Color image recording apparatus

Abstract: A color image recording apparatus such as an ink jet printer to form a color image by printing coloring agents such as inks of a plurality of colors comprises: recording heads (19A-C) to sequentially form coloring agents of a plurality of colors onto a recording sheet; a detector (20A) to detect a signal indicative of a print amount of the colorintg agent which is previously printed from among the coloring agents which are printed at the same position; a correction circuit (23) to correct the signal level (on 15b) indicative of the print amount of the coloring agent to be printed later at the same position on the basis of the output of the detector; an input circuit (13a-c) to input a color signal consisting of a plurality of color component signals; and a color processor (14) to output the signals indicative of the print amounts of those coloring agents and to perform a linear masking process. Since the optimum gradation correction is executed on the basis of the ink amount which has previously been printed, the good gradations and color reproducibility are obtained.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image recording apparatus and, more particularly, to a color image recording apparatus for forming a color image by printing inks of a plurality of colors. Related Background Art Hitherto, an ink jet recording apparatus for forming an image by spouting out inks from nozzles each having a small diameter has been well known. The ink jet recording apparatus is widely used as a color image recording apparatus because a color image can be easily obtained by printing inks of a plurality of colors so as to be overlapped. Fig. 5 shows a scanning method of ink jet heads in the case of obtaining a color image by overlapping the inks of three colors of yellow, magenta, and cyan. In the diagram, reference numeral 1A, 1B, and 1C denote multi-nozzle heads which are arranged at a distance d from each other. These heads are scanned on a recording sheet 3 at a velocity v in the direction of an arrow 4 while spouting out inks from orifices 2. The head 1A is used for the yellow ink. The head 1B is used for the magenta ink. The head 1C is used for the cyan ink. The inks are printed on the recording sheet 3 in accordance with the order of yellow, magenta, and cyan. Fig. 6 is a block diagram for the image signal processes by the foregoing ink jet recording apparatus. Input signals 5a to 5c indicative of image densities of three colors of yellow, magenta, and cyan are supplied to a color processing section 6 are subjected to color processes such as a masking process and the like. Therafter, the color processed signals are input to a gradation-correction section 7 and are γ corrected. Among the three-color signals after the correction, the yellow signal is directly sent to a recording head 9A. However, the magenta and cyan signals are once stored into buffers 8A and 8B and thereafter, they are delayed by the times corresponding to a distance d in the scanning direction of the recording head, i.e., by only the time of d/v in the case of the magenta signal and by only the time of 2d/v in the case of the cyan signal. The delayed magenta and cyan signals are sent to heads 9B and 9C. Thus, the inks of the respective colors of yellow, magenta, and cyan are printed in the same position on the recording sheet 3 and a color image is reproduced. In the gradation-correction section 7, the γ correction is performed in such a manner that the relation of the image density of the printed image for the input image density signal is linear with respect to each color of yellow, magenta, and cyan. The γ characteristics after the correction of yellow, magenta, and cyan become as shown in Fig. 7. However, these γ characteristics are obtained in the case where the inks of yellow, magenta, and cyan were printed as single colors respectively. The γ characteristics in the case where two or three colors are printed differ from them. In the case of such mixed colors, the γ characteristic of each color component depends on the amount of ink which was previously printed. Fig. 8 shows a change in γ characteristic of magenta to the amount of yellow in which has previously been printed. In Fig. 8, reference numeral 10a denotes a γ characteristics of magenta in the case where the magenta ink was first printed without printing the yellow ink. With an increase in the print amount of yellow ink, the γ characteristic of the magenta ink changes as shown in 10b to 10d. This change is not concerned with the kind of ink. The same tendency is obtained with the first or subsequent over-printing. It is considered that this phenomenon occurs because of the nonlinear way in which the ink is absorbed into a sheet. Thus, the relation between the output image density signal and the color component of the output image becomes nonlinear due to such a phenomenon. There is a drawback such that the color reproduction cannot be sufficiently performed by the linear color correcting processes such as the linear masking method and the like. For example in the case of the ordinary linear masking method, when it is assumed