JPS643108B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an image recording method that enables good intermediate expression in an image recording apparatus in which the density characteristic of the output with respect to the input has a threshold value that does not allow gradation expression in a low density region. There are two typical methods for expressing the halftone density of an image: a method in which the density is continuously changed as in photography, and a method in which the size of halftone dots is changed as in printing. Another method is to change the number of particles per unit area, and this method can be said to be effective for inkjet recording, where printing with small particles is not possible. The characteristics shown in FIG. 1 show the recording density characteristics with respect to the applied voltage of the recording signal in inkjet recording. In the case of an inkjet, the minimum and maximum diameters of the ejected ink particles are limited, so if the recording density on the recording surface is determined so that printing can be done at the maximum density at the maximum diameter, the recording density at the minimum diameter will be cannot be lowered. That is, as shown in the figure, the range in which the density cannot be reproduced becomes larger in bright areas, which significantly deteriorates the quality of the recorded image. In order to reproduce this bright area, a method is used to thin out the ink particles and lower their average density. Figure 2 shows its characteristics. This expresses one pixel information as a 4×4 matrix. The size of the 1×1 matrix corresponds to the size recorded by one ink droplet at the recording density of the maximum density. The drawback of this method is that the pixel information density on a unit recording surface becomes small. In the example shown in Figure 2, it has been reduced to 1/16. The present invention provides a method for suppressing the decrease in pixel information density on a unit recording surface as much as possible and reproducing the density of bright areas satisfactorily. In the method shown in Figure 2, all recording density levels are
4, but at high density levels it is also possible to express with a smaller matrix size. The basic idea of the present invention is to increase the average pixel information density on a unit recording surface by expressing the information with the smallest possible matrix size as long as the conditions for reproducing the same density level are met. An example is shown in FIG. In the example shown in the figure, the recording density is set to 16 levels of equal density difference, and the signal steps giving each density level are 16 steps from 0 to 15. Step 0 is the base density of the recording surface. Steps 7 to 15 give each concentration level only by changing the voltage, and the matrix size is all 1.
It is the minimum unit of ×1. In steps 1 to 6, each density level is given by a change in voltage and a change in matrix size, and the matrix size tends to increase toward the brighter part. The larger the matrix size, the smaller the pixel information density becomes, as shown by the dotted line. Assuming that each density level occurs with equal probability in one screen, the average image information density will be as shown by the dashed line.
Considering that generally parts of an image that require high resolution, such as outlines and characters, are often found in high-density areas (dark areas), it can be said that the present invention provides a method that is suitable for the actual situation. Next, a specific example for realizing the present invention will be described. Let us now assume that the highest recordable density is D _H and the lowest density (in this case, the density of the recording surface) is D _L , and the area between them is divided into N density levels with equal density differences. At that time, the concentration D _K of the Kth (K is 0 to N-1) is 1
It is expressed by the formula. D _K =KD _H -D _L /N-1+D _L ... (1) If D _H = 1, D _L = 0.1, and N = 16, D _K will be as shown in Table 1.

【table】

[Table] Among these, D ₇ to D ₁₅ can be expressed by one ink droplet. Next, the matrix size for expressing D ₁ to _{D 6} and the method of selecting each concentration in the matrix from D ₀ and D ₇ to _{D 15} will be explained. (In this example, in principle, one concentration in the matrix is considered to be 10 types from D ₀ and D ₇ to D ₁₅ , so D ₀ to
It is difficult to set the density difference up to _D6 to be completely equal due to errors. To reduce this error, D ₇ ~
It would be better to be able to select density types up to _D15 continuously. ) In Table 1, the light reflectance R〓 of each of D ₇ to D ₁₅ (α is 7 to 15) is R〓 = 1/10 ^D 〓 ...(2) The light reflectance of each of D ₁ to _{D 6} is Light reflectance R〓 (β is 1 to 6)
is R = 1/10 ^D = ... (3) The light reflectance R ₀ of D ₀ is R ₀ = 1/10 ^Do ... (4). Here, consider R〓 to be expressed as a combination of one R〓 and m pieces of R ₀ , and let the light reflectance obtained from this combination be R〓′, then R〓′=R〓+mR ₀ / m+1=1/10 ^D 〓+m/10 ^Do /
m+1...(5) In other words, 5 with the value closest to R〓 obtained by formula 3
Obtaining R〓' in the equation (m+1) is the matrix size, and D〓 is the density of one ink droplet entering the matrix. Table 2 shows the values of R〓' obtained by changing the values of m and D〓 in equation (5).

