Omni Systems, Inc. Mif2Go User's Guide, Version 55

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> 13 Converting to HTML/XHTML > 13.7 Defining and mapping colors for HTML > 13.7.6 Understanding CMYK-to-RGB conversion anomalies

### 13.7.6 Understanding CMYK-to-RGB conversion anomalies

CMYK and RGB are fundamentally different. CMYK is subtractive. It defines a color in terms of how the color reflects or absorbs light (most useful for printing). RGB is additive. It defines a color in terms of the light that is emitted (most useful for monitors). Any conversion from one to the other is bound to be an approximation.

However, Mif2Go encounters a bizarre CMYK-to-RGB conversion problem with colors defined in color libraries. HTML requires RGB. FrameMaker uses CMYK internally, though RGB is used for the screen display in Windows. The values that FrameMaker uses for CMYK do not always convert to the same RGB values as those FrameMaker displays; often, nowhere near the same.

If you choose a Pantone color, for example, and view it in RGB in FrameMaker, the percentages you see are not always the correct computed value (which is what Mif2Go uses). Also, you can define two visually very different colors that appear to have the same CMYK value in FrameMaker, but different RGB values.

This is different from what happens if you define a new color based, say, on black; you can work in either RGB or CMYK, and when you flip back and forth, FrameMaker uses the correct conversion (the same conversion Mif2Go uses).

But FrameMaker’s color databases do something very odd. If you look at a CMYK color that has no blacK, such as PANTONE 528 CVC (a maroon), and note the Cyan, Magenta, and Yellow percentages; then change to RGB, and note the Red, Green, and Blue percentages; each matching pair (Cyan-Red, Magenta-Green, Yellow-Blue) should total 100%, because aside from blacK, the two color models are mathematical complements. However, what you see instead are the percentages shown under FrameMaker RGB in Table 13-4. The color still looks maroon in FrameMaker.

With model RGB selected, choose Black, then choose New Color, and enter the computed values, from the Computed RGB column in Table 13-4. Change to model CMYK. The percentages now match the original FrameMaker CMYK values for PANTONE 528 CVC, but the color is no longer maroon, it is a deep violet/purple—with exactly the same CMYK values as the maroon.

Table 13-4 FrameMaker color conversion anomaly

Percentages for PANTONE 528 CVC (maroon)
FrameMaker CMYK | Computed RGB | FrameMaker RGB
Cyan: | 43.0 | Red: | 57.0 | Red: | 58.3
Magenta: | 56.0 | Green: | 44.0 | Green: | 35.5
Yellow: | 0.0 | Blue: | 100.0 | Blue: | 64.1