JPH0478068B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a color image encoding device that encodes periodic color component signals of a plurality of colors each having a different screen angle.

(Background Art) A method of binarizing image density data using a dither matrix is conventionally known. Predictive encoding is also known as a method for encoding the binarized data.

In addition, there is a method of producing an output image with halftone dots using a dot-concentrated dither matrix, and in the case of color image output, giving a screen angle to the halftone dots to avoid problems caused by diagonal movement of the halftone dots of each color. Methods for eliminating visual disturbances such as color moiré have also been proposed.

As mentioned above, the encoding method for image data having a halftone structure with a screen angle has the drawback that the processing is too complicated in the encoding method proposed in the past. In particular, the above drawbacks are significant when a different screen angle is provided for each color, such as in a color image.

(Objective) Therefore, an object of the present invention is to provide a color image encoding device that can highly efficiently encode periodic color component signals of a plurality of colors each having a different screen angle with a simple configuration.

(Example) The present invention will be described in detail below with reference to the drawings.

First, to generate halftone dots, for example, a dot-concentrated dither matrix DM as shown in Figure 1A is used.
Perform binarization using . Furthermore, in order to simply provide the screen angle, the two-dimensional arrangement of the dither matrix DM is determined in the main scanning direction and the sub-scanning direction, as shown in FIG. 1B.

Here, k indicates that one dither matrix DM has been moved by k dots in the sub-scanning direction in order to provide the screen angle.

FIG. 2 shows 2 by such a threshold matrix DM.
An example of a circuit that performs value conversion is shown. Here, the input data ID representing gradation image information usually consists of 6 to 8 bits of time-series pixel data, which is compared with each component of the threshold matrix circuit MTX in the comparator COM, and is converted into 1-bit data representing white or black. output data of
Output as OD.

As a result, if the threshold matrix DM is of the dot concentration type described above, a halftone image can be obtained.
By adopting the threshold matrix configuration as shown in FIG. 2 described above, a desired screen angle can be provided.

When converting the input image into a signal, the dot density is 1 mm.
When 8 to 10 points are set per pixel, the change in signal between adjacent pixels is small in such a high-resolution image signal. Therefore, when binarization is performed using a concentrated dot dither matrix DM as shown in FIG. 1A, the black portion is very often a point that spreads from the center depending on the signal level. Therefore, the binarized signal forms halftone dots, the interval between the halftone dots is determined by the size of the dither matrix, and the screen angle is determined by the two-dimensional arrangement of the dither matrix.

In other words, the information regarding halftone dots such as the halftone dot spacing of the data obtained by the above-mentioned binarization has little existence in the input image, and the input/output system mechanism as a part of encoding, especially the dither matrix Much depends on dimensional arrangement. Therefore, adaptation to the input image can be done by changing the reference point depending on the characteristics of the input image, or by performing preprocessing (for example, replacing image signals, changing the encoding method depending on the type of image) before normal encoding. 2 described above without using any processing.
Highly efficient encoding can be easily performed using dimensional arrangement.

Therefore, the dither matrix used for the above-mentioned binarization is an l×l array. Furthermore, if the matrix arrangement is moved by k dots in the sub-scanning direction as shown in FIG _.
Good prediction can be made by using the point A ₀ '(x+l, y-k) for (x, y) as a reference point during prediction.

Here, regarding the reference points, the reference pixels (hereinafter referred to as nearby reference points) when determining the pixel to be referred to for the predicted pixel within the dither matrix size l×l pixels, and the dither matrix A pixel (hereinafter referred to as a periodic reference point) in which the above reference pixel is extended outside the range of l×l pixels due to the periodicity of (hereinafter referred to as corresponding reference points) are collectively referred to as all reference points.

Furthermore, as an extension of the above, a reference point in the vicinity of the pixel X to be predicted is determined. Referring to FIG. 4, for the nearby reference points A ₁ , A ₂ , A ₃ , ..., A _N for the predicted pixel A ₀ ,
When A _i = (x _i , y _i ), consider the point A _i ′△ = (x _i +l, y _i −k) (1) {A ₁ , A ₂ , ..., A _N , A ₀ ′, A ₁ ′, A ₂ ′,
..., A _N ′} as all reference points to make good predictions.

As an example, a case where the dither matrix DM' having the shape shown in FIG. 5 is arranged as shown in FIG. 6 will be described. Here, instead of the matrix size l and the movement amount k, the movement vector (k ₁ ,
k ₂ ). As shown in FIG. 7A, the predicted pixel is A ₀ and the neighboring reference pixels are A ₁ , A ₂ , A ₃
shall be. For A _i = (x _i , y _i ), if A _i ′△ = (x _i +k ₁ , y _i −k ₂ ) (1′), then all reference points are {A ₁ , A ₂ , A ₃ , A ₀ ′, A ₁ ′, A ₂ ′, A ₃ ′}. A good compression ratio can be obtained by encoding the error signal between S ₀ ^ for the predicted pixel predicted using this reference point and the signal S ₀ of the actual predicted pixel.

