1. Field of the Invention
The present invention relates to an image processing method and apparatus for receiving a color image original in the form of R, G, and B color-separated signals, performing predetermined arithmetic processing, such as matrix conversion, for the input image signals, and outputting the result of the processing.
2. Related Background Art
Generally, a color copying machine desirably reproduces colors, gradation, and the like as faithful as possible in a hard copy output image with respect to those of an original image. In effect, however, problems are pointed out in the following three points because there is the difference between the color separation of an original image and the color reproduction range of a hard copy output device, and countermeasures against these problems have been proposed.
(1) In an original having a background portion (which is normally white), called a surface, in addition to an original image, it is desirable that this surface be reproduced in a color as white as possible (i.e., be not printed out). Additionally, even if the surface color is removed, color reproduction is desirably performed without degrading the color reproducibility of other colors. As a countermeasure against this, a method has been proposed by which the surface color is removed by detecting the surface level and performing nonlinear conversion for R, G, and B signals, as in Japanese Laid-Open Patent Application No. 5-63968.
(2) In an original in which the color distribution of an original image falls outside the color reproduction range of a hard copy output device, the gradation of that portion is degraded. In such an original, it is desirable to map the color distribution of an original image within the color reproduction range of a hard copy output device. In this connection, U.S. Ser. No. 38,898 (filing date: Mar. 29, 1993) proposes a method in which the color distribution of an original image is detected, and matrix conversion is performed in accordance with the detected color distribution to map the color distribution within the color reproduction range.
(3) A color copying machine compresses the expected dynamic range of an input image by converting the dynamic range in gradation. However, if an original image has a range wider than the expected dynamic range, the gradation is degraded in a dark portion. As a countermeasure against this, there is a method by which the compression ratio of the dynamic range is adjusted by modifying conversion expressions by using a logarithm conversion circuit in luminance-density conversion processing for an image signal.
A color copying machine includes an operation unit in addition to the above arrangement, so an operator can perform various color adjusting operations he or she desires by using this operation unit. For example, when an operator wishes to obtain a more reddish output image, he or she increases the M (magenta) density and decreases the C (cyan) density by operating an operation unit of a color copying machine as illustrated in FIG. 17. That is, in accordance with the setting in this operation unit, a color gradation modification circuit performs gamma conversion as in FIGS. 18A and 18B (in each of which a chain double-dashed line indicates a standard value, .gamma.=1). Consequently, the gamma curve of magenta rises to increase the magenta density as a whole, and the gamma curve of cyan falls to decrease the cyan density, thereby increasing reddishness in total.
The above countermeasures for effecting faithful color reproduction, however, have been separately proposed against the different problems. Therefore, if the above problems appear at the same time in practical situations (e.g., if the outlying color space of an original image and the batter of a dark level exist simultaneously), the signal conversion for a countermeasure against one problem produces an influence on the signal conversion for a countermeasure against the other, making it impossible to sufficiently achieve the respective effects of these countermeasures.
In addition, there are other problems in respect of the configuration of hardware. That is, the locations of the individual countermeasures are scattered at a plurality of points in an image processing signal system, resulting in increases in size, manufacturing cost, and difficulty in design of the circuit.
Generally, color adjustment is performed on the basis of a color sample image by using lightness, saturation, and hue as variables in many instances. As an example, the adjustment is to "increase the vividness (saturation) at a constant brightness (lightness) without changing the hue (at a constant hue)".
In any color adjusting method of the above conventional examples, however, the adjustment amounts of the lightness, the saturation, and the hue must be replaced with the combination of the color density adjustment amounts of Y, M, C, and Bk, as in FIG. 17. This makes desired color adjustment very difficult.
In addition, some image processing apparatuses employ a color adjusting method by which R, G, and B signals are converted in coordinates in a CIE-L*a*b* uniform color space to perform color conversion in the L*a*b* space, and then returned to the R, G, and B signals. However, the conversion from the RGB signal to the L*a*b* signal involves processing containing nonlinear arithmetic operations. Therefore, in an apparatus, such as a color copying machine, in which an enormous quantity of data (pixels) is processed at a high speed in real time, the load on the apparatus is increased to result in an increased size of hardware.
Furthermore, in the method of adjusting the color densities of Y, M, C, and Bk as in the above conventional examples, if the magenta density is increased and the cyan density is decreased in the adjustment for increasing reddishness, a red portion of an image becomes more reddish. However, green as a mixed color of cyan and yellow is rendered yellowish if the cyan density is decreased; that is, an undesirable color change takes place as a side effect.