JPH0374547B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a color image forming apparatus that forms color images using at least cyan, magenta, and yellow coloring materials.

The following will explain the case of an inkjet printer as an example, but it can also be applied to image forming apparatuses of other types such as a thermal type and an electrophotographic type.

<Prior Art> Conventionally, this type of inkjet printer uses cyan, magenta, yellow, and black inks to eject each color of ink from a nozzle for each pixel to form a color image on a paper image. recent years,
It has been proposed to print photographic images using such prints. Photography subjects can include landscapes, buildings, etc., but especially when personal use is considered, portraits are always the most common. Therefore, how well a printer can reproduce beautiful skin tones is an important criterion for evaluating printers. Beautiful skin color cannot be clearly defined, but
Generally, the OD value of cyan is much smaller than the optical reflection density (hereinafter referred to as OD value) of magenta and yellow. In one measurement example, the density of magenta
0.30, yellow density is 0.25, cyan density is
It was 0.06.

When attempting to reproduce such skin tones with a conventional inkjet printer, the corresponding OD values for magenta and yellow can actually be reproduced, but for cyan, the density will be lower than the minimum OD value that can be reproduced. By modulating, that is, as shown in Figure 1, the density of the dots is reduced,
Achieves an apparently low OD value.

Therefore, as in this example, the lowest OD value of cyan is
In the case of 0.27, the apparent OD value of cyan, which is almost a skin color, is adjusted by density modulation as shown in Figure 1, so the actual size of one pixel of cyan becomes large. This results in cyan ink being applied to the skin in some places on the actual image, and this roughness significantly degrades the image quality.

<Object of the Invention> In view of the above-mentioned problems, the present invention aims to provide a color image forming apparatus that is good in reproducing skin colors.

Embodiments of the present invention will be described in detail below with reference to the drawings.

<Description of Examples> FIG. 2 shows the relationship between the OD values of the cyan, magenta, yellow, and black inks used and the head drive voltage.

As shown here, magenta, yellow, and black each use one density ink;
For cyan, the lowest OD value was 0.08 using inks with dye concentrations of 3% and 0.4%. In this way, 0.08 to 0.27 of the area that conventionally relied on density modulation for areas below 0.27 can now be reproduced with one dot per pixel.

Also, since cyan uses two densities, it is possible to use a threshold L to print out dark ink when the cyan OD value is greater than L, and light ink when it is smaller than L. . Here L=
It is set at 0.30. Furthermore, when density modulation is performed using a 2×2 dither in the cyan region corresponding to 0 to 0.08 as shown in FIG. 3a, the minimum OD value is as small as 0.08, so that the roughness is hardly noticeable. Also, the minimum OD value for magenta and yellow is
Since it is 0.18, a 3×3 dither matrix is used to represent 0 to 0.18. Furthermore, since black expresses a high density area, only the area above a certain OD value is used, so there is no need to particularly perform density modulation on the area below the lowest OD value.

Figures 3a, b, and c show how to reproduce the areas below the lowest OD value for cyan, magenta, and yellow, respectively.

FIG. 4 is a control block diagram of an inkjet printer according to an embodiment of the present invention.

A video signal of an image, for example, a composite video signal including R, G, and B color signals and a synchronization signal is input to the video signal interface 41. Here, the signals are synchronized and sampled and held by the sample and hold circuit. This signal is guided to the AD conversion circuit 42, and the image signals R, G,
The B gradation signal is converted into a digital signal. This digital signal is stored in an appropriate number of lines in the next line memory 43. Here, the line is generally taken in the vertical direction, but it is clear that it is also good to take it in the horizontal direction. Next, the data in this line memory is processed by an image processing circuit for each pixel, such as color conversion, γ conversion, masking processing, undercolor removal, etc. Generally, cyan, magenta,
The signals are converted into yellow and black signals, and further converted into voltage values applied to each head and input to the head driver 46.

The ink jet head 49 ejects an amount of ink according to the applied voltage, and the hue and density are expressed by the amount of ink of each color.

On the other hand, the system controller 45, which controls the sequence of the printer, generates a head drive signal, a carriage motor drive signal, and a paper feed signal at timings corresponding to the input image signal. The signal is supplied to the feed motor driver 48, and the inkjet head 49, carriage motor and its mechanism 410, and paper feed motor and its mechanism 11 are controlled at the desired timing, thereby printing the reproduced image of the input video signal on the recording medium.

FIG. 5 is a block diagram of the image processing circuit 44. The R, G, B color separation signals corresponding to a certain pixel are
It is converted into CMY by the CMY converter, and γ-converted by the γ converter 52.

Next, the UCR 53 detects black components from C, M, and Y, and determines BK. Then, BK is output only when BK is greater than a certain value, and C,
Subtract BK from M and Y to find C, M, and Y. Thereafter, the masking section 54 performs various corrections. A comparator 55 determines whether the cyan density C is larger or smaller than a threshold L for switching between dark ink and light ink, and when the cyan density C is smaller than the threshold L, a high level "H" is output as the cyan control signal CC.

Further, the cyan density C is input to the dither generation circuit 61 and the comparator 58. Comparator 58
is compared with the output of the preset circuit 64, and when the output of the preset circuit 64 is larger, the dither generation circuit 61 is enabled. Similarly, the same applies to the magenta density signal M and the yellow density signal Y.

