Tonal Reproduction Curve
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Tonal Reproduction Curve is often referred to by its initials, TRC, and the 'R' is sometimes said to stand for Response, as in Tonal Response Curve.
A Tonal Reproduction Curve is applied to the luminance value of an electronic image which adjusts for dot gain of a particular printing method. Every printing method has a finite native dot size. The dot is not square as most people think of a pixel, nor any other shape that when stacked together perfectly fills an image area. Instead the dot will be larger than its target area and overlap its neighbors to some extent. If it were smaller than its target area, it would not be possible to saturate the substrate. A Tonal Reproduction Curve is applied to the electronic image prior to printing so that the reflectivity of the print closely approximates the intension of the electronic image.
It is easier to demonstrate this using halftoned printing methods such as inkjet, or xerographic technologies. However, the concept applies to continuous tone methods such as Photographic paper printing. As an example let's say we want to print an entire page at 50% reflectivity, assuming no ink is 100% reflective and saturated black ink is 0%, which of course they aren't. This could be approximated using digital halftoning by applying a dot of ink at every other dot target area, and staggering the lines in a brick-like fashion. In a perfect world, this would cover exactly half of the page with ink and make the page appear to have 50% reflectivity. However, because the ink will bleed into its neighboring target locations, greater than 50% of the page will be filled with black. To compensate for this, a TRC is applied and the digital image's reflectance value is reduced to something less than 50%. When digital halftoning is performed, we will no longer have the uniform on-off-on-off pattern, but we will have another pattern that will target less than 50% locations with ink. If the correct TRC was chosen, the page will be 50% filled with ink after the ink has bled, and will have a uniform pattern that gives the appearance of 50% reflectivity. A curve is needed to compensate for the non-linear effect of dot gain at all reflectance values.
A TRC can be applied when doing color space conversion. For example, by default, when transforming from L*A*B* to CMYK, Photoshop applies an ICC profile for SWOP standard inks and 20% dot gain for coated paper.