that the input yellow, magenta, and cyan signals are set to Y, M, and C, the conversion, is performed as follows. Y' = a 11 Y - a 12 M - a 13 C M' = a 21 Y + a 22 M - a 23 C C' = a 31 Y - a 32 M + a 33 C However, according to this method, the relation of the output signal to the input signal is linear and it is impossible to correct the printer characteristic which nonlinearly changes in accordance with the amount of ink which has previously been printed. To eliminate the above drawback, a method whereby the nonlinear conversion is performed for the input signal has been also proposed. However, this method has the drawback that the apparatus required becomes complicated and costly. In German patent application DE-A-3408108 (equivalent to GB-A-2141001) there is disclosed an image processing apparatus which includes a masking processing unit for each colour (e.g. yellow, cyan and magenta) to be printed, each unit processing a respective colour component signal according to the other colour component signals and according to user established code data. The yellow component masking processing unit, for example, receives a 6-bit yellow component signal Y,4-bit cyan and magenta components signals M and C, and 4-bit user established code data signals SYY, SYM and SYC which address operational ROMs 150-1, 150-2 and 150-3. The resulting 6-bit data signal Dy and 4-bit correction signals Dm and Dc are fed to a further operational ROM 150-4, the output of which after being synchronised with the video transmission clock in latch element 150-5 comprises the corrected yellow colour data used for printing. The apparatus also includes a UCR processing circuit for producing black printing plates, shading correction circuits, gamma correction circuits, and dither and multi-value processing circuits. German patent application DE-A-3408108 (equivalent to GB-A-2141001) discloses a multi-colour image processing apparatus in which individual colours are sequentially printed and corrections are made to the subsequently printed colour to compensate to the fact that the subsequently printed colour is being printed on top of an already printed colour and not onto plain paper. However, in this prior specification the correction which is made is only concerned with gamma correction. SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a colour image processing method as set out in claim 1. According to a second aspect of the present invention there is provided a colour image processing apparatus as set out in claim 8. How the invention may be carried out will now be described by way of example and with reference to the accompanying drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an image processing block diagram of the first embodiment of an image recording apparatus according to the present invention; Figs. 2(A) to 2(D) are γ correction curves according to the invention, respectively; Fig. 3 is a γ characteristic diagram in the case of the mixed-color printing according to the invention; Fig. 4 is an image processing block diagram of the second embodiment of the invention; Fig. 5 is a diagram showing an arrangement of recording heads and the scanning direction; Fig. 6 is an image processing block diagram of a conventional image recording apparatus; Fig. 7 is a γ characteristic diagram in the case of the single-color printing; and Fig. 8 is a γ characteristic diagram in the case of the mixed-color printing in a conventional apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is an image signal processing block diagram of the first embodiment of an image recording apparatus according to the present invention. In the embodiment, it is assumed that the ink jet heads are arranged and scanned as shown in Fig. 5. Namely, the inks are printed in the order yellow, magenta, and cyan. In Fig. 1, reference numerals 13a, 13b and 13c denote digital input signals of yellow,magenta, and cyan each consisting of six bits. These signals are input from an image reading apparatus, an image data storage device, or the like (not shown) to a color processing section 14. The color processing section 14 performs the color processes such as a masking process and the like to those signals and outputs respective signals 15a, 15b, and 15c of yellow, magenta, and cyan after the color processes as six-bit digital data. The yellow signal 15a to be printed first is input as the address data of a gradation-correction ROM 16A. The value which is obtained by multiplying a γ coefficient to address value is stored in each address in the ROM 16A. The signal 15a is γ corrected on the basis of these values. Together with the magenta signal 15b, the yellow signal 15a is input as the address data to a gradation-correction ROM 16B for magenta. The values which are obtained by multiplying a γ coefficient to the magenta signal 15b are also stored in the gradation-correction ROM 16B. However, a plurality of kinds of γ correction are prepared as shown in Figs. 2(A) to 2(D) instead of only one kind. The optimum γ correction is selected by the yellow signal 15a. There is selected such a γ correction curve as to obtain the linear γ characteristic of magenta when