Unsubtle: 12bit RGB to 1bit CMYK

M
Posted By
Mpaintnz
Aug 10, 2003
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384
Replies
1
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Closed
How do I downgrade a RGB photo (beige/blue plastic container with clear panes) so it prints off a CMYK deskjet printer (HP DJ670C) as a multicolor diagram without any dithering?

If I set the photo to indexed color /9color/primary (CMYK+RGB) /no dithering; the screen matches what comes out the printer, but I have no control on where the color borders are, unless I undo, twiddle the curves of the fullcolor version and reapply the color mode.

Ditto maxing out the contrast/brightness.

I’ve experimented with banding the colors in gradient mapping, <blue> – – – <blue><red> – – <red><white>- – -<white>

Converting the mode from RGB to CMYK

As well as FindEdges and Cutout filters.

In short I’m starting to suspect I’m going the wrong way about it by twiddling the RGB, and was wondering it there was some way of stating that you want the output to be undithered CMYK and dynamically steer ranges of RGB towards certain colors?

Any pointers?
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MR
Mike Russell
Aug 10, 2003
Mpaintnz wrote:
How do I downgrade a RGB photo (beige/blue plastic container with clear panes) so it prints off a CMYK deskjet printer (HP DJ670C) as a multicolor diagram without any dithering?

If I set the photo to indexed color /9color/primary (CMYK+RGB) /no dithering; the screen matches what comes out the printer, but I have no control on where the color borders are, unless I undo, twiddle the curves of the fullcolor version and reapply the color mode.
Ditto maxing out the contrast/brightness.

I’ve experimented with banding the colors in gradient mapping, <blue> – – – <blue><red> – – <red><white>- – -<white>
Converting the mode from RGB to CMYK

As well as FindEdges and Cutout filters.

In short I’m starting to suspect I’m going the wrong way about it by twiddling the RGB, and was wondering it there was some way of stating that you want the output to be undithered CMYK and dynamically steer ranges of RGB towards certain colors?

Any pointers?

Try fiddling with your driver settings, and see if you can get purer areas of color using the settings intended for charts and graphs. You are, of course, limited to pure swaths of cyan, magenta, and yellow (and their overlapping colors) using this method. If you want intermediate shades, or soft edges, there will be a certain amount of dithering.

I have thought about this problem for some time, and if you’re up something very unusual, here’s a possible way to accomplish 90 percent of what I believe you are after, which should give you very close to complete control over every dot of CMYK ink that reaches the paper.

Basically, use your printer as if it were a miniature printing press, running the paper through four times, printing a pass of yellow, cyan, magenta, and black. The colors will not be 100% pure, for example yellow areas will still have a few flecks of magenta and cyan, but you will get very close.

Start by separating your image into CMYK, and get it looking exactly the way you want.

Split the channels into separate documents, convert to bitmap, probably using threshold conversion, then convert back to RGB and colorize each of your "plates" to the appropriate cyan, magenta, or yellow, and print each one, one at a time, in the desired color.

It sounds from your description like you want solid areas of color and line art only, but if you want you may dither or halftone the individual colors to get intermediate densities.

Or course, as with a real printing press, registration becomes an issue, and you may find yourself learning about trap – expand or "trap" the lighter color where you have borders with adjacent contrasting colors. Print dark lines as pure black and not as overlapping CMY colors, which you do by separating using max GCR. These settings are in Edit>Color Settings, click on Custom CMYK and base your settings on SWP coated.

With the 670C, the black plate will print at a slightly higher resolution than your other colors. You may even want to sharpen it a bit by itself, as one would do for an image to be printed on a press.

You may be able to get purer colors by experimenting with the different driver settings – for example I believe there is one for printing charts and graphs that may yield a purer yellow.

I hope this response is on target. In any case, best of luck with your experimenting.

Mike Russell
http://www.curvemeister.com
http://www.zocalo.net/~mgr
http://geigy.2y.net

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