1. Field of the Invention
The invention relates to the field of printing systems, and in particular, to methods and systems for color conversions.
2. Statement of the Problem
In color printing, displaying, and reproduction, the term gamut represents the set of colors that a color-reproduction device is physically able to generate. Every device that displays or reproduces an image, such as a printer, monitor, scanner, or digital camera, may have its own unique color gamut. When an image is transferred from one device to another, the color gamut of each device is examined.
The color gamut generally has two components that are considered when performing a color conversion. Those components include the gamut boundary and the number of colors that are realizable within the gamut boundary. The gamut boundary of a device represents the outermost extent of the device's capabilities in a reference color space. Because of quantization in color reproduction systems, such as those of color halftone printers, not all colors within a device's boundary are realizable. Moreover, the shape of the boundaries can vary dramatically.
When an input color space is larger than the gamut of an output color device, gamut mapping algorithms are applied. The gamut mapping process transforms a point in the source gamut to a realizable color inside the gamut of the output device. The form of this transformation can dramatically impact the quality of the reproduced images. As such, care is taken in the design and implementation of gamut mapping transformations.
Certain current gamut mapping algorithms map all out-of-gamut points directly to the destination gamut. For example, clipping algorithms clip out-of-gamut points to the destination gamut boundary. Scaling algorithms, on the other hand, scale an input color gamut to an output color gamut such that some of the out-of-gamut points are mapped inside the destination gamut while others are mapped to the boundary of the destination gamut.
The large variability in past color gamut mapping studies suggests that ideal gamut mapping depends on image content, preservation of perceived hue throughout color space and the extent of the gamut mismatch in various regions of color space. Thus, image dependent and regional-dependent gamut mappings have been preferred. One such gamut mapping includes a linear conversion function that interpolates color space source data to printer specific color space data by subdividing the source data into a plurality of subsets and interpolating the data for each subset using one or more linear conversion functions. A problem, however, exists with the linear conversion function as it does not generate an optimum interpolation for each subset. To compensate for sub-optimal interpolations of subsets, one or more of a plurality of conversion functions may be selected for each subset, optimizing the interpolation for each subset. The converted subsets, however, still do not optimally combine to form a target data set.
Perceptual gamut mapping is also used in color reproduction. Perceptual gamut mapping modifies both in-gamut and out-of-gamut colors from their colorimetric representation in order to provide a pleasing or perceptual appearance. The results of the perceptual gamut mapping normally depend on the input color range and the device color gamut. However, if the input color range is much larger than the actual output color range, as is normally the case, the reproduced colors have lower chroma and are less vivid. Accordingly, there exists a need to map colors from the gamut of one device to the gamut of another device while ensuring that the colors remain aesthetically pleasing and vivid.