A specific example will be given below for explanation.

When the dither matrix DM' shown in FIG. 5 is used in the arrangement shown in FIG. 6, the movement vector becomes (4, 2). Also, if the coordinates of A ₀ are (x ₀ , y ₀ ), its nearby reference points A ₁ , A ₂ , A ₃ are A ₁ = (x ₀ -1, y ₀ ) A ₂ = (x ₀ , y ₀ −1) A ₃ =(x ₀ +1, y ₀ −1). From equation (1'), finding the reference points A ₀ ', A ₁ ', A ₂ ', A ₃ ' obtained from the two-dimensional arrangement of the matrix, A ₀ ' = (x ₀ + 4, y ₀ -2) A ₁ ′=(x ₀ +3, y ₀ −2) A ₂ ′=(x ₀ +4, y ₀ −3) A ₃ ′=(x ₀ +5, y ₀ −3).

All reference points obtained from this {A ₁ , A ₂ , A ₃ ,
A ₀ ′, A ₁ ′, A′ ₂ , A ₃ ′} are as shown in FIG. 7B.
Use this reference point to predict the value of A ₀ . The predicted signal S ₀ ^ is statistically determined from a lot of image data.
That is, P{A ₀ | A ₁ , A ₂ , A ₃ , A ₀ ′, A ₁ ′, A ₂ ′,
A ₀ with the state (black and white) _of the reference point as a variable.
is the conditional probability of becoming black, then S ₀ ^ = 0, P {A ₀ | A ₁ , A ₂ , A ₃ , A ₀ ′, A ₁ ′, A′ ₂ , A ₃
′}≦0.5, S ₀ ^1, P {A ₀ | A ₁ , A ₂ , A ₃ , A 0 ′, A ₁ ′, A _{2 ′} , A ₃ ′}
When >0.5, the predicted signal S ₀ ^ is determined.

Now, the reference points {A ₁ , A ₂ , A ₃ , A ₀ ′, A ₁ ′, A ₂ ′,
A ₃ ′} is 1 for black and 0 for white, and the state {1, 1,
0, 1, 1, 1, 0}, then P{A ₀ | 1,
1, 0, 1, 1, 1, 0} have been statistically determined in advance. Here, P{A ₀ | 1, 1, 0,
1, 1, 1, 0} = 0.8.

From the above, it can be seen that the predicted signal S ₀ ^=1. In reality, a fixed memory (ROM) is prepared in which seven signals from A ₁ to A ₃ ' are used as addresses and the prediction signal S ₀ ^ determined as described above is stored. Therefore, by reading the address 1101110(2) of the ROM, the predicted signal S ₀ ^=1 is obtained. Here, if the signal at the actual point A ₀ is 1, then the error signal E
=0, and if it is also 0, the error signal E is set to be 1, and this error signal E is set as the signal for the point _A0 . moreover,
Highly efficient encoding of the signal string consisting of this error signal E can be achieved using run-length encoding or MH code.

When handling color images, it is necessary to change the screen angle for each color, so the reference point must be changed for each color, but in the present invention, by changing the parameters k ₁ and k ₂ , Reference points for each color can be easily obtained.

FIG. 8 is an explanatory diagram of a color image recording apparatus to which the present invention is applicable. The color image recording device shown in FIG. 8 outputs color image information using an electronic copying device (laser beam printer) that includes a plurality of photosensitive drums arranged side by side, and sequentially prints different color images formed by this electronic copying device. This is a device that records in overlapping colors.

In FIG. 8, reference numerals 1a to 1d are scanning optical systems, which take out required image information from an image memory, etc. (not shown) as a light beam (laser beam), and this light beam is cyan (C),
Magenta (M), Yellow (Y), Black (Bl)
The image is formed on photosensitive drums 2a to 2d arranged in parallel corresponding to each other. Developing devices 3a to 3d are arranged near the photosensitive drums 2a to 2d, and each of the photosensitive drums 2a to 2d is placed on the side of a conveyor belt 7 for conveying recording paper (not shown).
Chargers 4a to 4d are arranged facing each other.

