under color removal (UCR)

There is often a direct relationship between the various color models that can be expressed mathematically. However, since certain restrictions occur in practice during printing, for example between the CMY color model and the CMYK color model, mathematical approximations must be used in this case where no mathematical relationship exists.

Under Color Removal (UCR) is such an approximation method, which is used for the conversion between the CMYK color model and the RGB color model.

Crucial to the CMYK color model, which is used in all color printers, is the additional black component, denoted by depth (K), for which the proportion must be determined. In the UCR process, the CMY values are calculated from the RGB values. Then the common portion of the three colors is determined and replaced by black.

Conversion from CMY to CMYK with the UCR method

This will be illustrated by an example, which is, however, theoretical in nature and modified in practice by empirical values: Cyan has 46%, Magenta 65% and Yellow 28%. All colors have at least 28%, which is the common proportion in which black is printed. Accordingly, the three primary colors are each reduced by 28%. Cyan is thus printed at 18%, magenta at 37% and yellow at 0%.

Characteristic curves of the colors and depth (K) in the UCR process

Like the alternative GCR process, the UCR process reduces the amount of ink and shortens its drying time. This process is particularly useful for gradients, as color gradients are printed without color deviations, known as color casts.