Undercolor reduction

Under color removal , often with the English word under color removal (UCR) designates a process that is in the separation of digitized image data in the CMYK color space used for the subsequent four-color printing is. Common image processing programs take over this task automatically.

Primary and secondary colors , i.e. those that only consist of one or two of the basic colors , are not affected. Only in the case of tertiary colors, which are made up of all three basic colors, can the resulting blackening of the mixed color be replaced by pure black. In order to print a medium gray without UCR, you need considerable amounts of the three basic colors, or even, with UCR, only black.

The term undercolor reduction is derived from the fact that in a mixed color of cyan, magenta and yellow, the common, lower portion is reduced, at most up to the complete replacement of the base color with the smallest portion:

UCR

From the real proportions in the graphic, it can be seen that the process is not trivial, since a neutral part of the original mixture that looks like pure black can always be seen, which can then be replaced by black. To take 10% in all 3 chromatic colors would also require replacing the color 10% C 10% M 10% Y, but this is not possible with pure black, since it is usually more of a beige. The proportion of colored inks to be extracted is therefore always dependent on the gray axis of the printing process concerned. With the color separation of an image using the CMYK ICC profile , the type and strength of the UCR or GCR is permanently implemented and cannot be changed.

Although four colors with a proportion of 0-100% each (i.e. max. 400%) are available in four-color printing, in practice, depending on the material, a maximum amount of color is only available in the range of about 240% to 340% for art paper, higher values are not necessary due to the undercolour reduction and would possibly only cause problems with the drying of the ink or the further processing of the printed matter (ink abrasion). If the printed matter is still folded, excessive ink application can lead to cracks in the ink.

The undercolor reduction cannot be reversed and can be problematic for further image processing, especially for color changes. Therefore it should only be done in the last step, after all (color) retouching.

A common misunderstanding in connection with UCR is based on the inconsistent designation in German and English. The German 'reduction' allows gradations, the English 'removal', translated as 'removal' or 'removal', rather implies a complete replacement of the lowest color.