Under color removal

Under color removal, are often referred to by the English term Under Color Removal (UCR ) is a method used in separation of the digitized image data in the CMYK color space for the subsequent four-color printing. Popular image editing programs perform this task automatically.

Primary and secondary colors, ie those consisting of only one or two of the primary colors are not affected. Alone at tertiary colors, which were made up of all three primary colors, you can replace the resulting grayness of the blend color by pure black. To print a medium gray it needs without UCR significant amounts of the three primary colors, or just with UCR, exclusively black.

The term under color removal derived from the fact that in a mixed color of cyan, magenta and yellow, the common bottom portion is reduced, a maximum of up to complete replacement of the base color with the lowest proportion:

From the actual proportions in the graphic you can see that the process is not trivial, there is always a neutral, like pure black acting, part of the original mixture can be seen, which can then be replaced by black. To remove 10% in all three chromatic colors would just require the color 10% C 10% M 10 % Y to replace, but this is not feasible with pure black, as it is more of a beige generally. Thus, the proportion of bright colors always to be taken from the gray axis of the respective printing process depends. In the color separation of an image using CMYK ICC profile, the nature and strength of the UCR or GCR is to be implemented and can not be changed.

Although the four-color printing four colors with a share of 0-100 % ( ie max. 400% ) are available, a maximum color application in practice is done pending on the material only in the range of from about 240% to 340 % for art papers, higher values are not required by the under color removal and would possibly cause only problems with the drying of the ink or the further processing of printed matter ( abrasion ). If the printed material still folded, may lead to high ink application to cracks in the paint.

The under color removal can not be undone and can be problematic for further image processing, especially for color change. Therefore, they should only before the last step, there are touch-ups after all ( color ).

A common misconception associated with UCR is based on the non-uniform designation in German and English usage. The German ' reduction ' allows for gradations, the English ' removal ', translated as ' Distance ' or ' withdrawal ', implies more of a complete replacement of the bottom color.

See also: achromatic, chromatic composition, offset printing

• Color reproduction
• Digital printing