the yellow signal has previously been printed. Thus, even if a print amount of yellow ink changed, the γ characteristic of magenta is always linear as shown in straight lines 11a to 11d in Fig. 3. The yellow signal 15a and magenta signal 15b together with the cyan signal 15c are also input to a gradation-correction ROM 16C. The optimum γ correction curve is selected by the signals 15a and 15b and the gradation is corrected. Among respective color signals 17a to 17c which were gradation corrected in this manner, the yellow signal is directly sent to a head 19A. However, the magenta signal 17b and cyan signal 17c are sent to buffers 18A and 18B and are delayed by the time corresponding to the distance between the heads and thereafter, they are supplied to heads 19B and 19C. The inks of respective colors are printed, so that a color image is reproduced. As described above, by performing the optimum γ correction in accordance with the size of image signal of the color ink which has previously been printed, even if the amount of ink to be previously printed changed, the linear γ characteristic is always obtained. Fig. 3 shows the situation in this case. When the γ characteristic changes as shown at 10a to 10d in Fig. 8 due to the amount of ink to be previously printed, by performing the γ correction as shown in Figs. 2(A) to 2(D), the linear γ characteristics are always derived as shown in the straight lines 11a to 11d in Fig. 3. As described above, when the γ characteristic of the output is linear, an image having a good gradation is obtained. The accurate color correction by the ordinary linear masking method can be performed. A color image can be reproduced with a good accuracy. The second embodiment will now be described. In the foregoing first embodiment, the image data to be previously printed is directly used as a selection signal to select the optimum γ correction curve. However, in the second embodiment, the image data to be previously printed is firstconverted into the data consisting of a smaller number of bits and thereafter,the converted data is used as the selection signal. Fig. 4 shows a schematic diagram in such a case. In Fig. 4, the same parts and components as those shown in Fig. 1 are designated by the same reference numerals. Among the signals processed by the color processing section 14, the yellow signal to be printed first is input to the gradation-correction ROM 16A and is also input to a converter ROM 20A and converted into a signal 21a consisting of a smaller number of bits. The number of bits of the signal 21a is determined in accordance with the number of γ correction curves to be prepared for the magenta and cyan signals. For example, even if the signal 15a consists of six bits and represents 64 gradations, there is no need to prepare 64 kinds of γ correction curves to be prepared for magenta and cyan. In general, it is sufficient to prepare about eight kinds γ correction curves. In this case, the number of bits of the signal 21a is set to 3. This 3-bit signal is used as the selection signal of the γ correction curve and input to the gradation-correction ROM for magenta, so that eight kinds of curves can be selected. The magenta signal 15b is also converted into a selection signal 21b consisting of a smaller number of bits by a converter ROM 20B. The γ correction curve for cyan is selected by the signals 21a and 21b. In the forgoing constitution, the number of bits of the selection signal is set to the necessary minimum number, so that there is an advantage such that it is sufficient that the memory capacity of the gradation correction ROM is small. As in the first embodiment, in the case of directly using the 6-bit image signal as the selection signal, the memory capacity of the γ correction ROM for magenta needs 2¹² x 6 = 24576 bits since the input signal consists of 12 bits and the output signal consists of 6 bits. The memory capacity of the ROM for cyan needs 2¹⁸ x 6 = 1572864 bits since the input signal consists of 18 bits and the output signal consists of 6 bits. However, in the second embodiment, since the 6-bit signal is converted into the 3-bit signal, the memory capacity of the gradation-correction ROM for magenta needs 2⁹ x 6 = 3072 bits since the input signal consists of 9 bits and the output signal consists of 6 bits. The memory capacity of the ROM for cyan needs 24576 bits since the input signal consists of 12 bits and the output signal consists of 6 bits. In addition to these bits, 2⁶ x 3 = 192 bits for the 6-bit input signal and 3-bit output singal are also necessary as a dividing ROM for yellow and magenta, respectively. However, as a whole capacity, the memory capacity of (24576 + 1572864) - (3072 + 24576 + 192 x 2) = 1569408 bits can be saved. On the other hand, in each of the foregoing embodiments, γ correction is used such that the γ coefficient is multiplied by the input signal and the resultant data is output. However, in a recording apparatus where the output gradations are derived by a dither method, by operating a threshold value of a dither matrix without changing a γ coefficient, a similar effect can be derived. For example, when the dither