Convert RGB to CMYK using custom tolerance for gray

In RGB gray is indicated by same (or similar) color values for all three colorants. The tolerance is important when you convert RGB to CMYK, at least when you want to keep gray so that RGB gray would only use the K colorant (or DeviceGray or Separation Gray depending on the other settings in the Convert colors Fixup). This is controlled via the "Preserve black objects" checkbox in Convert colors.

The internal tolerance in Convert colors for that is 2%. But pdfToolbox 11 introduces a new Process Plan "Convert RGB to CMYK (PSO Coated v3) with specified R=G=B tolerance" which allows you to modify this tolerance.

Via a variable you can specify the tolerance.

How it works

The first step uses the Convert colors Fixup with a filter Check. The filter uses the "Difference between colorant channels" property with a variable that defines the maximum difference between the R,G and B colorant to be matched by the filter. All such colors are converted to gray (Dot Gain 15%).

In the second step the remaining colors are converted to CMYK without preserving black objects. that means the tolerance can be either above or below 2% since what colors are considered to be gray is determined in the first step.

Example

This test chart has a R=G=B=50% patch in the center (indicated with a black stroke) and color shifts of 2.5% towards the outer edges. This can be used to visualize the effects of the Process Plan.

When you apply the Process Plan with a tolerance value of 25% the result looks like this.

Watch Dietrich von Seggern talk about RGB to Gray conversion in the video below:

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