Image communication method and apparatus

Abstract: According to this invention, when n-value image data is transmitted, data associated with its compression method is transmitted to a reception-side apparatus, so that the reception-side apparatus can perform proper image processing. More specifically, there is disclosed an image transmission method having the following advantages. That is, when a reception-side apparatus can select one of a plurality of image processing methods, it selects a processing method matching with an n-value compression method, and properly reproduces an m-value image. Even when the reception-side apparatus does not have an image processing method matching with the n-value compression method of a transmission-side apparatus, the fact can be informed to the transmission-side apparatus or to an operator of the reception-side apparatus, so that operators of the transmission- and reception-side apparatuses can recognize that a color difference will occur between the transmission- and reception-side apparatuses, and an effective countermeasure can be taken.

Description: BACKGROUND OF THE INVENTION : Field of the Invention The present invention relates to an image communication method and apparatus for transmitting image information, and the like. Related Background Art Conventionally, various color facsimile apparatuses for communicating color images have been proposed. In a color image, since each of three color data of red (R), green (G), and blue (B) has 256 gradation levels between 0 to 255, the data volume is very large, and the communication time is prolonged as compared to those of a black-and-white image. For this reason, it is very difficult to put such an apparatus for directly transmitting multi-value data into practical applications. As a method of compressing a data volume of a color image, each of R, G, and B data is binarized from 256 gradation levels between 0 to 255 to 2 gradation levels of 0 and 1, and binary data is coded by a conventional coding method in a facsimile apparatus such as MMR, MR, or the like. However, when a received color binary image is color-processed, and is printed out, colors vary depending on the type of binarizing method of binarising an original multi-value image, and an image in colours different from those of a colour image to be transmitted is undesirably printed out in a receiver. Colours of an image to be transmitted are different from those of an image printed out by a receiver like in a case wherein image data which is binarised by a Fatning type dithering method is transmitted to a receiver which is adjusted to reproduce appropriate colours of an image which is binarised by a Bayer type dithering method. GB-A-1401637 discusses an arrangement in which a run-length coded signal is accompanied from time to time by an instruction indicating a shift in the code word size used in the coding. SUMMARY OF THE INVENTION According to the present invention there is provided an image data transmission method as set out in claim 1 and a receiver as set out in claim 11. The remaining claims set out optional features. An embodiment of the present invention provides an image reception method or apparatus capable of dealing with various compression methods. The preferred embodiment of the present invention allows a reception side to appropriately deal with transmission data. According to a preferred embodiment of the present invention, when n-value image data is transmitted, data associated with its compression method is transmitted to a reception-side apparatus, so that the reception-side apparatus can perform proper image processing. More specifically, there is disclosed an image transmission method having the following advantages. That is, when a reception-side apparatus can select one of a plurality of image processing methods, it selects a processing method matching with an n-value compression method, and properly reproduces an m-value image. Even when the reception-side apparatus does not have an image processing method matching with the n-value compression method of a transmission-side apparatus, the fact can be informed to the transmission-side apparatus or to an operator of the reception-side apparatus, so that operators of the transmission- and reception-side apparatuses can recognise that a colour difference will occur between the transmission- and reception-side apparatuses, and an effective countermeasure can be taken. An embodiment of the present invention provides an image communication method which can satisfactorily perform a printing operation when printing out a colour image which has been compressed to n-value data and transmitted. An embodiment of the present invention provides an image communication method and apparatus which can satisfactorily transmit a colour image signal using an ISDN. Embodiments of the present invention will be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS : Fig. 1 is a block diagram showing an arrangement according to an embodiment of the present invention; Fig. 2 is a block diagram showing an arrangement of a binarizing unit of the embodiment shown in Fig. 1; Fig. 3 is a block diagram showing an arrangement of an image processing unit of the embodiment shown in Fig. 1; Fig. 4 is a schematic flow chart showing a transmission