Here, the output level of the preset circuits 64 and 65 corresponds to the lowest OD value when reproduced with one pixel and one dot.

Therefore, the digital output level of preset circuit 64 corresponds to the lowest cyan OD value of 0.08, and
A digital output level of 5 corresponds to the lowest OD value of 0.18 for magenta and yellow. If C, M, and Y are less than or equal to the minimum OD values L _C , L _M , and L _Y , C,
Compare the values of M and Y with the values of each cell in the dither matrix of each dither generation circuit, and if the values of C, M, and Y are larger, switch the terminals of switches 66, 67, and 68 to the a side. Connecting. Also, if it is smaller, connect it to the b side. When connected to the a side, the output voltage of the preset circuit 57, that is, the digital value corresponding to representing the minimum dot of each ink is output to the DA converter 5.
6, and the respective lowest OD values are expressed on the recording paper. Also, when connected to the b side, C, M, Y
Although each value is directly transmitted to the DA converter 56,
When connected to the b side, the values of C, M, and Y are necessarily insufficient to express the smallest dot, so they are not recorded on the recording paper.

Moreover, each comparator 58, 59, 60 is C,
If it is determined that the values of M and Y are larger than the digital value equivalent to the lowest OD value, each dither generation circuit 6
1, 62, and 63 are in a disable state, and switches 66, 67, and 68 are connected to the terminal b side.

At this time, the values of C, M, and Y are larger than the digital value equivalent to the lowest OD value, so the DA converter 56 converts C,
Analog voltages corresponding to the values of M and Y are output to the head driver 46, and one dot of a size corresponding to the values of C, M and Y is formed for each pixel. At this time, there are two types of heads for ejecting cyan ink, dark and light, so when the cyan control signal is "H", the head for light ink is selected, and when it is "L", the head for dark ink is selected. .

By the way, the minimum OD value here is 1 per pixel.
It refers to the smallest OD value achieved when dots are struck, and does not include OD values achieved by density modulation, dither methods, etc.

Incidentally, the components of the dither matrix are selected by the main scanning direction address X and the sub-scanning direction address Y.

In other words, magenta and yellow use a 3x3 dither matrix, so the address (X,
The comparison thresholds for Y) are _{M M} (mod ₃ X, mod _{3 Y} ), M _Y (mod ₃ X, mod ₃ Y) for each matrix M M , M Y
It can be expressed as Cyan uses 2×2 dither matrix Mc, so Mc(mod ₂ X, mod ₂ Y)
It is. FIG. 6 shows the dither matrix used for cyan, magenta, and yellow.

In this embodiment, the invention was achieved using a hardware circuit, but it is also possible to implement the invention using software using a computer. Also, RGB
This can also be realized using a table that stores YMC output digital data for digital inputs. As the values in the table, values obtained by simulation using the circuit of this embodiment may be stored.

In addition, in this example, by using ink with a light ink density for cyan ink, the lowest density can be achieved.
Although the OD value is lower than that of other magenta and yellow inks, this can also be achieved by making the minimum drive voltage of the magenta and yellow ink heads higher than the minimum drive voltage of the cyan ink heads, as shown in Figure 7. can. This can also be achieved by making the ejection orifice diameter of the cyan ink head smaller than that of the other heads.

As for the cyan ink, instead of using two types of ink, dark and light, one type of ink having a lower dye concentration than the yellow and magenta inks may be used.

Also, although this embodiment has been explained using an inkjet printer as an example, it is possible to reduce the density of cyan toner,
It can also be applied to electrophotographic printers by reducing the size of the cyan-forming light beam. Furthermore, in a thermal printer, this can be achieved by making the density of the cyan ink ribbon thinner than the density of other ink ribbons.

The present invention is applicable to all printers that can control the density of one dot in this way.

<Description of Effects> According to the present invention, by lowering the minimum OD value of cyan, the cyan component in skin color can be expressed without reducing its resolution, so an image forming apparatus with good skin color reproduction can be realized. Therefore, image reproduction suitable for reproducing human images is possible.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the dot pattern by density modulation, Fig. 2 is a diagram showing the relationship between the OD value of cyan, magenta, yellow, and black ink used in this example and the head drive voltage, and Fig. 3 a, b, c are diagrams showing the dot patterns of cyan, magenta, and yellow inks, FIG. 4 is a control block diagram of the inkjet printer, FIG. 5 is a detailed block diagram of the image processing circuit 44 in FIG. 4, and FIG. 7 is a diagram showing the dither matrix of each ink, and FIG. 7 is a diagram showing the relationship between ink density and head drive voltage in another embodiment. In the figure, 54 is a masking circuit, 55, 5
8, 59, 60 are comparators, 61, 62, 6
3 indicates a dither generating circuit, respectively.

Claims (1)

[Claims] 1. In a color image forming apparatus that forms color images using at least cyan, magenta, and yellow coloring materials, means for forming dots of each of the cyan, magenta, and yellow coloring materials, and a means for forming dots of each of the coloring materials within a predetermined area of each color dot. means for varying the optical reflection density expressed by , respectively, within a limited range, and in order to make the minimum value of the optical reflection density of cyan smaller than that of magenta and yellow, 1. A color image forming apparatus characterized in that the density of a coloring material or the size of a dot that defines cyan is smaller than that of magenta or yellow.