Color expression method and image processing apparatus thereof

Abstract: In a color expression method and an image processing apparatus thereby, characteristics of human vision which responds in proportion of a cube root of a luminance signal indicated by CIE L*a*b* colorimetric system is utilized. In look-up tables 101 ∼ 103a, a cube-root conversion is performed and a difference signal of data on which the cube-root conversion is performed is generated in subtraction circuits 104, 105. That is, each of 8-bit data of R, G, B is reduced to 7-bit data in the look-up tables, and each of color signals C1, C2 after a subtraction operation is requantized to 6-bit data. Accordingly, color signals of 24 bits/pixel is reduced to 19 bits/pixel, thus resulting in eliminating color data redundancy.

Description: BACKGROUND OF THE INVENTION The present invention relates to a color expression method and image processing apparatus thereof. In a conventional color expression method of a color image reading apparatus, there is a method which adopts colors of R (red), G (green), B (blue) signals standardized by television broadcasting, as shown in Fig. 7, for color sensitivity of color separation system. This is determined to correspond to characteristics of color development of R, G, B fluorescent materials in a cathode-ray tube which are used as luminous materials in a television receiver. There is another color expression method using spectral sensitivity of CIE 1931 standard colorimetric system or color separation system having spectral sensitivity of a narrow bandwidth in order to measure the density of ink used for an original (by transmission or reflection) or density of Y (yellow), M (magenta), C (cyan) fluorescent materials. In a video camera/player, for example, there is a method in which color image signals read by the aforementioned method using spectral sensitivity are once stored in a magnetic tape and the R, G, B signals read out of it are output to a color monitor and the like. There is another method, such as a tintometer, where the read XYZ signal values are calculated and the calculated values are plotted on a chromaticity coordinate system of the CIE L*a*b* colorimetric system. There is another method where a color correction processing is performed so as to correspond to the characteristics of luminous material, e.g. ink or toner, and corrected color image signals are outputted by a color printer, a color copier. There is still another method that a series of color image processing, such as color image reading, color correction, and output, is performed in a manner such that color image signals are stored in a hard disk of a computer or work station, and editing such as image synthesis and color transformation is performed on a color monitor of the computer. In the above color image reading apparatus, however, as shown in Fig. 7, each of chromaticity values of R, G, B signals are located inside of a spectrum locus. Therefore, theoretically, spectral sensitivity characteristics of a color separation system used for the color image reading apparatus which generates R, G, B signals to satisfy the chromaticity values should lie in part in a negative area. However, it is impossible to realize the spectral sensitivity characteristics located in part in a negative area. Accordingly, as shown in Fig. 8, approximate values are obtained by spectral correction, that is, the characteristics in the negative area is eliminated, the characteristics are corrected as indicated by a dotted line, or a primary transformation is performed for correction. In the above-described methods, however, color characteristics of an objective original or an object is read with a considerable amount of error. Even if an original is accurately read, as shown in an xy chromaticity diagram of Fig. 9, color corresponding to a notation x which is located outside of the triangle which is formed by chromaticity values developed by the aforementioned fluorescent materials will have a negative signal value. This indicates the fact that when an object is exposed by a color video camera and played back by a video player, colors of the object cannot be accurately reproduced. In case of color copiers, the problem is that colors of the original image differ from those of a copy, that is, it is impossible to obtain an accurate color reproduction. To solve the above problem, it has been suggested to use color data expressed by a combination of reference color stimuli indicated by vertices of a triangle which is substantially circumscribed about a spectrum locus of the CIE chromaticity diagram as color data of data input/output in each unit such as an image input unit, image output unit, and other image processing unit of the color image processing apparatus. However, even if such color data is used, since color discrimination ability of human being is not considered, there is an inconvenience when image data is stored in a magnetic disk as a file or transmitted through a telephone communication lines. That is, when each of tristimulus values of the R, G, B signals expressed by the three primary colors is expressed in 8 bits, bits are allotted to colors which cannot be discriminated by human eyes and data redundancy is increased. Consequently, the amount of data to be storeed or transmitted becomes considerably large. Accordingly, it is a concern of the present invention to provide a color expression method and image processing apparatus thereof capable of reducing color data redundancy, improving color reproduction in various media, and performing efficient data storage and data transmission whilst maintaining the quality of image data. An article at pages 93-100 in Electronics & Communications Engineering, No. 1, 1989, discloses an XYZ color system in which a quantisation process is carried out in which (RGB)vectors→(XYZ)vectors→(Y ξ η)vectors. However, it does not provide a solution to the problem posed earlier in the present specification. In accordance with a first aspect of the present invention there is provided image processing apparatus as set out in claim 1. In accordance with a second aspect there is provided an image processing method as set out in claim 5. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Fig. 1A is a block diagram illustrating the construction of a color image processing apparatus according to a preferred embodiment of the present invention; Fig. 1B is a block diagram illustrating the color signal conversion unit comprising the color image processing apparatus of the embodiment of Figure 1; Fig. 2 illustrates a block diagram of a general look-up table for converting color image signals R, G, B to primary signals Y, M, C, K; Fig. 3 illustrates a block diagram of a look-up table for converting color image signals R, G, B to primary color signals Y, M, C, K according to the embodiment of Figure 1; Fig. 4 is a diagram illustrating a color space of the basic six primary colors according to the modification 1; Fig. 5 is a block diagram illustrating the color signal conversion in the color image processing apparatus according to the modification 2; Fig. 6 is a reference color stimuli coordinate system according to the present embodiment; Fig. 7 is a diagram illustrating a spectral characteristic of color signals R, G, B which are standardized for TV broadcasting; Fig. 8 is a diagram illustrating a characteristic of spectral sensitivity of signals R, G, B; and Fig. 9 is an xy chromaticity diagram. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. In the present embodiment, three points indicated by notation × are expressed on a reference color stimuli coordinate system as shown in Fig. 6. These coordinates on the xy coordinate system are: ( 0.7347, 0.2653) (-0.0860, 1.0860) ( 0.0957, -0.0314) A triangle (numeral 2) formed by connecting these three points is circumscribed about an xy chromaticity value spectrum locus (numeral 1) approximately at 505 nm and 525 nm, and almost overlaps with a purple boundary connecting two points at 380 nm and 780 nm. Color signal values which can be obtained from the reference color stimuli coordinates are expressed by R, G, B, and a color signal expression which is suitable for data storage and data transmission can be obtained by performing nonlinear transformation on R, G, B values. Following are the equations to transform R, G, B values to color signal values L, C1, C2: L = 255 × (G/255) 1/3 C1 = 255 × ((R/255) 1/3 - (G/255) 1/3 ) C2 = 255 × ((G/255) 1/3 - (B/255) 1/3 ) The reason for taking the cube root of each signal value is that human eyes respond proportionally to the cube root of the luminance signal as shown in the CIE L*a*b* colorimetric system. That is, human beings judge that an image quantized to 7 bits by the color signal value L is equivalent to an image quantized to 8 bits by the signal value G. Accordingly, the amount of information of the image is reduced to 7/8 without deterioration of an image quality by using the color signal value L. In the above equations (1), the reason why taking differences of cube root of R, G, B values as color signal values C1, C2 is that the ability of color discrimination by human eyes decreases as chroma becomes higher. Accordingly, even if the number of quantizing bit of the color signal values C1, C2, or a spatial resolution is reduced, the image quality will not deteriorate. Fig. 1A is a block diagram illustrating a construction of a color image processing apparatus according to a preferred embodiment of the present invention. In Fig. 1A, an image signal is inputted to an image input unit 11 from an image signal source (not shown). An A/D conversion and sample-and-hold are performed in the image input unit 11. In an image processing unit 12, shading correction or black correction is performed on the output signal from the image input unit 11 and the corrected signals are input to a color signal conversion unit 13 as R, G, B image signals. The color signal conversion unit 13 performs quantization which is described later on the input R, G, B signals and outputs color signals, that is, L, Cl, C2 signals, in which the number of bit per pixel is reduced. A look-up table (LUT) 14 transforms these input color signals L, C1, C2 into primary color signals Y, M, C, K as needed in a hard copy output unit 15. Fig. 1B is a block diagram illustrating the color signal conversion unit 13 performing a conversion expressed by the above equations (1) and comprising the color image processing apparatus according to the present embodiment. In Fig. 1B, numerals 101 ∼ 103 are look-up tables which perform cube-root conversion, and numerals 104 and 105 are subtraction circuits for generating a difference signal. In the circuit of Fig. 1B, with respect to each 8-bit input of the R, G, B values, data on which a cube-root conversion is performed is quantized to 7 bits and each of the color signal values C1, C2 after the subtraction operation is requantized to 6 bits. Accordingly, a color signal of 24 (8 × 3) bits/pixel can be reduced to that of 19 (6 + 7 + 6) bits/pixel, thus resulting in reduction of data amount to 19/24. Next, a table conversion based upon the color signal L, C1, C2 is described. In order to output a hard copy of color image signals expressed by the signals R, G, B on a recording paper, the color image signals are transformed to primary color signals Y, M, C, K by the LUT 301 shown in Fig. 2. However, according to