threshold values are set at regular intervals, if it is assumed that the γ characteristic as shown in Fig. 2(A) was obtained, by setting the interval between the dither threshold values to a wide value in the low density portion and to a narrow value in the high density portion, the γ characteristic as shown in Fig. 2(B), 2(C), or 2(D) can be obtained. Therefore, even by preparing a few kinds of dither matrices of magenta and cyan and by switching them by a selection signal, the present invention can also be obtained. Although in the foregoing embodiments, the number of kinds of inks has been set to three colors of yellow, magenta, and cyan the invention can be also similarly applied to the case where the inks of four colors of yellow, magenta, cyan and black are used. The printing order of the inks is not limited to the foregoing order of yellow γ magenta → cyan but can be arbitrarily set. In addition, although the invention has been described with respect to the example of ink jet recording, the invention is not limited to ink jet recording but can be also applied to printers such asa thermal copy transfer type printer, an electrophotographic printer, and the like in which each coloring agent is sequentially formed. As described above, in the recording apparatus where a color image is obtained by sequentially printing the inks of a plurality of colors, a good gradation characteristic and good color reproducibility can be obtained by changing the gradation correction in accordance with the amount of coloring agent which has previously been printed.

Claims: A colour image processing method comprising the steps of: inputting a plurality of first colour component signals (13) representing a color image; generating second colour component signals (15) by performing a colour masking process on the first colour component signals, the second colour component signals being used for recording the colour image by depositing a plurality of colouring agents; correcting the second colour component signals (15) so as to correct the quantity of the plurality of colouring agents to be deposited; and recording the colour image in response to the corrected second colour component signals (17) by depositing the plurality of colouring agents in a predetermined order of recording and in a superposed orientation; characterised in that, in said correcting step, each of the second colour component signals (15) is subjected to different gradation corrections, and that for each pixel the correction of each second colour component signal representing a colouring agent deposited after the first colouring agent in said predetermined order has been deposited is performed in accordance with one of a plurality of gradation correction curves, said one curve being selected in dependency upon the second colour component signals representing the colouring agents which have previously been deposited. A method as claimed in claim 1 comprising supplying the first color component signals from an image reading apparatus. A method as claimed in claim 1 or 2 in which the generating step utilises a linear masking circuit. A method as claimed in any previous claim in which the correcting step utilises a memory for storing a table of values. A method as claimed in any previous claim comprising the step of printing in response to a corrected second color component signal. A method as claimed in claim 5 in which the printing step utilises ink jet recording. A method as claimed in claim 6 in which the ink jet recording is performed by an ink jet device with multiple heads. A color image processing apparatus comprising: means for inputting a plurality of first colour component signals (13) representing a colour image; means (14) for generating second colour component signals (15) by performing a colour masking process on the first colour component signals, the second colour component signals being used for recording the colour image by depositing a plurality of colouring agents; means (16) for correcting the second colour component signals so as to correct the quantity of the plurality of colouring agents to be deposited; and means (19) for recording the colour image in response to the corrected second colour component signals (17) by depositing the plurality of colouring agents in a predetermined order of recording and in a superposed orientation; characterised in that, the correcting means (16) is adapted to operate in such a way that each of the second colour component signals is subjected to different gradation corrections so that for each pixel the correction of each second colour component signal representing a colouring agent deposited after the first colouring agent in said predetermined order has been deposited is performed in accordance with one of a plurality of gradation correction curves, said one curve being selected in dependency upon the second colour component signals representing the colouring agents which have previously been deposited.

Inventors: OHKUBO MASAHARU, SUZUKI AKIO, TAKADA YOSHIHIRO, OHKUBO, MASAHARU, SUZUKI, AKIO, TAKADA, YOSHIHIRO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: H04N1
IPC codes: H04N1, B41J2