To explain the operation of the color image recording apparatus having such a configuration, the modulated light beams output from the scanning optical systems 1a to 1d are sent to each of the photosensitive drums 2a to 2d.
The optical image is formed on the top, and then, through an electrophotographic process, this formed image becomes an electrostatic latent image,
Developed by the developing devices 3a to 3d, and charged by the charging devices 4a to 3d.
4d, each color is sequentially transferred onto the recording paper held on the conveyor belt 5 to form a color image.

Figure 9 shows the four scanning optical systems 1 shown in Figure 8.
1 is a schematic perspective view showing a detailed example of one of a to 1d (generally referred to as 1), in which a light beam modulated by a semiconductor laser 11 is a rotating polygon that is collimated by a collimating lens 10; The light is deflected by mirror 12. The polarized light beam forms an image on a photosensitive drum 2 (the above-mentioned photosensitive drums 2a to 2d are collectively designated by reference numeral 2) by an imaging lens 13 called an fθ lens to perform beam scanning. During this beam scanning, one of the light beams
The tip of the line scan is reflected by the mirror 14,
Light is guided to a photoelectric detector 15. This photoelectric detector 15
The detection signal from is used as a well-known synchronization signal in the scanning direction H (horizontal direction).
This signal name will hereinafter be referred to as the BD signal or horizontal synchronization signal.

FIG. 10 is an overall block diagram showing the signal processing system of the present invention.

In FIG. 10, 21 is an input device, and the color image information of blue (B), green (G), and red (R) outputted from this input device is, for example, 8 bits (256 levels) each. It is quantized to represent the key. Each of these 8 bits B,
The G and R outputs are supplied to a masking/inking circuit 22 to be converted into 8-bit yellow (Y), magenta (M), cyan (C) and black (Bl) signals, and then to a binarization circuit 23. and convert them into predetermined 1-bit binary signals Y2, M2, C2 and Bl2, respectively.

These binary signals are supplied to the compression encoding circuit 24, and compressed signals Yc of 1 bit or less per pixel,
Mc, Cc and Blc are formed and these compressed signals are stored in the storage device 25. From the storage device 25,
Compression signal Yc, according to a predetermined readout control signal
Read Mc, Cc, Blc. The read compressed signal is converted into binary signals Y2, M2, C2 and before the compression encoding circuit 24 by the decoding circuit 26.
The signal is reproduced to Bl2 and then taken out to the outside by the output device 27.

The compression encoding circuit 24 described above is configured as shown in FIG. Ce and
These prediction error signals are supplied to a run length encoding circuit or MH encoding circuit 32 to obtain compressed signals Yc, Mc, Cc and Blc.

Furthermore, the above-mentioned predictive encoding circuit 31 can be configured as shown in FIG. 12, for example. 1st
In Fig. 2, input binary signals Y2, M2, C2, and Bl2 are stored in a line memory 33, and are moved by an address signal from a reference point ROM 35, an output from a ROM (not shown), or a diversion of a control signal. Vector signals IV are added by an adder 34, and the contents of the line memory 33 are read out using the addition result signal as an address signal. The prediction ROM 36 uses the read contents as an address signal.
The prediction signal read from the input binary signal Y2, M
2. Matching circuit 3 determines the match with C2 and Bl2.
7, and the prediction error signals Ye, Me, Ce and
Output Ble.

In the present invention, since the address signal for the line memory 33 is obtained as a sum signal of the output signal of the reference point ROM 35 from the adder 34 and the movement vector signal, the arrangement of the dither matrix or the matrix itself can be changed. can be encoded easily and efficiently.

In the present invention, as a further extension of the above example, for a plurality of dither matrices, e.g., A=(a _ij ), B=(b _ij ), the If the value ε can be expressed as a _ij = b _ij + ε (2), then the reference point can be determined by the method of the present invention using the movement vector (k ₁ , k ₂ ) without distinguishing between A and B. can.

Examples of dither matrices A and B in this case are shown in FIGS. 13A and 13B, respectively. Here, ε=1. As another example,
Considering the case where the dither matrix DM'' shown in Fig. 14 is used, this means that the small matrices A, B, C, and D of Figs. and is arranged in the formula
It can be seen that (2) is satisfied.

From this, an error signal sequence can be obtained by performing prediction using a single reference point using a method similar to the encoding method of the above-described specific example. Highly efficient encoding can be performed using such a method using MH encoding, run-length encoding, or the like.

Figure 16 shows the dither matrix of Figure 14.
As is clear from the figure, which shows an example of binarization using DM'', the small matrices A,
B, C, and D are A→B→A→B or C→D→C→
It can be seen that the positions are periodically repeated with a movement vector (2, 0) as shown in D. That is, here, predictive encoding is performed using a pixel two pixels before in the main scanning direction as a reference pixel (corresponding pixel). In other words, the reference point is □×, which is one point before the predicted point □× in the main scanning direction.