operation of the embodiment shown in Fig. 1; Fig. 5 is a schematic flow chart showing a reception operation of the embodiment shown in Fig. 1; Fig. 6 shows a threshold value matrix of a Bayer type dithering method; Fig. 7 shows a threshold value matrix of a Fatning type dithering method; Fig. 8 shows image data binarized by the Bayer type dithering method; Fig. 9 shows image data binarized by the Fatning type dithering method; Fig. 10 is a table showing code data corresponding to four binarizing methods of the embodiment shown in Fig. 1; Fig. 11 shows code data for informing binarizing methods of an image communication apparatus of the embodiment shown in Fig. 1 to a destination apparatus; Fig. 12 is a flow chart showing an operation of a transmitter so as to discriminate whether or not a receiver can deal with the binarizing methods in the embodiment shown in Fig. 1; Fig. 13 is a flow chart showing an operation of a receiver so as to discriminate whether or not a transmitter can deal with the binarizing methods in the embodiment shown in Fig. 1; Fig. 14 is a block diagram showing another arrangement of an image processing unit according to another embodiment of the present invention; Fig. 15 shows an arrangement of a smoothing filter for the Bayer type dithering method in the embodiment shown in Fig. 14; Fig. 16 shows an arrangement of a smoothing filter for the Fatning type dithering method in the embodiment shown in Fig. 14; Fig. 17 is a flow chart showing an operation of a transmitter according to still another embodiment of the present invention; and Fig. 18 is a flow chart showing an operation of a receiver of the embodiment shown in Fig. 17. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS : Fig. 1 is a block diagram showing an arrangement according to an embodiment of the present invention. An image communication apparatus of this embodiment comprises a scanner unit 1 for reading a color image on an original, a binarizing unit 2 for binarizing color image data input from the scanner unit 1, a coding unit 3 for coding the binary data by MMR, MR, or the like to compress a data volume, a transmission control unit 4 for performing a communication with another image communication apparatus through a telephone line or an ISDN network to exchange coded data, a decoding unit 5 for decoding received coded data, an image processing unit 6 for processing image data in correspondence with characteristics of a printer unit 7, and the printer unit 7 for printing out image data. Note that the binarizing unit 2 outputs input multi-value image data as multi-value image data without binarizing it. Fig. 2 is a block diagram showing an arrangement of the binarizing unit 2. The binarizing unit 2 of this embodiment comprises four binarizing means 21 to 24 for respectively binarizing data by a Bayer type dithering method, a Fatning type dithering method, an error diffusion method, and a simple binarizing method. The unit 2 selects one of these four binarizing units 21 to 24 to binarize image data, and can also supply image data to the coding unit 3 without binarizing it. Fig. 3 is a block diagram showing an arrangement of the image processing unit 6. The image processing unit 6 of this embodiment comprises four image processing means 61 to 64 corresponding to the four binarizing methods, i.e., the Bayer type dithering method, the Fatning type dithering method, the error diffusion method, and the simple binarizing method, and also comprises an image processing means 65 for a non-binarized 256-gradation image. The image processing unit 6 selects one of these image processing means 61 to 64 to perform image processing of coded image data, and supplies the processed data to the printer unit 7. Fig. 4 is a schematic flow chart showing a transmission operation, and Fig. 5 is a schematic flow chart showing a reception operation. In a transmission mode, the scanner unit 1 reads a color image, and supplies R, G, and B 256-gradation data (0 to 255) corresponding to dots at, e.g., 400 dpi to the binarizing unit 2 (S1). The binarizing unit 2 binarizes the R, G, and B 256-gradation data of the color image data, and sends them to the coding unit 3 (S2). In this manner, the scanner unit 1 requires 8 bits for each of R, G, and B data in units of dots, i.e., a total of 24 bits. However, when these data are binarized, each of R, G, and B data requires one bit, i.e., these data require a total of only 3 bits. Thus, these data are compressed to 1/8. The coding unit 3 encodes input data by a coding method such as MMR, MR, or the like, and sends the coded image data to the transmission control unit 4 (S3). The transmission control unit 4 exchanges procedure signals with a receiver, and then transmits data associated with a binarizing method and image data to the receiver (S4). In Fig. 5, when the transmission control unit 4 receives the data associated with the binarizing method and the image data (S11), the received image data is sent to and decoded by the decoding unit 5, and the decoded data is supplied to the image processing unit 6 (S12). The image processing unit 6 performs image processing in accordance with the data associated with the binarizing method received from the transmission control unit 4 (S13), and supplies the processed data to the printer unit 7 to