK output profile (or list it at all, for that matter). I'm thinking of contacting the CMS author to see whether it's possible to do this.
Obviously, I would rather use such a profile to do the conversion, and I realize the "Custom CMYK" option is not ideal, but it's the only automatic way I have right now to remove the black plate completely and still have control over ink limit.

 

[meddington]

I'm using Argyll CMS (http://www.argyllcms.com) to generate my profiles.

You might also try posting your Argyl issues to the Colorsync users Mailing list, or the Argyl CMS Mailing list.

 

[Schnitzel]

Thanks, I didn't know about the Colorsync mailing list.