Image encoding apparatus

Abstract: An image encoding apparatus which enables to improve an efficiency of a motion compensate encoding includes a frame memory unit (12) for storing a preceding encoded and partially decoded image signal, a first and a second motion compensated prediction units (11, 14) connected to the frame memory unit (12) for predicting a motion compensation between predetermined image signals and the image signal stored in the frame memory unit (12), and a predicted value selection unit (15) connected to the first and second motion compensated prediction units (11, 14) for comparing a first prediction value outputted from the first motion compensated prediction unit (11) with a second prediction value outputted from the second motion compensated prediction unit (14) and for selecting the first prediction value or the second prediction value in accordance with a compared result.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image encoding apparatus for encoding information contained in a video signal (time-varying image) so as to reduce an amount of information contained in the video signal. 2. Description of the Related Art Recently, with developments of high-speed digital signal processing and LSI techniques therefor, with the progress of image processing techniques, making efficient use of image information is expected. In particular, in the field of communication, active studies of a high-functional network enabling to transmit image information efficiently are carried on, thereby, for B-ISDN which is a wide-band communication network of the next generation, it is expected that very high quality image transmission service is supplied. In general, it is unpractical to process a video signal as it is, since information contained in an image information is too much. However, since much redundancy is involved in its information of a video signal, amount of information can be reduced by removing the redundancy. Because of this, (compressing) encoding technique of video is very important for efficiently processing the video information, and active studies are conducted about a video encoding system therefor. A video signal comprises temporal information about movement, change, etc. and spatial information about the contents of a piece of video frame, both of which include redundancy. The amount of information can be greatly reduced by reducing time redundancy since successive video frames have much correlation, particularly in a video (time-varying) signal. A motion compensated interframe prediction encoding system is widely used in general as an encoding system by means of such time correlation. An example of a know image encoding apparatus is described below. An inputted video signal is sent to a motion compensated predictor in non-overlapping rectangular blocks by each video frame. The motion compensated predictor effects a motion compensated prediction between the inputted video signal and a video signal inputted prior to the inputted video signal and stored in the frame memory, to detect and output a motion vector. A block judged to be the most appropriate is read out as a predicted value from the frame memory, resulting from the above motion compensated prediction. The difference operator obtains the difference between the predicted value and the inputted image signal to output a prediction error signal which is sent to an error encoder. The error encoder effects appropriate encoding such as DCT encoding by means of a correlation of a prediction error signal inside a video frame, resulting in a encoded signal to be outputted to the outside of the device together with a motion vector from the motion compensated predictor. On the other hand, the encoding signal is locally decoded in the error decoder to output a prediction error decoding signal which is added at the adder again to a predicted value read out from the frame memory to obtain a video signal reproduced after encoded. This reproduced video signal is sent to and stored in the frame memory. Upon completion of encoding a video frame, the frame memory stores a locally decoded and reproduced video signal of the video frame which is used as a predicted value of motion compensated prediction of the following video frame to be encoded. However, in the above-mentioned known image encoding apparatus in which encoded video frames are used one after another for prediction encoding of the following video frames, there might be a problem of uncertainty about the effects of motion compensated prediction encoding. In this respect, document EP-A 0 420 627 discloses a video signal coding method using a moving video signal transmission system comprising a delaying means, a frame memory means, two motion compensated prediction means and a prediction value selection means. The prediction values outputted from the respective motion compensated prediction means are compared and selected. Thereby, two prediction values of the current image frame to be encoded are formed from two already encoded image frames, respectively. The current image frame to be encoded is between a preceding and a next frame chronologically, and the order of input and display is different from the order of encoding. In addition, document EP-A-0 397 206 discloses an adaptive interframe prediction coded video communications system in which first and second motion vectors are derived and an interframe predicted error signal representative of the difference between an input frame and a motion compensated previous frame is generated. SUMMARY OF THE INVENTION The object of the present invention