[Table] Table 3 shows the values of the density D〓 from D ₁ to _{D 6} , the value of its reflectance R〓 obtained from equation (3), and the value of R〓 close to R〓. The value obtained from Table 2 (marked with *), the value obtained from equation (6) for the concentration D〓′ for R〓′, and
This is a summary of m and D〓 when D〓′ is selected. D〓′＝−logR〓′……(6)

[Table] In Table 3, D〓″ for D ₅ and D ₆ is not determined. This is because the lower limit of the density value that can be expressed by one ink droplet in this example is too high. The present invention In this case, the following special provisions are provided.In other words, the concentrations of D ₅ and D ₆ are
Let us express it with one D ₀ and two D 〓 (therefore, the matrix size is 3). This D〓 is D〓 ₁ , D〓 ₂
(The reflectances are R〓 ₁ and R〓 ₂ respectively.) If the density value expressed by this is D〓'' and the reflectance is R〓'', then R〓''=R ₀ +R〓 ₁ +R〓 ₂ / 3=1/10 ^Do +1/10 ^D 〓 ¹
+1/10 ^D 〓 ^2/3
...(7) D〓''=-logR〓'' ...(8) Table 4 shows the calculation results of equation (7).

【table】

[Table] The * mark is R〓″ which gives D ₅ and D _6. D〓, R〓, R〓 _″ , D〓″ regarding D ₅ and D 6 and D〓 ₁ at that time,
D〓 ₂ is shown in Table 5.