the construction of Fig. 2, each of the number of quantizing bit of the input/output signals R, G, B is 8 bits. The capacity necessary for the LUT is 64 megabytes and this is not practical. In the present embodiment, as shown in Fig. 3, after the input signals R, G, B are transformed to color signals L, C1, C2, these signals are input the LUT 401. The capacity necessary for the LUT is 2 megabytes when L = 7 bits, C1 = 6 bits, and C2 = 6 bits. Consequently, a circuit size of the LUT can be reduced to 1/32 of the LUT shown in Fig. 2. Accordingly, the amount of information of the image can be reduced without deterioration in the image quality by using color signals in which the color data redundancy has been removed in a manner such that a transformation is performed, with a consideration for the ability of color discrimination by human eyes, on the signals R, G, B expressed by a combination of the reference color stimuli data indicated by vertices of a triangle which is substantially circumscribed about a spectral locus of the CIE chromaticity diagram. [Modification 1] A modification of the above embodiment is described below. In this modification, the color signal values L, C1, C2 are obtained by performing nonlinear transform on the signals R, G, B. L = 255 × ((R/255) 1/3 + (G/255) 1/3 + (B/255) 1/3) /3 C1 = 255 × ((R/255) 1/3 - (G/255) 1/3 ) C2 = 255 × ((R/255) 1/3 /2 + (G/255) 1/3 /2 - (B/255) 1/3 ) By using the equations (2), basic six primary colors such as red, green, blue, yellow, cyan, magenta are expressed by the signals R, G, B, that is, six colors expressed by (R, G, B) = (255, 0, 0), (0, 255, 0), (0, 0, 255), (255, 255, 0), (0, 255, 255), (255, 0, 255) form a hexagon in a color space, as shown in Fig. 4. Consequently, signal processing can be easily performed. When the hexagon in Fig. 4 is expressed by color signal values L, C1, C2, simple values can be obtained as follows: Red : ( 255/3, 255, 255/2) Green : ( 255/3, -255, 255/2) Blue : ( 255/3, 0, -255 ) Yellow : (255 × 2/3, 0, 255 ) Cyan : (255 × 2/3, -255, -255/2) Magenta: (255 × 2/3, 255, -255/2) In this way, according to the present modification, signal processing can be easily performed by using simple color signal values. [Modification 2] As a second example of modification, the color signal values L, C1, C2 are obtained from the color signal values R, G, B which can be obtained from the same reference stimuli coordinates as the above embodiment. That is: L = -255/Dmax × Log 10 (G/255) C1 = -255/Dmax × (Log 10 (R/255) - Log 10 (G/255)) C2 = -255/Dmax × (Log 10 (G/255) - Log 10 (B/255)) As a model of visual characteristics of human eyes, Weber-Fechner's logarithmic response model is utilized. In equations (3), Dmax indicates a maximum density included in an image signal and value of 1.5 ∼ 2.0 is normally set. Fig. 5 is a block diagram illustrating the color signal conversion in the color image processing apparatus according to modification 2. In Fig. 5, numerals 201 ∼ 203 are look-up tables which perform logarithmic transformation. Numerals 204 and 205 are subtraction circuits which generate a difference signal of the transformed data. In Fig. 5, with respect to each 8-bit input of the R, G, B values, data after the logarithmic transformation is quantized to 7 bits, and the color signal values C1, C2 after the subtraction operation are quantized to 6 bit. In this embodiment, the amount of image information can be reduced without deterioration in image quality. The present invention can be applied to a system constituted by a plurality of devices, or to an apparatus comprising a single device. Furthermore, it goes without saying that the invention is applicable also to a case where the object of the invention is attained by supplying a program to a system or apparatus.

Claims: An image processing apparatus characterized by comprising: means (11, 12) for obtaining first color signal data in a color space (2 of Fig. 6) defined by three stimulus components (R,G,B) located at ( 0.7347, 0.2653) (-0.0860, 1.0860) ( 0.0957, -0.0314) in CIE 1931 xy chromaticity coordinates; data transform means (13, 101-105) for obtaining second color signal data by performing a nonlinear transformation on the first color signal data; and outputting means (15) for outputting a visual output based on the second color signal. Apparatus according to claim 1, wherein said data transform means performs nonlinear transformation so that the quantizing step becomes larger as the luminance of said first color signal data is higher. Apparatus according to claim 1, wherein said data transform means further comprises a memory table (14, 401) to input said second color signal data and output a predetermined signal data. Apparatus according to any preceding claim, wherein said nonlinear transformation is a logarithmic transformation. An image processing method characterized by comprising the steps of: obtaining first color signal data in a color space (2 of Fig. 6) defined by three stimulus components (R,G,B) located ( 0.7347, 0.2653) (-0.0860, 1.0860) ( 0.0957, -0.0314) in CIE 1931 xy chromaticity coordinates; obtaining second color signal data by performing a nonlinear transformation on the first color signal data; and outputting a visual output based on the second color signal. A method according to claim 5, wherein said nonlinear transformation is a transformation such that the quantizing step becomes larger as the luminance of said first color signal data is higher. A method according to claim 6, wherein said nonlinear transformation is logarithmic transformation.

Inventors: OHTA KEN-ICHI, OHTA, KEN-ICHI, Ohta, Ken-Ichi, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N1
IPC codes: H04N1, G06T1, G06T7