FI92271B - Coding method - Google Patents

Description

92271 CODING METHOD

The invention relates to an encoding method as defined in the preamble of claim 1, in particular for encoding a live video signal for a narrow transmission channel.

The transmission of a video signal typically requires a transmission channel with a throughput of 2 to 70 Mbit / s.

When the video signal is compressed by detecting changes or differences in successive picture fields by encoding and transmitting them, the transmission channel capacity can be reduced between 64 Kbit / s and 2 Mbit / s.

Known methods for compressing a live video signal are generally based on the procedure of dividing an image field 15 consisting of e.g. 256 x 256 pixels, i.e. pixels, into groups of pixels, i.e. clusters, e.g. containing 8x8 pixels. The use of clusters can reduce the number of address bits to be transmitted. The address of a single pixel, typically determined by 6 bits, located at an arbitrary location in the 256 x 256 pixel image field, requires 16 bits. When using clusters such as 8x8 pixel clusters, a common address of 10 bits is used for all pixels in the cluster, such as 64 pixels.

25 The disadvantage of cluster-based coding methods is their inability to deal efficiently and accurately with the edge regions of moving objects.

They either waste the capacity of the transmission channel by retransmitting permanent information or they cause the edge of the moving object to blur, i.e. the so-called dirty window effect.

The object of the invention is to provide a new coding method by means of which the disadvantages of the cluster-based coding methods described above can be eliminated. The object of the invention is in particular to provide a new coding method by means of which successive image fields are coded on the basis of pixels and 92271 2 in which the address of the pixels in particular is compressed so as to achieve a clear saving in channel capacity.

The method according to the invention is characterized by what is stated in claim 1.

In the encoding method according to the invention, in particular for encoding a live video signal for a narrow transmission channel, each image of the video signal to be transmitted is sequentially processed by retrieving a reference image stored in the memory and comparing the images and transmitting only the changed image areas to the receiver. According to the invention, each image is handled in pixel format; the image and the reference image are compared with each other by a pixel and a difference image is formed based on the differences in the grayscale of the pixels; the difference image consisting of a continuous grayscale scale is quantized according to a preset grayscale scale into a set of 20 sub-images, each sub-image being a binary image, each pixel having a binary value of 1 or 0 depending on whether a certain amount of grayscale grayscale is valid on that pixel; all subpictures are then encoded using commonly known-25 and binary image encoding methods and transmitted via a transmission channel to the receiver; the binary images are received at the receiver and decoded by a corresponding binary image decoding method, after which the image information 30 obtained from the image memory of the receiver is updated with the received difference information pixel by pixel and read from the image memory as a video signal for further processing.

In one application of the method, the difference image is divided according to the magnitude of the grayscale difference into 35 limited numbers of 2k + 1 independent sub-images, where • j * 92271 3 • difference 0 zero 5 1 smallest difference (positive) 2 second smallest difference (positive) 3 third smallest difference (positive)

• · I

k maximum difference (positive) 10 k + 1 smallest difference (negative k + 2 second smallest difference (negative) • »· 2k maximum difference (negative) 15 and where each subimage 2k + 1 comprises nxm pixe-1s, each pixel having binary value 1 or 0 depending on whether a certain amount of grayscale difference is valid in the pixel in question or not.

In one embodiment of the method, the sub-images 20 are encoded with a pass code. In the run code, bits 1 and 0 are grouped into suitable groups having the same binary value. The number of ones or zeros in the groups is reported as a number forward through the transmission channel to the recipient.

25 In one embodiment of the method, the bridging method uses a bridging method to shorten the coding. This means that separate opposite binary values in a given group of ones or zeros are eliminated and interpreted as the same binary values that the other members of the majority of the group represent.

In one application of the method, the color image is divided, as is known per se, into three sub-color images, such as RGB or YUV, each of which is treated separately as its own grayscale image. In this way, the color video image can be transmitted to the transmission channel encoded by the method according to the invention.

Regarding the advantages of the invention, we state the following. Although the 92271 4 image is quantized into a set of binary images, the coding of address information is significantly saved so that the amount of information input to the transmission channel is clearly reduced. A further advantage of the invention is that the quantization of the image into binary images is a straightforward operation and no distortions can occur. All grayscale differences are transmitted according to the quantization steps and a similar exact copy of the difference image is produced at the receiver. A further advantage of the invention is that if the limited distortion is accepted, the bridging method can be used efficiently in connection with the compression method according to the invention.