[Table] Figure 4 corresponds to Figure 3 above.
It is a figure expressed based on Table, Table 3, and Table 5.
Steps 7 to 15 are D〓 (matrix size is 1), steps 5 and 6 are D〓'' (matrix size is 3), steps 3 and 4 (matrix size 2), step 2 (matrix size 3) and step 1. (matrix size 6) is D〓', and step ₀ is the base concentration level D 0.Next, Fig. 5 shows an example of the matrix shape and concentration arrangement of each step. The double frame indicates the current position of the recording signal on the recording surface, and the other elements in the matrix are set in the future direction in terms of time. Some conventions are necessary when actually recording in the arrangement shown in Figure 5. For example, if step 11 occurs after step 3 in the signal train, the position of D ₀ of step 3 and the position of D ₁₁ of step 11 will be the same, so at what density will this same position be recorded? Now, with reference to Figures 6 and 7, we will explain the specific rules (principles) when using the concentration array shown in Figure 5. Promise 1> *When the density at the position where processing is currently being performed has been designated by the density array that has already been processed, <Promise> for determining the density at that position. For example, as shown in A in Figure 6. , when the concentration at position X ₀ is step 3, the concentration at position _{X 0 becomes} D ₈ , while _the concentration at _position Processing is determined by the following conventions: Concentration D _x1 at position X ₁ where processing is currently being performed.
When is a density that can be determined with one matrix size, that is, when it is the density D _x1 of steps 8 to 15 as shown by B in FIG. 6, the step density D x1 is one step lower than that step density D _{x1 . -1} is determined to be the concentration at the position _X1 . For example, step 15 becomes step 14, and step 14 becomes step 13. (However, step 7 follows the process ``'' below.) Density D _x1 of position X ₁ where processing is currently being performed
When is a density determined by multiple matrix sizes (including the density in step 7),
That is, when the density D _x1 of steps 1 to 7 is as shown by C in FIG. 6, the step density D _x1 is ignored and the density at the position X ₁ is
D _x1 follows the density specification processed at the already processed position X ₀ . <Promise 2> *The density array of the position to be processed now specifies the density of the position to be processed in the future, and the density array of the position that has already been processed specifies the density.
When the density of the position to be processed in the future has been specified, <promises> for determining the density of the position to be processed in the future. For example, as shown in A in FIG. 7, the density at position X ₀ is determined in step 1, _the position
The density at the positions _{X 1 ,} X ₂ . . . is also designated as D ₀ (B in FIG. 7). Next _, when processing the density at position X ₁ , if the density at position _{X 1} is at step ₆ as shown in FIG. Due to <Promise 1>-, the concentration is actually
D becomes ₀ . Now, the concentration at position X ₂ is as shown in D in Figure 7.
Following the density designation by processing at position X ₀ , density designation is also received by processing at position X ₁ . In such a case,
Step 12, which is one step lower than step 13 whose density is specified by the position X ₁ , is set to the position X ₂
Specify the concentration of (In _addition , _position The density processing result up to ₁ is as shown in E of FIG. Note that the * mark in E of FIG. 7 indicates a position for which density has been specified in advance (a position for which density determination processing has not been performed). The above <Promise 1> and <Promise 2> can be briefly summarized as follows. In other words, <Promise 1> When the current recording position on the recording surface has a density specified by a past signal sequence, if the signal to be recorded is below the lowest density level in the high density area, the signal will not be recorded. ,
When the density level is higher than the minimum density level, the signal is recorded at a density level one step lower than the signal. <Promise 2> If a future density specified position by a signal to be currently recorded on the recording surface has already been specified by a past signal train, then the density level at that future position by the current signal is the base density level. If the density level is a density level other than the base density level, the density level is one step lower than the density level. As described above, in the matrix pattern of FIG. 5 of this embodiment, actual image recording can be performed with the above <provision 1> and <provision 2>. Further definition may be required depending on the shape of the pattern. FIG. 8a shows an example of a pattern expressed in step values. Consider this to be the data value of the original image. The pattern shown in FIG. 2B is obtained by converting the data of the pattern a as described above. Since the pattern b has all step values of 7 or more except for the step value 0, it can be recorded using an ink jet or the like according to the step values. A place marked with a ■ mark in the upper left of the square b indicates that the concentration was specified by a past signal string. Since it is difficult to understand the shape of the pattern using the numerical values shown in Figure 6, I will try redrawing it by giving it a visual density change. To express the density of D _K in Table 1, the area ratio b of unit area is black density 1.0, and (1-b) is white density.
Assuming that it is 0.1, D _K can be expressed as in Equation 9. D _K = -log (1 - b / 10 ^0.1 + b / 10 ¹ )... (9) Insert the values in Table 1 into Equation 9, find the black area b, and create a pattern using the value of b. Record. Hereinafter, a specific configuration for realizing an image recording method according to an embodiment of the present invention will be briefly described. FIG. 9 is a block diagram of an apparatus for realizing an image recording method according to an embodiment of the present invention.
In the figure, 1 is an input terminal for an input image signal, 2 is a current position density determining circuit that performs signal processing according to <Provision 1>, and 3 is a future position density setting circuit that performs signal processing according to <Promise 2>. The circuit 4 is an output image signal output terminal. Note that the operation of the configuration shown in FIG. 9 is clear from <Promise 1> and <Promise 2>, so a description thereof will be omitted. Also,
The signal processing of <Promise 1> and <Promise 2> above can be configured by logical design of ICs, LSIs, etc. using existing technology.
Nowadays, it can be easily realized in software using a microcomputer. As described above, the present invention provides an image recording method for an image recording apparatus in which the density characteristic of the output with respect to the input has a threshold value that makes it impossible to express gradation in a low density region.
By recording the shading of the high density area above the threshold value by changing the voltage, and recording the shading of the low density area below the threshold value by a combination of the density level of the high density area and the base density level, Even in inkjet recording devices that cannot express bright areas, for example, it is now possible to easily express bright areas, and this has great industrial value.

[Brief explanation of drawings]

Fig. 1 is a recording density characteristic diagram with respect to the applied voltage of a recording signal in inkjet recording, Fig. 2 is a recording density characteristic-information density characteristic diagram in a constant size matrix recording method, and Fig. 3 is an image in an embodiment of the present invention. A conceptual diagram of the recording method, FIG. 4 is a recording characteristic-information density characteristic diagram based on specific numerical values of the same image recording method, FIG. 5 is a conceptual diagram showing a specific density arrangement of the same image recording method, and FIG. , Figure 7 is a conceptual diagram for explaining the conventions of the image recording method, and Figure 8 is a conceptual diagram for explaining the conventions of the image recording method.
The figure is a conceptual diagram of an image processed according to the same convention.
FIG. 9 is a block diagram of main parts of an apparatus for realizing the image recording method. 2...Future position concentration setting circuit, 3...Current position concentration determining circuit.

Claims (1)

[Scope of Claims] 1. In an image recording method for an image recording device in which the density characteristic of the output with respect to the input has a threshold value that makes it impossible to express shading in a low density area, the shading in a high density area above the threshold value is changed to each density level by a voltage change. An image recording method characterized in that the shading of the low density area lower than the threshold value is recorded by a combination of the density level of the high density area and the base density level.