is to provide an image encoding apparatus which enables to improve the efficiency of the motion compensated encoding. According to the present invention, this object is accomplished by an image encoding apparatus as defined in claim 1. Further developments of the present inventions are subject of the accompanying dependent claims. In operation, a first motion compensated predicting means performs a first motion compensated prediction a second motion compensated predicting means performs a second motion compensated prediction, a prediction value selecting means compares the results of prediction by the above two motion compensated predicting means so as to enable a good efficiency of the motion compensated prediction. As explained above, the image encoding apparatus of the present invention enables better efficiency in encoding, resulting in better quality of encoded video in a system relating to video information. In addition, it enables also very good effects in a motion compensated prediction encoding in a high quality video encoder expected to be realized, resulting in great improvement in encoding efficiency, with application of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of an embodiment of an image coder of the present invention; Fig. 2 is a flowchart showing the working of an embodiment of an image coder of the present invention; Fig. 3 is a flowchart showing the working of predictive signal selector in an embodiment of an image coder of the present invention; and Fig. 4 is an explanatory drawing to explain the principle of a motion compensated prediction encoding system. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the accompanying drawings, an embodiment of an image encoding apparatus according to the present invention will be described in details. Fig. 1 shows a construction of the image encoding apparatus according to an embodiment of the present invention. As shown in Fig. 1, the image encoding apparatus includes a first motion compensated predictor 11 serving as first motion compensated predicting means, a frame memory 12 connected to the first motion compensated predictor 11 and serving as frame memory means, a frame delay unit 13 serving as frame delaying means, a second motion compensated predictor 14 connected to the frame delay unit 13 and serving as second motion compensated predicting means, a prediction signal selector 15 connected to the frame memory 12 and serving as a prediction value selecting means, an error encoder 16 connected to the prediction signal selector 15, an error decoder 17 connected to the error encoder 16, and an adder 18 connected to both the prediction signal selector 15 and the error decoder 17. The first motion compensated predictor 11 performs a motion compensated prediction between an inputted video signal and a video signal stored in the frame memory 12. The frame delay unit 13 outputs an inputted video signal with a time delay equivalent to an appropriate number of frames. The second motion compensated predictor 14 performs a motion compensated prediction between a delayed video signal and the video signal stored in the frame memory 12. The prediction signal selector 15 compares a predicted signal outputted from the first motion compensated predictor 11 and a predicted signal outputted from the second motion compensated predictor 14 for an appropriate selection to output two types of prediction error signals. The error encoder 16 appropriately encodes the prediction error signals to output encoded signals corresponding to the prediction error signals, respectively. The error decoder 17 partially decodes one type of encoded signal to output a prediction error decoded signal. The adder 18 adds a prediction error decoded signal to a predicted value to output a reproduced video signal after encoded. The reproduced video signal is sent to and stored in the frame memory 12. Now, an operation of the image encoding apparatus shown in Fig. 1 is described by referring to a flowchart as shown in Fig. 2 . The frame delay unit 13 delays an inputted video signal (step 201) with a time equivalent to an appropriate number of frames (step 203). The second motion compensated predictor 14 performs a motion compensated prediction between a delayed video signal and a video signal stored in the frame memory 12 (step 204) to read out an appropriate area, while the first motion compensated predictor 11 parallelly performs a motion compensated prediction between an inputted video signal and a video signal stored in the frame memory 12 (step 204) to read out a predicted value in the same way. Both of these predicted values are compared with each other at the prediction signal selector 15 (step 205) to separate the prediction error signals strongly correlated with video frames to be subsequently encoded and those weakly correlated therewith for a selection (step 206). The prediction error signals are appropriately encoded at the error encoder 16 e.g. by DCT encoding by means of correlation inside an image frame (step 207) to be outputted to an outside of the device in the form of encoded signals (step 208). An encoded signal corresponding to the above strongly correlated prediction error signal is decoded at the error decoder 17 (step 209) in a manner inverse to the encoding at the above error encoder 16. A prediction error decoded signal is added at the adder 18 again to a predicted value (step 210) to obtain a reproduced signal after encoding. This reproduced video signal is sent to the frame memory 12 to be stored (step 211) in order to be used as a predicted value for motion compensated