Image transmission device and method

Abstract: An image transmission device is capable of transmitting image data in different resolutions with different pixel number of the image. The gamma correction values are converted in proportion to the transmittal resolution of the image data and the quasi half-tone which closely follows the original image is reproduced with less degree of omission of minute lines.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image transmission device capable of transmitting image data in different resolutions with different pixel numbers of the image. Related Background Art In order to transmit an image of characters, tables, pictures etc. recorded on books, documents etc., there is used a facsimile apparatus which transmits and receives image data in the form of electric signals by photoelectric conversion of the image. In recent years, there are known similar transmission devices, for example a Group-4 facsimile apparatus equipped with a high speed digital circuit, having usually 400 DPI (dot per inch) of reading resolution. The subscribers, however, may use a Group 4 facsimile apparatus with 200 DPI resolution or a Group 3 facsimile apparatus with 8 pel/mm x 3.85 line/mm capacity. Such being the case, the receiver has to adjust the resolution of the receiving apparatus by thinning out, etc. or by speeding up the rotation of motor attached thereto so as to convert a sub-scanning resolution to 200 DPI or 8 pel/mm x 3.85 line/mm. There is also a widely spread facsimile apparatus with a half-tone mode, utilizing an error-diffusion method or the like which enables to reproduce a clear-cut image of very sensitive materials such as a photograph. These conventional devices, however, have been These conventional devices, however, have been associated with the following drawbacks, although the transmittal resolution thereof is converted in the manners mentioned above. (1) In case that the image data, read in a resolution of 400 DPI, is transmitted in 200 DPI or 8 pel/mm x 3.85 line/mm, the operation for thinning out and so on causes omission of image information, and thus provides a light image as a whole. (2) In case that the sub-scanning resolution is converted to 200 DPI or 8 pel/mm x 3.85 line/mm by speeding up the rotation of the motor, the facsimile apparatus skips in reading minute lines, and thus the image is transmitted in incomplete manners. Namely, in case that a transmittal resolution does not match to that of the receiving apparatus, it may cause inconvenience in transmitting a clear-cut image of materials. DE-A-3 739 030 discloses an image processing device comprising generating means for generating image data representing an image to be output, output resolution converting means for converting an output resolution of the image data generated from the generating means, and output means for outputting image data with one of plural output resolutions, resolution-converted by the output resolution converting means. Moreover, US-A-4 841 375 teaches that a modification means modifies a parameter for density conversion in accordance with a transmission resolution. The term "density" denotes a resolution, while the expression "parameter" specifies a ratio for resolution conversion performed by a pixel density converter, which changes a binarization threshold during conversion. Furthermore, JP-A-2 053 382 discloses a control system for facsimile equipment wherein a receiving device receives a picture with a high concentration from a transmitting device. The gamma correction characteristic is changed in accordance with a recording characteristic informed from the receiving device. That is, the resolution of the plotter constituting the recording characteristic is notified to the transmitting device which selects the gamma correction characteristic in accordance with the notified plotter resolution. It is an object of the present invention to provide a device and a method of processing images which enable to transmit an image with less lost information even in case the reading resolution of the receiver is different from that of the transmitter while, at the same time, keeping the structure of the device simple. According to the invention this object is accomplished by an image processing device according to claim 1. According to the invention this object is also achieved by an image processing method according to claim 4. This invention provides an image transmission device and method which enable to transmit a clear-cut image, thus preventing any omission of standard value of resolution for judging black and white by quantifying the difference in transmittal resolutions. The present invention provides an image transmission device and method which enable to transmit an image of a half-tone mode in a manner of copying as it looks like its original image, thus preventing from causing any omissive reading of the image data by adjusting the gamma correction value which is designed to meet the characteristics of the image density in proportion to the selected transmittal resolution. The above-mentioned and other collateral objects and effects of this invention shall be further clarified through the following explanations. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing the principle of a projective technique. Fig. 2 is a block diagram of a type of the facsimile embodying the present invention. Fig. 3 is a flow chart indicating the motion of said facsimile device shown in Fig. 2. DESCRIPTION OF THE PREFFERD EMBODIMENT Fig. 2 is a block diagram showing a type of the facsimile embodying the present invention. In the figure, the element 1 is a scanner which scans optically the original manuscript for transmission or copying by its CCD image sensor or other device; the element 11 in the scanner is an image sensor which enables to scan the image at 400 DPI resolution; the element 12 is an A/D converter unit which converts the analog image signal transmitted from the image sensor 11 to a digital image signal; and the element 13 is a shading correction unit which corrects the level of the image signal by every bit. The element 10 is a converter unit for converting the binarized 400 DPI resolution of image data, constituting the element 15 which is a 400/200 converter unit for converting 400 DPI image data to 200 DPI image data, the element 16 which is a G4/G3 converter unit for converting 400 DPI image data to 8 pel/mm x 7.7 line/mm image data applicable for G3 facsimile, the element 18 which is a line to transmit 400 DPI image data directly and the element 17 which is a selector to select one of those three transmitted data, namely 400/200 converter 15, G4/G3 converter 16 and line 18. The 400/200 converter unit 15 thins out every other line of 400 DPI image data to be transferred into each line and further thins out every other pixel of such lost image data. This operation is designed to reduce the resolution to half in both main scanning direction and auxiliary scanning direction. Accordingly, 400 DPI image data is converted to 200 DPI image data. It provides not only a simple thinning out operation but other selective functions for thinning out in consideration of the neighboring image data by film treatments or the like. The G4/G3 converter unit 16 is required to have a function to convert 400 DPI image data to 8 pel/mm x 7.7 line/mm which performance is not attainable through the thinning out process with a simple integral operation by use of the 400/200 converter unit 15. Consequently, the G4/G3 converter unit 14 employs a method of projective technique for converting the resolution. Following is a succinct explanation of the principle upon the operation of resolution conversion by the projective technique. Fig. 1 shows an original pixel before it is converted by the projective technique and the converted pixel after it is converted by magnification of 2/3. The projective technique is a method to regard the shape of pre-converted pixel as a square (Fig. 1-(a), and place a rectangle of a longer side having such a length of the pre-projected pixel multiplied by the reciprocal number of conversion magnification, namely 2/3, in both main and auxiliary scanning directions (Fig. 1-(b), on top of the pre-projected pixel as shown in Fig. 1-(c), thereby measuring a ratio of the black area contained in said rectangle as the information of density. The G4/G3 converter unit 16 employs not only the thinning out operation but the above-mentioned projective technique which enables to convert the density of such fractions excluded those converted by a ratio of the reciprocal number of integer which is obtained by the thinning out process. In other words, the G4/G3 converter unit 16 converts 400 DPI image data to 8 pel/mm x 7.7 line/mm image data by applying a certain magnification in accordance with the above-mentioned principle of projective technique. For a more detailed explanation of this technique, refer to U.S. patent US-A-4 150 401 which was published on April 17, 1979. As a means for converting resolutions, there is a method for converting them in the auxiliary scanning direction by changing a speed of reading the image, besides the electrical operations to convert them in both main and auxiliary directions. The element 2 is a compression unit which compresses the image data transmitted from the converter 10 into the signal of MH, MR and MMR. The element 3 is a communication control which is equipped with a function of image transmission, constituted by the control of communication protocol built in Group 4 facsimile system, the control of communication protocol built in Group 3 facsimile system and the conversion-recovery processing unit. The element 4 is an expansion unit which expands the receiving image data transmitted by the communication control 3. The element 5 is a printer unit which prints the image data transmitted from the expansion unit 4. The element 6 is a central control unit which controls the whole functions of the illustrated system. The element 9 is an integrated service for digital transmission network (ISDN). The element 20 is an operation unit equipped with keys to input for instructing the addressees of information transmission, and starting the transmitted operation as well as the displays on which various information is indicated. The illustrated facsimile system employs a gamma correction unit 21 which converts the density of the digital image signal transmitted from the A/D converter 12, a gamma table controller 22 which controls the conversion properties of the gamma correction unit 21 and a half-tone processing unit 23 which accommodates dummy half-tone processing such as error-diffusion treatment and dither-processing for the signals of the digital image. The element 24 is a control signal which controls the conversion of a correction value table of the gamma correction unit 21 determined by the gamma table controller 22. The control signal is regulated by the central control unit 6. The element 25, consisting of the central control unit 6, the gamma table controller 22 which enables to set a plural number of the gamma table and the gamma correction unit 21 which converts the transmittal density of the image data, has a function to convert the gamma correction values. The element 9 is an integrated service for digital communication network (ISDN). In the following, there will be explained the functions of the above-mentioned facsimile system according to the present invention. Fig. 3 is a flow chart showing the motion of said facsimile device. After the manuscript to be transmitted is set in the scanner 1 and the addressee is instructed by the operating unit 20, the communication control unit 3 starts transmitting the material (S11). Then the transmittal resolution is identified through the communication with the receiving apparatus and an appropriate transmitted resolution is set to meet that of the receiving