print it out (S14). Conventionally, many methods have been proposed as multi-value image binarizing methods. However, these methods have both merits and demerits, and no optimal binarizing method in all cases and in all respects is proposed. As typical binarizing methods, (1) the Bayer type dithering method, (2) the Fatning type dithering method, (3) the error diffusion method, and (4) the simple binarizing method are available. In the Bayer and Fatning type dithering methods, an image and a threshold value matrix are compared with each other, and if image data is larger than a corresponding threshold value, "1 is set; otherwise, "0" is set. Fig. 6 shows a threshold value matrix of the Bayer type dithering method, and Fig. 7 shows a threshold value matrix of the Fatning type dithering method. The feature of the Bayer type dithering method is that dots tend to be scattered upon binarizing of this method. Contrary to this, the feature of the Fatning type dithering method is that dots tend to be concentrated. Figs. 8 and 9 show image data obtained by binarizing a flat image having a luminance of 160 by the above-mentioned methods. Portions having "0" data, i.e., dark portions are scattered when an image is binarized by the Bayer type dithering method, while they are concentrated by the Fatning type dithering method. When these two binarized images are printed by, e.g., an ink-jet printer, considerably different images are obtained due to the influence of a blurred ink. More specifically, in an image binarized by the Bayer type dithering method, since dark portions, i.e., dots formed by an ink are scattered, an ink is considerably blurred. Contrary to this, in an image binarized by the Fatning dithering method, since dots are concentrated, an ink is not so blurred. As a result, the image binarized by the Bayer type dithering method becomes a darker image than that by the Fatning type dithering method. In this manner, if images binarized by the different binarizing methods are subjected to the same image processing, different colors are undesirably obtained. In order to obtain an optimal color image, image processing matching with a binarizing method used must be performed. In this embodiment, as the binarizing methods, the Bayer type dithering method, the Fatning type dithering method, the error diffusion method, and the simple binarizing method are available. In general, when a resolution is low, the Bayer type dithering method can provide a good image; otherwise, the Fatning type dithering method can provide a good image. The dithering methods are suitable for halftone images such as photographs, but are not suitable for images having clear contrast such as character images. Furthermore, the error diffusion method suffers from a large data volume after coding although a good image can be obtained. The simple binarizing method is not suitable for halftone images. Therefore, in this embodiment, a binarizing method is selected by an operator, or by automatic discrimination based on a resolution or a text or photograph mode, or by communicating with a receiver in step S2 in Fig. 4. More specifically, in this embodiment, the binarizing method can be selected manually or automatically in step S2. In this embodiment, binarizing methods of transmission data in a transmitter, and binarizing methods which are available upon image processing of received data in a receiver are expressed by predetermined codes, and these codes are exchanged between the two apparatuses as the above-mentioned data associated with the binarizing method. Thus, the binarizing means 21 to 24 and the image processing means 61 to 65 are selected with reference to the code data. Fig. 10 shows code data corresponding to the four binarizing methods described above. In this embodiment, a four-bit code corresponds to each binarizing method. As described above, a code is also assigned to a case wherein data is not binarized. When the code data is sent to a receiver upon transmission of image data, the binarizing method of the image data can be identified. Fig. 11 shows code data for informing the binarizing methods of an image communication apparatus to a destination apparatus. The code data is obtained by logically ORing the codes shown in Fig. 10. When this code data is sent to a destination apparatus, the binarizing methods of this image communication apparatus can be informed to the destination apparatus. In this embodiment, code data representing binarizing methods of the image processing unit 6 when the image data is received is common to the code data shown in Fig. 11. Note that the code data shown in Fig. 11 is registered in advance in a predetermined memory area of the transmission control unit 4. Figs. 12 and 13 are flow charts showing operations for discriminating, using the code representing the binarizing methods, whether or not a receiver can deal with a binarizing method of a transmitter. Fig. 12 shows an operation of the transmitter, and Fig. 13 shows an operation of the receiver. The transmission control unit 4 informs the code representing the binarizing methods to the receiver by means of a predetermined procedure signal (S21). More specifically, when the receiver is an image communication apparatus complying with the G3 facsimile standards, the code representing the