JPH0147946B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an image gradation reproduction method using a dither method. Among the image gradation expression methods, a dither method is used to achieve this using only digital information. This dither method uses a dither matrix, as will be described later, and the gradation expression differs depending on the level at which this matrix is set. Therefore, this level setting is an important point in gradation expression.
Furthermore, if the background color level of the original can be set, it is possible to erase the background stains on the original and also scan the original with beautiful gradation in the image. First, a brief description of dither will be given. Digital information itself cannot produce shading, but if black dots are densely gathered, it will appear black, and conversely, as the black dots become sparse, it will move from gray to white. In other words, if you look at it from a close distance, it looks like just a collection of black dots, but if you look at it a little further away, it looks gray. In other words, the human eye is fooled. This condensed dot method is shown in FIG. 1 and is called a dither matrix. In the illustrated block, 2×2=4 cells constitute one block, and gradation is expressed depending on which cell is set to “ON” (black). Next, when trying to express gradations using this dither matrix, as shown in Figure 1, each cell A is numbered as shown, and a threshold value is assigned to each cell. If a value is set, and if the pixel level exceeds this threshold, that cell will be expressed as black, then for example, as shown in Figure 2, gradations from white to black will be expressed. Can be done. As mentioned above, the dither method is effective for expressing gradation in digital processing, but it is difficult to set the threshold value for each cell, and when trying to express gradation, the threshold value has to be changed depending on the original. There must be. The present invention takes the above points into consideration and attempts to set an optimal combination of threshold values for the dither method. For this purpose, the present invention provides an image gradation reproduction method that uses a dither matrix to perform dither processing on an original image to reproduce a gradation image. A determination means for determining the density of the ground color of a document by comparing it with a plurality of comparators having a The present invention provides an image gradation reproduction method characterized in that one of the plurality of types of dither matrices is selected based on the number of dither matrices. Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. FIG. 3 is a circuit diagram showing an example of a device implementing the present invention, in which R is a resistor, Q is a comparator,
DE indicates decoder. According to this embodiment, first, the original is scanned in order to detect the background color of the original. This reference level is applied to the input Vin in Figure 3, which is connected to comparator Q1.
-Distributed to Q4. Also, Vo is the resistance R1 to R
4, each comparator Q
1 to Q4, and the reference level and resistance division level are compared. This resistance division level is used to determine the density of the scanned reference document. If the standard manuscript is pale,
Comparator Q1 becomes "ON" and output D1 of comparator Q1 becomes "0". Similarly, when the reference original is a little dark, the comparator Q2 turns "ON" and the output D2 of the comparator Q2 also becomes "0". Therefore, comparators Q3, Q4
Considering this, the following five possibilities can be considered.

[Table] Next, when outputs D1 to D4 take such values, a decoder with the truth table shown in Figure 4 is used.
When this is input to DE, the output of this decoder DE becomes Y1 to Y5. Any one of Y1 to Y5 becomes "0", meaning that it is selected.
For example, D1=0, D2=0, D3=1, D4=1
In this case, Y3 is selected. Here, five types of dither matrices are prepared in advance, and one is selected according to the reference document. For example, as shown in FIG. 5 or 6, five types of dither matrices are prepared,
Each is compared with Y1 to Y5. In the figure, the cell numbers represent thresholds. That is, if the background color of the document is white and light, the threshold value around the dither matrix is lowered, and if the background color of the document is dark, the threshold value around the dither matrix is raised. are doing. With this configuration, the threshold value around the dither matrix changes depending on the background color of the original, so the edge portions of the image are clearly reproduced. What is shown in FIG. 6 is a 3×3 dither matrix. Here, for example, when an image is reproduced by scanning in the direction of arrow B, if the threshold level in the scanning direction is the same and is the lowest, lines are likely to appear in the scanning direction. In order to prevent such a situation, the dither matrix shown in FIG. 6 has the same threshold level in the direction perpendicular to the scanning direction and is set to the lowest level. As described above, according to the present invention, it is possible to select dither matrices with different threshold arrays based on the density of the background color of the original, so that, for example, higher quality gradation can be achieved by removing the background color of the original. Images can be reproduced. In this embodiment, the background color of the document was detected by scanning the document in advance, but the operator may determine the threshold value of the dither matrix by comparing the background color of the document with a reference chart, for example. You may.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a dither matrix, Fig. 2 is a diagram illustrating gradation expression made using the dither matrix, and Fig. 3 is a circuit diagram showing an example of a device implementing the method of the present invention. , FIG. 4 is a truth table of the decoder, and FIGS. 5 and 6 are diagrams showing the correspondence between five types of dither matrices and decoder outputs. A...Dither matrix cell, R...resistance, Q
…Comparator, DE…Decoder.

Claims (1)

[Claims] 1. In an image gradation reproduction method in which a gradation image is reproduced by dithering a document image using a dither matrix, level signals obtained by scanning the document each have different comparison levels. A determining means for determining the depth of the ground color of the original by comparing it with a plurality of comparators, and a plurality of types of dither matrices having mutually different threshold value arrays are provided, and the aforementioned method is provided based on the determination result of the determining means. An image gradation reproduction method characterized in that one of a plurality of types of dither matrices is selected.