Image Signal Compressing Device

1. Field of the Invention

The present invention relates to an image signal compressing device adapted for use in a facsimile machine and an electronic filing machine.

2. Description of the Prior Art

Recently, a device has been developed which, when used in a facsimile machine or an electronic filing machine, operates to convert an image of intermediate gradation into a binary image signal according to a dither process.

It has, however, been found that, when the binary image signal, converted by the above described device, is compressed according to an MR code or a modified Hoffman (MH), which is a coding (compressing) system standardized in the facsimile machines, the compression ratio tends to be considerably lowered.

Accordingly, the present invention has been devised with a view to substantially eliminating the above described problem and provides, as its essential object to, an improved image compressing device effective to increase the compression ratio of the binary image signal of an intermediate gradation without being accompanied by a reduction in image quality.

In order to accomplish the above described object, the present invention provides an improved image compressing device which includes a means for dividing the binary signal into predetermined areas and for converting the binary signal into codes corresponding to predetermined patterns one for each predetermined area. It further includes means for detecting a difference between the pattern corresponding to the code and the binary image signal, a multi-digit compressing device for compressing the code, and a binary compressing device for compressing the difference.

According to the present invention, the compression ratio can be increased, without being accompanied by reduction in image quality, by separating the binary signal, according to a dither process, into the pattern obtainable from a dither matrix and the difference, and then compressing them individually.

This and other objects and features of the present invention will become clear from the following description taken in conjunction with a preferred embodiment thereof with reference to the sole accompanying drawing which illustrate, in schematic block circuit representation, an image signal compressing device according to the present invention.

Referring to the sole accompanying drawing, reference numeral 1 represents a memory unit including a binary image data storage area, a multi-digit compressing data storage area and a binary compressing data storage area. Reference numeral 2 represents a dither pattern coding circuit adapted to receive a binary data read out from the binary image data storage area of the memory unit 1 for each 4.times.8 bit area, which coincides with a dither matrix, for coding the binary data according to a predetermined algorithm. Reference numeral 3 represents a dither pattern generator for developing a dither code, outputted from the dither pattern coding circuit 2, into a pattern of 4.times.8 bits. Reference numeral 4 represents an error detector operable to determine an exclusive OR logic between an output data from the dither pattern coding circuit 2 and the original binary data, outputted from the dither pattern generator 3, and then to output a difference data representative of the exclusive OR logic. Reference numeral 5 represents a binary compressing circuit operable to compress the difference signal from the error detector 4 into binary compressing data, which is in turn stored in the binary compressing data storage area of the memory unit.

Reference numeral 6 represents a multi-digit compressing circuit operable to compress a dither code, outputted from the dither pattern coding circuit 2, into multi-digit compressed data, which is in turn stored in the multi-digit compressing data storage area of the memory unit 1.

While the image signal compressing device is so constructed as hereinabove described, if all of the image data coincide with, for example, 33 4.times.8 bit matrixes obtained from the dither matrix, all of the difference data will be "0". Accordingly, the amount of data which is binary compressing data will be substantially "0". The number of the dither codes is 33 and the amount of data is compressed to about 5/32 in this instance. Therefore, further multi-digit compression can result in an increase of the compression ratio.

According to this technique, since the difference data is subjected to the binary compression separately, to provide the binary compressing data which is subsequently stored, no error will occur relative to the original binary image state when expanded.

In the foregoing embodiment of the present invention, although the area to be coded is made to coincide with the dither matrix, a similar effect can be obtained by coding only into a pattern of high frequency of reproduction if the area to be coded is not made to coincide with the dither matrix.

From the foregoing description, it is clear that, according to the present invention, the compression ratio of the binary image signal representative of the image having an intermediate gradation can be increased. Also, the data which has been compressed and then expanded can completely match with the original binary image signal, with no substantial reduction in quality of the image.

Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawing, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.