JPH0365066B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a recording method in an inkjet recording apparatus, and provides a new signal processing method that improves the halftone reproduction characteristics of grayscale images.

There are various types of nozzles used in inkjet recording devices, and the ability to express light and shade varies depending on the characteristics of the nozzle and the recording method, but it is generally difficult to express low density areas as continuous tones. For example, if we take a piezo-excited nozzle called on-demand type as an example, the relationship between the amplitude of the input signal voltage and the optical density of the ejected ink droplets on the recording paper surface is as shown in Figure 1. have. That is, the ejection of droplet particles occurs at a certain threshold voltage.
Since this is performed for inputs greater than V _L and the particle size has a finite size, the concentration region below D _L in FIG. 1 is not reproduced and becomes discontinuous. Therefore, conventionally, the recording of grayscale images with halftones has been done using D _L
This was carried out in the density range of ~D _H , and only a dark screen was reproduced overall.

The present invention compensates for the above-mentioned density region below _DL through simple processing, and can particularly improve the expressive characteristics of the highlight portion of an image. A detailed explanation will be given below using the drawings.

Figure 2 shows the principle of halftone expression in the present invention, and shows the density level D _i of a certain picture element.
For example, let it be expressed by a combination of two different density values D _M and D _n . Since the image is distributed on a two-dimensional plane, in this case, when viewed macroscopically, the original density D _i is approximated in units of 2×2=4 picture elements on average.

Now, if the average density of two picture elements with density values D _M and D _n is D _i ′, and D _i ′ is obtained as the average reflectance of D _M and D _n , then D _i ′=− log ₁₀ [1/2 (10 ^-DM +10 ^-Dm )] (1) The following relationship holds true.

Here, if D _M >D _n ≧0, clearly D _M >D _i ′>D _n ≧0 (2) holds true.

Therefore, let us consider conditions such that the target D _i ' is in the range D _L > D _i '>0 and reproduces the density between 0 and D _L in FIG. 1. Since D _M and D _n follow the characteristics shown in Figure 1 and D _L > D _n , D _n
Naturally, the possible value of D _n =0. However, D _n =
0 means a blank state with no printing, and the base density of the recording paper is assumed to be 0. Substituting this condition into equation (1) yields D _M =−log ₁₀ (2×10 ^−Di ′−1) (3), and D _M is obtained for the target D _i ′. However, since the condition D _M ≧ D _L is imposed,
Letting D _K be the lower limit of D _i ′ reproduced by equation (3), D _i ′≧D _K =−log ₁₀ [1/2 (10 ^−DL +1)] (4).

To summarize the above, by setting D _n to 0 and defining D _M as shown in equation (3), D _i ' can be reproduced to a desired density value in the range D _L ≧D _i ′≧D _K. FIG. 3 illustrates a relationship in which the reproduced density D _i ' with respect to the input density D _i is a straight line within the above range, that is, D _i '=D _i . In the input range where D _i is less than or equal to D _K , (4)
According to the formula, the limiter characteristic is D _i '=D _K , but considering the error from the straight line, D _i '=0 may be set below an appropriate level as shown in the figure.

Next, signal processing for performing the conversion as shown in FIG. 2 will be explained. On the recorded image plane, the matrix shown in Figure 2 is arranged vertically and horizontally, so D _M and
The light and shade dots for D _n are connected in a direction of ±45° to form a pattern similar to a mesh pattern in printing plate making. Such a mesh is said to be suitable for visually reproducing smooth halftones on a two-dimensional surface. Figure 4a shows the arrangement of light and shade dots, and figures b to e show the signal processing method. b
As an example, the stepwise input signal D _i , c indicates a clock having twice the period of the picture element, and d and e indicate the processing results for b. In the section indicated by B or C in b, the input concentration D _i is in the range D _L > D _i > D _K , so according to the concentration curve in Fig. 3 above,
Concentration modulation of ΔD ₊ =D _M -D _i in the positive direction and ΔD _- =D _i -D _n in the negative direction is applied to D _i . Modulation is performed so that the positive and negative signals alternate every half period of the clock c, but since D _n =0 in the negative direction, the input may actually be held at 0. d corresponds to the odd-numbered scanning line of a, and e corresponds to the even-numbered scanning line, and the clock phase is inverted by 180° for each scanning line, resulting in a modulation signal. By this operation, a mesh-like shading dot array a can be created.
Modulation of ΔD ₊ in the positive direction can be achieved by, for example, superimposing c with sufficient amplitude on b and then applying a limiter so that the envelope of the positive amplitude follows the curve _DM in Figure 3. It can be realized. In section A of FIG. 4b, since D _K >D _i, the amount of modulation in the positive direction is ΔD ₊ =D _L -D _i , and in section D, since D _i >D _L , no modulation is performed.

Note that a specific device for realizing the image recording method of the present invention will be described below.

FIG. 7 is a block diagram of main parts of an image recording apparatus for realizing the image recording method of the present invention.