binarizing method can be transmitted to the receiver using an initial identification signal of a non-standard function, e.g., specific four bits of an NSF signal. When the receiver is an image communication apparatus complying with the G4 facsimile standards, the code representing the binarizing method can be transmitted to the receiver using, e.g., specific four bits of a user-to-user signal based on the TTC recommendation, ISDN network interface part 3, layer 3 specification 4.5.24. On the other hand, the receiver receives the specific four bits of the above-mentioned NSF signal or the user-to-user signal (S31), and decodes it to identify the binarizing method (S32). When the receiver can deal with the binarizing method (S33), it transmits the binary code using the NSF signal or the user-to-user signal (S34); otherwise, it transmits a binary code other than the received binary code using the NSF signal or the user-to-user signal (S35), thereby informing whether or not the receiver can deal with the informed method. The transmitter receives the NSF signal or the user-to-user signal from the receiver (S22), and discriminates its specific four bits (S23), thereby discriminating whether or not the receiver can deal with the binarizing method represented by the transmitted binary code (S24, S25). Thereafter, if the transmitter discriminates that the receiver can deal with the informed method, it transmits image data binarized by the binarizing means corresponding to the informed binarizing method (S26). When the receiver cannot deal with the informed method, in step S35, it can transmit the code (in this embodiment, 1111 shown in Fig. 11) which is registered in the transmission control unit 4 and represents binarizing methods which are available in the receiver using the NSF signal or the user-to-user signal. In this case, the transmitter discriminates the specific 4 bits of the NSF signal or the user-to-user signal, thereby discriminating binarizing methods available in the receiver. Therefore, the transmitter compares the binarizing methods available in the receiver, and those available in the transmitter, and selects an optimal one. Thus, the transmitter can binarize data by a binarizing method available in the receiver, and can transmit the binarizing method and image data to the receiver by the above-mentioned transmission operation. When the transmission control unit 4 of the receiver discriminates the binarizing method informed from the transmitter, it supplies the discriminated binarizing method to the decoding unit 5 and the image processing unit 6. The image processing unit 6 selects the corresponding image processing means on the basis of the input binarizing method to perform image processing of the received image data, and supplies the processed data to the printer unit 7 to print it out. Note that the above-mentioned code data may be exchanged in advance with a specific image communication apparatus, and binarizing methods of the specific image communication apparatus, and corresponding binarizing methods may be registered in, e.g., a memory area of the transmission control unit 4. Thus, the binarizing means 21 to 24 and the image processing means 61 to 65 may be selected based on the registered code data. Code data of an image communication apparatus, which data cannot be registered in advance, can be exchanged in a communication protocol when image data is transmitted/received, as described above. Fig. 14 is a block diagram showing another arrangement of the image processing unit. More specifically, the image processing unit comprises a multi-value converting unit 71, a density conversion unit 72, a color masking unit 73, a γ correction unit 74, and a binarizing unit 75. The multi-value converting unit 71 performs multi-value conversion by smoothing processing. Another arrangement of the multi-value converting unit 71 will be described later. Figs. 15 and 16 show arrangements of smoothing filters for image data which are binarized by the Bayer type dithering method and the Fatning type dithering method, respectively. In this manner, when a multi-value conversion method is switched in accordance with the type of transmitted binary signal, a high-quality image signal can be obtained. For data binarized by the error diffusion method, a window for filtering binary data may be continuously moved, while for data binarized by a dithering method, the window may be intermittently moved without being moved continuously. The density conversion unit 72 searches a look-up table to convert image data from luminance (RGB) data to density (cmy) data. Different look-up tables may be prepared in accordance with binarizing methods, or the same table may be used for the different methods. The color masking unit 73 performs masking processing for correcting muddiness of inks in the printer unit 7. The γ correction unit 74 performs γ correction in correspondence with characteristics of the printer unit 7, and the binarizing unit 75 performs binarization for a printer output. Different sets of the masking unit 73, the γ correction unit 74, and the binarizing unit 75 may be prepared in accordance with binarizing methods, or they may be common to different binarizing methods. Alternatively, a common circuit may be employed, and only parameters may