Furthermore, when block encoding is performed for each piece of data corresponding to a small matrix as described above, the present invention takes into account the arrangement of the matrix and its periodicity when performing encoding, resulting in better or better compression. rate can be given.

As explained above, since the present invention performs encoding based on the periodicity of the matrix, all dither matrices other than dot-concentrated matrices are effective if they are arranged with periodicity. It turns out that it is applicable.

According to the embodiments of the present invention, it is possible to easily obtain reference points for making predictions for dot-concentrated dither matrices, and it can also be applied to matrices that are not square matrices. It is also possible to perform good encoding using a simple method. Furthermore, even when multiple dither matrices are used, encoding can be performed using the same method using the parameters (k ₁ , k ₂ ).
This is particularly effective when a different matrix is used for each color in a color image, or when a matrix suitable for the input image is used depending on the nature of the input image.

Moreover, since the present invention encodes the two-dimensional arrangement of the matrix using periodicity, by extending its application to the case where all the threshold values of the matrix satisfy equation (2), a very wide variety of Another advantage is that matrices can be encoded using the same method.

Note that the present invention can be applied not only to the above-mentioned dither method but also to a case where a density pattern method or the like is used to achieve the same effect.

(Effects) As described above, according to the present invention, the input means (line memory 33 in FIG. 12) inputs periodic color component signals of a plurality of colors each having a different screen angle.
and an encoding means (prediction ROM 3 in the figure) for predictively encoding the color component signal inputted by the input means.
6, a coincidence judgment circuit 37), and the encoding means inputs first information (shown in the same figure) indicating a screen angle corresponding to the color component signal inputted by the input means and receives the input of the color component signal. The second indicating the period
By having a reference pixel determining means that determines pixels to be referred to during prediction based on the input of information (reference point ROM 35 in the figure), periodic color component signals of multiple colors each having a different screen angle can be easily generated. Highly efficient encoding can be achieved with this configuration.

[Brief explanation of the drawing]

FIG. 1A is a diagram showing an example of a dot concentrated dither matrix, FIG. 1B is a diagram showing an example of a two-dimensional matrix arrangement, and FIG. 2 is a block diagram showing an example of a binarization circuit. 3 is an explanatory diagram of reference points in the present invention, FIG. 4 is an explanatory diagram of nearby reference points in the present invention, FIG. 5 is a diagram illustrating an example of a dither matrix that is not a square matrix, and FIG. 6 is a diagram illustrating a matrix in the present invention. A line diagram showing an example of the two-dimensional arrangement of
FIG. 7B is an explanatory diagram of the nearby reference point in the present invention.
is an explanatory diagram of all reference points in the present invention, FIG. 8 is a diagram schematically showing the color image recording device according to the present invention, FIG. 9 is a perspective view showing the schematic configuration of the scanning optical system, and FIG. 10 is the same diagram. A block diagram showing an example of the signal processing system, FIG. 11 is a block diagram showing a detailed example of the compression encoding circuit, FIG. 12 is a block diagram showing a detailed example of the predictive encoding circuit, and FIGS. Figure 13B shows multiple dither matrices A
and B, FIG. 14 is a diagram showing another example of the dither matrix, and FIGS. 15A to 15D are diagrams showing the 14th
FIG. 16, a diagram showing small matrices constituting the illustrated dither matrix, is an explanatory diagram of encoding according to the present invention. DM, DM′, DM″……Dather matrix, ID
...Input data, MTX...Threshold matrix circuit, COM...Comparator, OD...Output data, 1a
~1d...Scanning optical system, 2a-2d...Photosensitive drum, 3a-3d...Developer, 4a-4d...Charger, 5...Transport belt, 10...Collimating lens, 11...Semiconductor laser, 12... Rotating polygon mirror, 13... Imaging lens, 14... Mirror, 15
...Photoelectric detector, 21...Input device, 22...Masking/inking circuit, 23...Binarization circuit, 2
4...Compression encoding circuit, 25...Storage device, 26
... Decoding circuit, 27 ... Output device, 31 ... Predictive encoding circuit, 32 ... Run length or MH
Encoding circuit, 33...line memory, 34...adder, 35...reference point ROM, 36...prediction
ROM, 37... Matching judgment circuit.

Claims (1)

[Claims] 1. An input means for inputting periodic color component signals of a plurality of colors each having a different screen angle, and an encoding means for predictively encoding the color component signals inputted by the input means. , the encoding means performs prediction based on an input of first information indicating a screen angle corresponding to the color component signal input by the input means and an input of second information indicating a period of the color component signal. 1. A color image encoding device comprising reference pixel determining means for determining a pixel to be referred to in the process.