prediction of a video frame to be subsequently encoded. Now, an operation of the prediction signal selector 15 is described referring to a flowchart shown in Fig. 3. A predicted value of the above inputted video signal and a predicted value of the above inputted and delayed video signal are inputted to the prediction signal selector 15 (step's 301 and 302), while a delayed original video signal is also inputted (step 303). The delayed video signal is a current video frame to be encoded, in the present invention taking an advantage of a correlation between a video frame to be subsequently encoded and a temporarily prior video frame. Then, an appropriate motion interpolation is effected to a predicted value so as to carry out a time-position adjustment of a predicted value of an inputted video signal (without delay) for a video frame to-be encoded (step 304), followed by obtaining the respective differences between both of the above predicted values and the delayed original video signal (steps 305 and 307) and computing the respective differential squared sums (steps 306 and 308). The above respective differential squared suns are compared (step 309) to select and output the predicted value of the smaller differential squared sum (step 310), while the difference between the selected predicted value and the delayed original video signal is outputted as a prediction error signal (step 311). for the purpose of avoiding reduction in encoding efficiency at a current video frame to be encoded and keeping a correlation with the following image frame. Fig. 4 shows the principle of the motion compensated prediction encoding system. In the motion compensated prediction encoding system, an image frame to be encoded and a previously encoded image frame are respectively divided into appropriate rectangular blocks to make comparison at each block. A block well compatible with a block to be encoded is found out of previously coded image frames to detect a motion vector indicating the direction of the position of the compatible block relative to a block in the same position with a block to be coded. Then, the motion vector is used to represent motion information involved in a video signal to represent a predicted value of the block to be encoded by the compatible block in the said image frame.

Claims: An image encoding apparatus comprising: a frame delaying means (13) for delaying an inputted video signal for a predetermined time and for outputting a delayed video signal; a frame memory means (12) for storing a locally decoded video signal; a first motion compensated prediction means (11) for providing a first motion compensated prediction signal, said first motion compensated prediction means (11) being connected to said frame memory means (12) and providing a first predicted value; a second motion compensated prediction means (14) for providing a second motion compensated prediction signal, said second motion compensated prediction means (14) being connected to said frame memory means (12) and providing a second predicted value; a prediction value selection means (15), connected to both said first motion compensated prediction means (11) and said second motion compensated prediction means (14), for receiving said first and second motion compensated prediction signals, for performing a comparison based on said motion compensated signals, for selecting said first predicted value or said second predicted value in accordance with said comparison, for outputting a selected prediction value, and for outputting the difference between said selected prediction value and said delayed video signal as a prediction error signal; characterized in that: said first motion compensated prediction means (11) is arranged to derive said first motion compensated prediction signal from said inputted video signal and said locally decoded video signal stored in said frame memory means; said second motion compensated prediction means (14) is arranged to derive said second motion compensated prediction signal from said delayed video signal and said locally decoded video signal stored in said frame memory means; and said prediction value selection means (15) is arranged for performing said comparison by obtaining the respective differences between said delayed video signal and both of said first and second predicted values, by computing the respective differential squared sums, and by comparing said respective differential squared sums, and is further arranged for selecting the predicted value corresponding to the smaller differential squared sum as said selected prediction value. An image encoding apparatus according to claim 1, characterized by an error encoding means (16) connected to said prediction value selecting means (15) for encoding said prediction error signal. An image encoding apparatus according to claim 2, characterized by an error decoding means (17) connected to said error encoding means (16) for locally decoding said encoded prediction error signal so as to output a prediction error decoded signal. An image encoding apparatus according to claim 3, characterized by an adding means (18) connected to both said prediction value selection means (15) and said error decoding means (17) for adding said prediction error decoded signal to said prediction value. An image encoding apparatus according to one of the preceding claims, characterized in that said video signal is a time-varying image signal.

Inventors: HIBI KEIICHI, HIBI, KEIICHI, HIBI, KEIICHI, SHARP-KASHIWA-RYO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N7
IPC codes: H04N11, H04N7, G06T9, H03M7