be changed in such a manner that a color masking parameter varies depending on binarizing methods. The image processing unit discriminates the binarizing method of a received image, and selects the image processing means 61 to 65 or the multi-value converting unit 71, the density conversion unit 72, the color masking unit 73, the γ correction unit 74, and the binarizing unit 75, thus performing optimal image processing. Figs. 17 and 18 are flow charts showing other transmission/reception operations. Fig. 17 shows an operation of a transmitter, and Fig. 18 shows an operation of a receiver. In this embodiment, when the transmitter transmits a binary code (S41), and the receiver receives the binary code (S51), if the receiver can deal with a binarizing method represented by the code (S52), it sends back the received binary code (S53); otherwise, it sends back a logical sum code (Fig. 11) representing all the usable binarizing methods of the receiver (S54). When the transmitter receives the code sent back from the receiver (S42), it compares the transmitted binary code and the received binary code (S43). If a coincidence is found between the two codes, the transmitter binarizes image data by one of the binarizing methods which can be dealt with in the receiver (S46); otherwise, the transmitter checks if the received binary code includes a usable binarizing method (S44). If the binary code includes a usable binarizing method, the transmitter selects the binarizing method (S45) to binarize image data, and then transmits the binary image data (S46); otherwise, it transmits image data without binarizing it. As still another embodiment, the image processing unit may comprise a correction unit for performing correction in correspondence with binarizing methods, and an image processing means common to the binarizing methods. For example, correction corresponding to the binarizing methods may be performed as follows. That is, a look-up table according to a 3 x 3 dot pattern having a pixel of interest as the central pixel is prepared, and a value is assigned to the pixel of interest. Furthermore, the Bayer type dithering method may be further classified to some compression methods, such as a 4 x 4 Bayer type dithering method, an 8 x 8 Bayer type dithering method, and the like. In the above embodiment, a code representing a binarizing method is transmitted to a receiver by means of a predetermined procedure signal. However, a binary code may be transmitted as first 4 bits of image data. In the above embodiment, when data associated with an n-value compression method is transmitted, it may be transmitted in advance or after image data is transmitted. Alternatively, the data may be transmitted during transmission of image data. In the above embodiment, in order to discriminate whether or not a receiver can deal with an n-value compression method of image data upon transmission of n-value data, a method based on the G3 or G4 recommendation may be employed. Alternatively, a code in the above-mentioned layer of the ISDN may be used. Such a protocol may be variously changed. In the above embodiment, a receiver informs a usable n-value compression method to a transmitter upon communication with the transmitter. In this information, for example, a user-to-user signal of the layer 3 specification in the TTC recommendation as described above, or other signals may be used. As the image processing means of the above embodiment, all or some of a 2 → multi-value converting method, γ correction, masking, and the like may be used. The above-mentioned embodiment has the following effects. Since data associated with an n-value compression method is transmitted together with n-value image data, a receiver can perform proper image processing. More specifically, the receiver can select an image processing means matching with the n-value compression method of received image data. Since image data is transmitted when a receiver can deal with n-value image data to be transmitted, transmission of n-value image data which cannot be dealt with by the receiver can be prevented. Image data can be transmitted by selecting a usable n-value compression method, and proper reproduction of an m-value image at a receiver can be guaranteed. In this embodiment, since a receiver transmits code identifying usable n-value compression method to a transmitter, the transmitter can perform proper n-value compression. In the above embodiment, data associated with a compression method executed by a transmitter is transmitted to a receiver. In the above embodiment, binarization is performed as n-value compression. However, the present invention is not limited to binarization, but may be three-value conversion, four-value conversion, and the like. In the above embodiment, image data is input from the scanner unit 1. However, the present invention is not limited to the scanner. For example, image data may be input from a video camera, a still video camera, or an image database. In the above embodiment, an image is output to the printer unit 7, but may be output to a CRT, an image database, or the like. In the above embodiment, R, G, and B data are processed as image data. Similarly, X, Y, and Z data or L*, a*, b* data representing color data, or Y, I, Q data used in a television signal may be used.

