JP2928786B2 - Expression method of halftone image - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for expressing a halftone image, and to a so-called dither process for pseudo-halftone image representation using a binary image.

[Conventional technology]

The dither method is well known as a method for expressing a halftone image. Generally, a threshold value of each pixel in a square dither cell of nxn pixels is determined, and the dither cell is applied to the image to be processed. Binarize. Here, in the case of a dispersed dither (eg, Bayer type), when trying to increase the number of gradations, a large dither cell is used, so that black pixels do not concentrate on edges and the like, resulting in a blurred image and peculiar to tissue dither. Cycle becomes noticeable. On the other hand, a halftone dot or spiral type dither can provide a more natural pseudo-halftone expression, but an attempt to increase the number of gradations results in an image with extremely coarse dot intervals.

[Problems to be solved by the invention]

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional problem, and provides a method for expressing a halftone image for generating a dither image capable of giving a natural impression while increasing the number of gradations. Aim.

[Means for solving the problem]

A method for expressing a halftone image according to the present invention defines a non-rectangular dither cell and a macro dither cell formed by arranging a plurality of adjacent dither cells, evaluates the density of a processing target image in units of macro dither cells, and It defines the threshold order of the dither cells in the cell and the threshold order of each pixel in the dither cell.

[Action]

According to the method for expressing a halftone image according to the present invention, since the density of the processing target image is evaluated in units of macro dither cells, the number of tones of the dither image can be increased, and
Since the threshold order of the dither cell in the macro dither cell and the threshold order of each pixel in the dither cell are defined,
Pixels are dispersed in the macro dither cell in the dither image, which can give an overall natural impression.

〔Example〕

Next, an embodiment of a method for expressing a halftone image according to the present invention will be described with reference to the drawings.

FIG. 1 shows a dither cell and a macro dither cell of the first embodiment. One macro dither cell MD as a whole and four dither cells D _{1 to} D ₄ in the macro dither cell MD are indicated by solid lines. Is shown.

Each of the dither cells D _{1 to} D ₄ is a pattern of eight pixels having a substantially regular hexagonal shape, and has a shape in which two pixels are arranged in three columns vertically and one pixel is added to each of the left and right of the center two pixels in the vertical direction.
In each dither cell, the left pixel of the center two pixels in the vertical direction → the right pixel → the right pixel of the bottom row → the left pixel of the bottom row → the pixel added to the left of the center in the vertical direction → the left of the top row The threshold order is defined in the order of the pixel, the right pixel in the top row, and the pixel added to the right of the center in the vertical direction. This for each dither cell corresponds to gave spiral pattern, however, the examples are given threshold ranking relative dither cell D ₁ to D _4, circulates dither cell D ₁ to D ₄ Then, one pixel is given to each dither cell to express the density in units of macro dither cells, so that a spiral pattern does not occur in the dither image, giving a natural impression. The density of the processing target image is evaluated in units of macro dither cells, and density expression of 8 × 4 = 32 gradations is possible.
According to experiments performed by the inventor, a natural image is generally required to have a density gradation of at least 32 gradations, and even with dither processing of 32 gradations, a natural impression can be obtained with the conventional dither method. No dither image can be provided.

The dither cells D _{1 to} D ₄ have their centers (indicated by crosses in the figure).
Are arranged so as to be located at the vertices of a rhombus, and the center of each dither cell is arranged at an extremely short distance (two pixel width) from the center of the macro dither cell (indicated by a circle in the drawing), and the dither cells D _{1 to} D _The pixels arranged cyclically in ₄ give a very dense impression.

Various shapes of the dither cell are conceivable as shown in FIGS. 2 to 9, and generally, a non-rectangular dither cell gives a natural impression in which pixels are dispersed.

FIG. 3 shows an approximately square dither cell composed of 11 pixels. A macro dither cell including four dither cells is configured as shown in FIG. 10, for example.

In the macro dither cell of FIG. 10, it is possible to express density of 11 × 4 = 44 gradations.

FIG. 4 shows a cross-shaped dither cell consisting of five pixels,
A macro dither cell including four dither cells is configured, for example, as shown in FIG. The macro dither cell shown in FIG. 11 is capable of expressing 5 × 4 = 20 gradations, and the dither image gives an extremely dense impression as a whole.

In the above-described first embodiment and its modifications, the threshold pattern of each dither cell in the macro dither cell is set in advance. However, if randomness is given to the threshold pattern of each dither cell, the dither image will have a more natural feeling. Will give.