In FIG. 7, reference numeral 71 denotes a determiner consisting of a comparator, which compares the input density value D _i with the values of D _L and D _K. 72 is a predetermined value (for example, the fourth
This is an arithmetic unit that performs calculations on whether to add to the input density value D _i or to set the input density value D _i to zero using the clock (as shown in FIG. 3c) having sufficient amplitude. A limiter 73 limits the value of the output 72a of the arithmetic unit 72 based on the information of the judgment result 71a of the judgment unit 71. 74 generates timing pulses as shown in FIG.
1, a control unit that controls the operations of the arithmetic unit 72 and the limiter 73;

The operation of the above configuration will be explained below.

First, the determiner 71 compares the input density value D _i with D _L and D _K , respectively, and determines the magnitude relationship between them (A) to (C).
Judgment is made as follows.

(A)...D _i >D _L (B)...D _L >D _i >D _K (C)...D _K >D _i Next, the arithmetic unit 72 and limiter 73 perform the judgment of the judge Each concentration modulation is performed based on the information of the result 71a.

(A) In the case of D _i > D _L (in the case of section D in Fig. 4b) The arithmetic unit 72 and limiter 73 output the input density value D _i as it is as the output density 73a without adding any density modulation. do. That is, based on the information of the judgment result 71a of the judgment unit 71, the calculation unit 72 does not perform any calculation on the input concentration D _i . Further, the limiter 73 also outputs the input density value D _i as an output density 73a without imposing any restrictions on the input density value D _i based on the information of the judgment result 71a of the judger 71.

(B) In the case of D _L > D _i > D _K (in the case of sections B and C in FIG. 4b)
In accordance with _the timing pulse _shown in FIG. The limiter 73 modulates the density by applying a limit along the curve _DM in FIG.

When the density modulation (ΔD) is expressed by the arithmetic unit 72 and limiter 73 using the following formulas, ΔD ₊ =D _M −D _i (positive direction) ΔD ₋ =D _i −D _n (negative direction).

(C) In the case of D _K > D _i (in the case of section A in FIG. 4b) In the arithmetic unit 72, the fourth
In accordance with _the timing pulse _shown in FIG. The limiter 73 modulates the concentration by applying a limit along the curve _DK in FIG.

The concentration modulation (ΔD) by the calculator 72 and the limiter 73 is expressed by the following equations: ΔD ₊ =D _K −D _i (positive direction) ΔD ₋ =D _i −D _n (negative direction).

As described above, by performing the conversion processing as shown in (A) to (C) based on the information of the judgment result 71a of the judgment unit 71, it becomes possible to convert the density with the characteristics shown in FIG.

In the above embodiment, as shown in FIG. 3, although the concentration range of D _L > D _i > D _K is improved, it is impossible to reproduce the concentration below D _K. In order to further expand the playback range, as shown in Figure 5, the brighter the screen, the more the _DM
It is sufficient to limit the amount of D _n mixed. 6th
The figure is an example showing to what extent the lower limit value D _K of the reproduced density can be lowered with respect to _DL with respect to FIGS. 2 and 5 a and b. In principle, this method can cover the entire low-density area, but as a matter of course, image sharpness is sacrificed in the area to which this process is applied. However, since there is generally little edge information that requires sharpness in bright areas of an image, the present invention has a practically sufficient effect of improving image quality.

As described above, the present invention enables a stepwise change in density when looking macroscopically at a portion below the threshold in a recording device with a limited range of gradation expression in which the density sacrifice of the output relative to the input has a threshold. This provides a method for recording grayscale images, which can expand the range of grayscale expression and greatly improve image quality.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the characteristics of an on-demand inkjet recording device with a discontinuous recording density range, FIG. 2 is a diagram explaining the principle of the present invention, FIG. Figures a to e are diagrams showing the signal processing method of the present invention, Figures 5a and b are diagrams showing other embodiments of the present invention, and Figure 6 is a numerical sequence showing the effect of improving the reproduction range according to the present invention. FIG. 7 is a specific circuit diagram for realizing the image recording method of the present invention.

Claims (1)

[Claims] 1. Using a recording device in which the density expression range of recording with respect to the density range of input information is limited in a highlight part, in order to record the highlight part, the density signal in the input information is In an image recording method that converts and provides an output density signal to the recording device, an image of n×m (with the exception of , n, and m are positive numbers, and n×m is an integer of 2 or more) The density of the target picture element before density conversion in the plural picture element blocks, and the average density of the plural picture element blocks after the density conversion. An image recording method characterized in that positive or negative density modulation is applied to the density signal of the pixel of interest so that the density signal of the pixel of interest is approximated to obtain the density signal of each pixel in the plurality of pixel blocks. 2. The image recording method according to claim 1, wherein density modulation is performed in synchronization with a pixel clock. 3. The image recording method according to claim 1, wherein the positive and negative polarities of the density modulation are reversed every certain number of picture elements.