The invention will now be described with reference to the accompanying drawing, in which Figure 1 schematically shows the conversion of a grayscale image into a set of binary images; Figure 2 shows in block diagram form a transmitter using the method according to the invention; and Figure 3 shows 20 receivers using the method according to the invention.

A digital video signal containing image information to be transmitted by the pulse code modulation method requires a transmission channel capacity, the magnitude of which is determined by the number of pixels to be transmitted, grayscale, and the number of images to be transmitted in se-25 municipalities. The information to be transmitted can be reduced without degrading the image quality by transmitting only a video signal representing the change between successive images. To do this, one way or another is to look for areas in the image that have changed from the previous 30 images, which are thus only transmitted. In addition, the information of the transmitted changed image areas must be provided with addresses indicating which area of the image is in question. This makes proper image reconstruction possible. One such video compression method is disclosed in Finnish Patent Publication 70662.

A pixel or pixel located in a man-made position in the image field / can be represented by the following data: pixel value (gray level) + pixel location address (position coordinates, i.e. x-address + y-address). For example, if the image comprises 256 x 256 pixe-5, the total pixel address is 16 bits (the x and y addresses both require an 8-bit address) and the pixel value is mapped at 64 grayscale levels (6 bits), then the amount of information to be transmitted is next: 6 bits (pixel value) + 16 bits (address) = 24 bits / pixel.

10 From this example, it can be seen that a large part of the transmission capacity is dedicated to the transmission of pixel addresses (75% in the example).

The coding method according to the invention is directed in particular to the addresses of the pixels. According to the invention, the 15-pixel coding method specifically implements address compression so that considerably fewer bits per pixel are transmitted on the transmission path. The invention relates to a method for converting an image, in which a grayscale image is converted into an organ-20 set of binary images which carries address information and which can be further processed using binary image coding methods known per se.

The method according to the invention will be described below with reference to Figure 1. The method according to the invention utilizes a video compression method in which only a video signal representing a change between successive images is transmitted to the transmission channel. Thus, a difference image 3 is formed from the grayscale image 1 and the reference image 2 of the video signal 3. All said images are formed of n x m pixels, where n 'and m are integers. The difference image 3 is formed by comparing the grayscale image of the video signal from 1 pixel pixel to the corresponding pixels of the reference image 2. Only the quantized image 4 is formed by the difference 3. This means that the difference image 3 is divided according to a predetermined grayscale scale into a limited number 2k 92271 6 + 1, where k = an integer, independent sub-images.

This can be presented in the form of the following table: # difference 5 0 zero 1 smallest difference (positive) 2 second smallest difference (positive) 3 third smallest difference (positive) 10 k maximum difference (positive) k + 1 smallest difference (negative) k + 2 second smallest difference (negative) • · «15 2k largest difference (negative)

Each subimage 2k + 1 comprises n x m pixels, each pixel having a binary value of 1 or 0. The binary value of 1 describes the existence of 20 grayscale differences of a certain magnitude. A binary value of 0 describes the absence of this difference.

The grayscale of the image is then, for example, divided into 64 levels, and during the quantization process the grayscale of each pixel is checked and then classified into a specific sub-image corresponding to this grayscale. The other sub-25 differences have a binary value of 0 for this pixel. Thus, all sub-images have n x m pixels with a binary value of 0 or 1 for each pixel.

All binary images 2k + 1 implemented as described above can now be coded using coding methods known per se, which are specifically suitable for coding binary images. We refer to e.g. publications: 1) Hunter, R. & Robinson, A.H .: International Digital Facsimile Coding Standards, Proc. Of IEEE, Vol. 68, no.

35 7, July 1980, p. 854-867; and 2) Yasuda, Y .: Overview of Digital Facsimile Coding Techniques in Japan, Proc. of IEEE, Vol. 7, J »92271 7

July 1980, ss. 830-845.

Each binary image is most preferably encoded using a passcode. This means that the binary values of each binary image are grouped into groups of zeros 5 and ones, from which the binary value and their number are successively transmitted to the transmission channel. Such pass codes are in general use e.g. facsimiles.

In connection with the coding method according to the invention, a bridging method can be used if limited distortion in the picture transmission is allowed. By bridging method is meant a general process by which the coding of a pass code can be improved. In the bridging method, for example, bit "0" is changed to bit "1", 15 if the two bits preceding it are both "1" and the next two bits are also "1". For example, if the bit string is: 11011, the intermediate bit "0" is changed to one when encoding the run code. It should be noted that bridging rules can vary considerably.