FIGS. 12 to 18 show a second embodiment for generating the randomness, in which a reference image RIM having the same size as the processing target image IM is generated, and each pixel of the reference image IRM is generated. Random numbers r ₁ , r ₂ ,... Are given. Image IM to be processed
When the macro dither cell MD is applied to a certain area, the random number of each pixel of the reference image is simultaneously referred to. This random number is stored in each dither cell D _{1 to} D ₄ in the macro dither cell MD (FIG.
Numerical values of 0 to 3 are given corresponding to FIG.
D ₁ when "0", D ₂ when "1", D ₃ when the "2", the D ₄ when "3" is designated.

In FIG. 12, each pixel in the macro dither cell MD is represented by P1 to P32
In FIG. 13, a macro dither cell is added to the image IM to be processed.
The example of the random number corresponding to each pixel in the state where MD was applied is shown. For example, focusing on the pixel P ₁ of the left top of the macro dither cell MD, in the FIG. 13 random number corresponding to that pixel is "0". Random number "0" corresponds to the dither cell D _1, dither cell D ₁ is applied for the pixel P _1. The position of the pixel P ₁ corresponds to the top left pixel of the dither cell D ₁ , and the threshold value is “20”. Therefore, the pixel P ₁
Is binarized by the threshold “20”. By randomly changing the applied dither cell for each pixel in this way, the variance of the pixels of the dither image becomes random, and a more natural impression can be given. Of course, pseudo-random numbers often include some periodicity, and a natural image with high randomness is taken from a camera and quantized to 0 to 3 as a reference image. As a result, a more natural dither image is obtained.

Depending on the distribution state of the random numbers, there is a possibility that the density may be biased. However, if the frequency of the random numbers is made uniform, such an obstacle can be completely prevented. For example, reference image RIM
By distributing the random numbers so that the number of random numbers 0 to 3 becomes uniform in each macro dither cell corresponding area, the frequency of occurrence becomes uniform. In addition, if the distribution of random numbers for equalizing the frequency of occurrence is the same for each macro dither cell, periodicity will occur in the dither image, but this random number distribution is converted to a random number for each macro dither cell. If it is changed based on this, such periodicity is also eliminated. It is not always necessary to make the random number appearance frequency uniform in units of macro dither cells, but may be made uniform for each region including a plurality of macro dither cells.

〔The invention's effect〕

As described above, the halftone image expression method according to the present invention defines a non-rectangular dither cell and a macro dither cell formed by arranging a plurality of adjacent dither cells, and evaluates the density of the processing target image in units of the macro dither cell. In addition, since the threshold order of the dither cells in the macro dither cell and the threshold order of each pixel in the dither cell are defined, dense dot expression is possible in each local area while increasing the number of gradations by the macro dither cell. , A natural pseudo-halftone expression with high image quality can be realized.

Further, by setting the threshold order of the dither cells in the macro dither cell based on the random number sequence, a more natural halftone expression can be realized.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a macro dither cell of the first embodiment,
2 to 9 are conceptual diagrams showing various dither cells, FIG.
FIG. 11 is a conceptual diagram showing a macro dither cell using the dither cell of FIG. 3, FIG. 11 is a conceptual diagram showing a macro dither cell using the dither cell of FIG. 4, and FIG. FIG. 13 is a conceptual diagram showing an example in which a macro dither cell is applied to an image to be processed in the second embodiment, FIG. 14 is a conceptual diagram showing a reference image of the same embodiment, FIG. FIG. 16 is a conceptual diagram showing one dither cell in the macro dither cell of FIG. 12, FIG. 16 is a conceptual diagram showing another dither cell, FIG. 17 is a conceptual diagram showing still another dither cell, and FIG. It is a conceptual diagram which shows a dither cell. MD ...... macro dither cell, D ₁ ~D ₄ ...... dither cell, IM
... Processing target image, RIM... Reference image, r ₁ , r ₂ .

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-163959 (JP, A) JP-A-57-185768 (JP, A) JP-A-63-164758 (JP, A) JP-A-57-185758 142072 (JP, A) JP-A-61-15467 (JP, A) JP-A-63-245174 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04N 1 / 40-1 / 409 H04N 1/46 H04N 1/60

Claims (1)

(57) [Claims] 1. A regular hexagonal dither cell comprising 8 pixels,
And a macro dither cell in which four dither cells are arranged in a diamond shape so that the distance between the centers becomes minimum, the dither cell order in the macro dither cell is set, and the threshold order of the pixels in the dither cell is randomized. Is set based on the density of the natural image having high probability, and while each dither cell in the macro dither cell is circulated according to the dither cell order, a threshold value of each pixel in each dither cell is set according to the threshold order, and this circulation is performed. A method for expressing a halftone image, wherein threshold values of all pixels in a macro dither cell are set by repetition, and each pixel of the image is binarized based on the threshold value.