Claims: Image data transmission method in which m-value data representing intensity gradations are compressed to n-value data for transmission to a receiver where m and n are both positive integers where m is greater than n, the method being characterised in including a step of transmitting from the receiver to the transmitter data indicating n-value compression methods which can be dealt with by the receiver. A method according to claim 1 including the steps of: (a) transmitting from the transmitter to the receiver a code representing an n-value compression method available at the transmitter; (b) comparing the code received from the transmitter with codes stored at the receiver representing n-value compression methods which can be dealt with by the receiver; and (c) transmitting, from the receiver to the transmitter, either, (i) the received code, indicating that the compression method thereby represented can be dealt with by the receiver, or (ii) a code different from the received code, representing an alternative compression method which can be dealt with by the receiver. A method according to claim 1 including the steps of (a) transmitting from the transmitter to the receiver a code representing the totality of compression methods available at the transmitter; (b) comparing the code received from the transmitter with codes stored at the receiver to produce an output comprising a code representing which n-value compression methods which can be dealt with by the receiver; and (c) transmitting the output code from the receiver to the transmitter. A method according to claim 3 in which, if the output code produced in step (b) represents that none of the compression methods can be dealt with by the receiver, the m-value image data is transmitted uncompressed. A method according to any preceding claim in which the n-value image data are binary data. A method according to any preceding claim in which the m-value data are colour image data. A method according to any preceding claim in which the compression method includes a binarizing method based on a dithering method. A method according to any of claims 1 to 6 in which the compression method includes a density preservation type binarizing method. A method according to any preceding claim in which the m-value image data is a user-to-user signal provided on an ISDN network. A receiver for receiving compressed image data and a code representing a compression method, the receiver comprising: means for receiving compressed image data; and means for receiving a code representing the compression method by which the received image data has been compressed; and means for processing data in accordance with the compression method; the receiver being characterized in further comprising: means (103-107) for storing codes representing compression methods which can be dealt with by the receiver; and means for transmitting data indicating n-value compression methods which can be dealt with by the receiver, where n is a positive integer. A receiver according to Claim 10 further comprising: comparison means for comparing the code representing the compression method with the stored codes to determine whether the received code represents a compression method which can be dealt with by the receiver; and means for transmitting a code to indicate a compression method which can be dealt with by the receiver depending on the comparison result in the comparison means. A transmitter for transmitting compressed image data characterised by comprising: means for transmitting to a receiver a code representing an n-value compression method available at the transmitter, where n is a positive integer; and means for receiving code from the receiver indicating whether the compression method represented by the transmitted code can be dealt with by the receiver.

Inventors: KOBAYASHI SHIGETADA, SUGIYAMA MITSUMASA, KOBAYASHI, SHIGETADA, SUGIYAMA, MITSUMASA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N1
IPC codes: H04N1, G06T9, G06K9