apparatus (S12). In case the transmittal resolution is set as 400 DPI, the gamma table controller 22 functions to convert the resolution by the gamma table correction control signal 24 so that the gamma correction value is set to meet 400 DPI resolution at the gamma correction unit 21 (S13, S14). Similarly, in case the transmittal resolution is set as 200 DPI or 8 pel/mm x 7.7 line/mm, the gamma table controller 22 functions to convert the resolution by the gamma table correction control unit 22 so that the gamma correction value is set to meet 200 DPI resolution at the gamma correction unit 21 (S13, S15). As explained above, the transmittal density of reproduced image is lighter (or lower) at the resolution of 200 DPI or 8 pel/mm x 7.7 line/mm, in comparison with the case of 400 DPI resolution. Accordingly, when 200 DPI resolution or 8 pel/mm x 7.7 line/mm is adopted for transmission, it is necessary to apply the gamma value correction to the image data so as to increase the transmittal density thereof. Namely, the central control unit 6 adjusts the gamma correction table to provide a lighter density of the transmitted image than that of 400 DPI resolution. In so doing, even if the transmittal resolution is lower than the level applied for 400 DPI, the density of the transmitted image is not lowered. In other words, such an appropriate adjustment of the gamma correction table makes it possible to receive the transmitted image at almost the same reproducing density even if the transmittal resolution of the receiving device is not identical to that of the sender. After the above-mentioned procedures are completed, the central control unit 6 instructs the scanner unit 1 to start reading the material (S16). Then the central control unit 6 further sends an instruction to the communication control 3 to transmit smoothly the manuscript read by the scanner unit 1 through the line 9 (S17). As explained above, it is possible to transmit a clear-cut image and a half-tone image close to its original material with less missing information, even if the transmittal resolution is kept coarse, which performance is attained by converting the gamma correction table selectively and appropriately when the manuscript is read with either 400 DPI, 200 DPI or 8 pel/mm x 7.7 line/mm resolution. The above said examples are used to explain a conversion of gamma correction values. The present invention is not limitedly applied to those examples but enables to convert the gamma correction values into more than three values. For example, in case the data is to be transmitted by a Group 3 (G3) facsimile of 8 pel/mm x 3.85 line/mm of standard resolution, it is appropriate to set an additional gamma correction value. In summary, the present invention enables to lessen the omissive transmission of minute lines and to reproduce a similar half-tone image which looks exactly as the original image by converting the gamma correction values in response to the transmittal resolution of the image data to be transmitted. The present invention is explained as above with reference to the appropriate examples. Needless to say, however, the applications of this invention are not limited to the above-mentioned examples but can be further reformed and modified in various ways within the scope of claims specified hereto.

Claims: An image processing device comprising: generating means (11-13) for generating image data representing an image to be output; output resolution converting means (10) for converting an output resolution of said image data generated from said generating means (11-13); and output means (3) for outputting image data with one of plural output resolutions, resolution-converted by said output resolution converting means (10); characterized by data density conversion means (21, 22, 24) for converting a data density of said image data generated from said generating means (11-13) by using a gamma correction parameter, wherein said data density conversion means (21, 22, 24) change said gamma correction parameter for the conversion of data density according to an output resolution of said output resolution converting means (10); wherein, when a value of said output resolution is lowered, said data density conversion means (21, 22, 24) convert the data density of said image data by using such a gamma correction parameter that causes the data density to be higher. A device according to claim 1, characterized by halftone processing means (23) for converting image data density-converted by said data density conversion means (21, 22, 24) into binary data. A device according to claim 2, characterized in that said output resolution converting means (10) convert an output resolution of image data halftone-processed by said halftone processing means (23). An image processing method comprising the steps of: generating image data representing an image to be output; converting an output resolution of said generated image data to one of plural output resolutions; and outputting said image data with said one converted resolution; characterized by further comprising the step of converting a data density of said generated image data by using a gamma correction parameter, wherein said gamma correction parameter for the conversion of data density is changed according to an output resolution of said converting step; wherein, when a value of said output resolution is lowered, the data density of said image data is converted by using such a gamma correction parameter that causes the data density to be higher. A method according to claim 4, characterized by further comprising a halftone-processing step of converting said density-converted image data into binary data. A method according to claim 5, characterized by further comprising the step of converting an output resolution of said halftone-processed image data.

Inventors: NAOTO KAGAMI, NAOTO, KAGAMI, NAOTO, KAGAMI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N1
IPC codes: H04N1, G06T3