JPS6367394B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an image information encoding method for optimally encoding image information sampled in units of pixels and subjected to multilevel quantization. In general, image input/output devices such as facsimiles scan and sample the original image pixel by pixel on the input side, and then transmit the image information to the output side. How can we improve transmission efficiency through encoding, reduce storage capacity when it is necessary to store the data, and reproduce high-resolution, high-quality images on the output side? The big question is whether it can be done.
In this case, in order to improve the resolution of images such as characters and line diagrams that are reproduced by binary output, it is necessary to increase the sample rate on the input side or perform multi-value quantization of the read image information. In any case, the amount of information transmitted to the output side tends to increase, and there is a strong demand for improvement in data compression. Conventionally, when encoding a document image consisting only of white and black levels, encoding is performed using the number of consecutive pixels of the same code (run length) in image information expressed in binary. run-length encoding method or a variation thereof
The Modified Huffman method (hereinafter referred to as the MH method) is widely adopted, but although this method has a good data compression rate, it cannot express halftones, and therefore its encoding The quality of the images reproduced based on the data, especially the resolution, has deteriorated. Furthermore, conventionally, in order to encode image information expressed in multi-values, a differential encoding method has been widely adopted in which, for example, the difference from the previous pixel is encoded according to each gradation level. In this case, it is possible to encode halftones, but since the state of change in the density level is simply expressed at each pixel, not only is the data compression rate poor, but also the sudden change in density level is The quality of the reproduced image deteriorates because it is unable to follow the fluctuations. On the other hand, recently, in multi-value quantized image information, for example, as shown in Table 1, a bit plane code in which each eight-value quantized density level is weighted by 2 ⁰ , 2 ¹ , and 2 ² is used. Use each
Focusing on the number of consecutive bits that are "1" or "0" in the bit pattern when looking at each weight of 2 ⁰ , 2 ¹ , and 2 ² , we calculated the run length for each weighted bit pattern. An encoding method has been proposed that aims to improve data compression by encoding (Japanese Patent Laid-Open No. 49-58705
reference).

[Table] However, even with such data compression methods, the amount of information when compressing binary image information is 2, compared to the amount of information when compressing binary image information.
In the case of double and 8-value data, there is a drawback that as the number of bits increases, the amount of information increases by a factor of 3, resulting in a decrease in data compression ratio. The present invention has been made in consideration of the above points, and enables encoding with a good data compression rate even when the number of bits increases when encoding multilevel quantized image information. The present invention provides an image information encoding method that can reproduce high-resolution, high-quality images based on the encoded data. The image information encoding method according to the present invention focuses particularly on the characteristics of binary images such as line drawings or character images, and
As a first process, image information for one line in the main scanning direction for each multi-level quantized pixel is processed according to the run length at the change point between the white level pixel and the black level pixel. By executing encoding, rough data compression is performed according to the distribution of each pixel between the quantized minimum level (white) and maximum level (black), and the image information of the same line is As the second process, the interval at which halftone level pixels appear is encoded according to the run length, and then a code for identifying the halftone level is added immediately after the run length encoded code. Then, as a third process, the binary data encoded by each of these processes is appropriately combined according to a fixed rule, and the data of the line is compressed. Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings. Figure 1a shows part of the image information of one line in the main scanning direction for each pixel unit that has been quantized with four values.The density level of each pixel ranges from white (level 0) to halftone (level 1 or level 2)
This shows an example of information in the case where when the color changes to black (level 3), it always changes to black (level 3) and then returns to white, and conversely, when it changes from black to intermediate tone, it always changes to white and then returns to black again. In addition, FIG. 1b shows run-length encoded data obtained by subjecting the image information shown in FIG. 1a to the first process, and FIG. Each figure shows length-encoded data. That is, as a first process, first, the run length at the change point between the white level pixel and the black level pixel is encoded according to the Weyl code (WYC) shown in Table 2.

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Claims (1)

[Claims] 1. Prior to encoding the halftone level, perform run length encoding according to the white level run and the black level run in the change point section of the point changing from the white level to the black level or from the black level to the white level. In the first process, for the section where halftone level pixels appear, run length encoding is performed according to the halftone level run, and then the middle level is identified immediately after the run length encoded code. a second process that adds a code for each scan line; and a third process that encodes one scanning line by combining the image data encoded by the first process and the second process, respectively.
An image information encoding method characterized in that multi-level quantized image data is compressed by sequentially executing the above process and the first to third processes.