The coded binary images are received by a receiver, where the used code is decoded and the information corresponding to the difference image is obtained. The difference is compared with the reference image of the receiver's image memory pixel by pixel, and thus a new image is formed, which is stored in the image memory. This image is still available from the receiver's memory.

A video signal transmitter for carrying out the method according to the invention is shown in Figure 2. The video signal is fed to the transmitter via an A / D converter 6, in which converter it is converted to digital form. The digital video signal is further input to the first image memory 7. A reference image is stored in the second image memory 8. In each image memory 7, 8, the image is in pixel format as an n x m matrix. The digital video-35 image is input from the first memory 7 pixels at a time to the comparator 9 simultaneously and in the same phase as the second image memory 8, the reference image is also fed to the comparator 9. The comparator 9 generates a difference image which is further fed to the grayscale comparator 10. The grayscale comparator 10 includes ROM from the grayscale scale of Table 5 stored in memory. This table includes a total of 2k + 1 shades of gray according to the table above. The difference image input to the grayscale comparator 10 is compared with the grayscale stored pixel by pixel, and the image is divided into sub-images, i.e., 10 binary images, for a total of 2k + 1, or as many grayscales stored in the ROM. Each sub-picture 2k + 1 has its own memory unit 11; li1 - ll2k + 1. These memory units are preferably RAM memories. The memory units 11 of the sub-images 15 are each connected to their own flow code 12; 12 to 12 and further to the selector 13 connected to the transmission channel 14. The sub-picture memories 11 are individually scrolled in a certain order and the bit maps of the binary images containing 20 n x m bits are encoded by the flow encoders 12 and further fed to the transmission line 14 via the selector 13.

The receiver applying the method according to the invention is shown in Fig. 3. The coded video signal from the transmission scheme 14 is input to the decoder 15 and further to the comparator 16. The comparator 16 is fed with the received decoded video signal a pixel by pixel stored in the image memory 17 and compared with each other. The resulting new image data is fed back to the image memory 17 and thus the image memory is updated. The updated image memory 17 is decoded through the D / A converter 18 to form a video signal which is fed to a display device or a suitable image processing device or the like.

The transmitters and receivers described above further comprise a control unit 19 and 20, respectively, by means of which the transmitter and receiver & 92271 9, respectively, are suitably controlled.

The invention has been described above only in one preferred embodiment by overcoming it, but it is clear that the invention can be modified in many ways within the scope of the appended claims.

Claims (5)

1. Encoding method in particular for encoding a live video signal for a narrow transmission channel, wherein the video signal method to be transmitted each image is taken for successive processing such that a reference image previously recorded in the image memory is retrieved from the memory and the images are compared. with each other, and only the altered image areas via the transmission channel are transmitted to the receiver, by which method - each image is processed in pixel form (nxm); and - the image and the reference image are compared between pixels by pixel and due to the differences in their gray color tones, a quantized difference image is formed; characterized in that - the difference image, which consists of a limited quantized gray hue scale, is transmitted to a plurality of sub-difference images, each of which is a binary image, each of which has a binary value of 20 or 0 all because whether a difference value of some size is in force in this pixel or not; - all sub-difference images are encoded using the close-up coding method and transmitted via the transmission channel to the receiver; 25. The binary images are received in the receiver and decoded by the corresponding binary image decoding method, after which the image information obtained from the receiver image memory is updated with the pixel received pixel difference information and is extracted from the image memory as a video signal for further processing. 2. A method according to claim 1, characterized in that the difference image is divided according to the gray color scale according to the size of the difference into a limited number of 2k + 1 independent sub-difference images, where: 13 92.71 # difference 0 noil 1 the smallest difference ( positive) 5. the second smallest difference (positive) 3 the third smallest difference (positive) k the largest difference (positive) 10 k + 1 the smallest difference (negative) k + 2 the second smallest difference (negative) ♦ · · 2k is the largest difference (negative) and wherein each subdifference image 2k + 1 comprises nxm pixels, each pixel having the binary value 1, which describes a certain gray hue difference in magnitude in force in the respective pixel, or 0, which describes the lack of this difference . 20 Method according to claim 1 or 2, characterized in that the sub-difference images are encoded with run codes (RLC). Method according to Claim 3, characterized in that the bridging code is used in the running code for shortening the coding. Method according to any of the preceding claims, characterized in that the color image is divided into three sub-color images (RGB or YUV) known per se, each of which is treated separately as a separate grayscale image.