JPH0797861B2 - Interframe coding method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to digital transmission of an image signal, and particularly to a high-efficiency band by an inter-frame coding method that performs different quantization depending on a moving region of an input moving image signal. It relates to compression transmission technology.

(Prior Art and Problems) Conventionally, as an inter-frame coding method in which an input moving image signal is separated into motion and motion, and different quantization characteristics are applied to each motion region, Japanese Patent Application No. 59-169011 “Motion image signal” is used. Predictive Coding Device "is known, which applies a quantization characteristic having a large dead zone (a range in which an output is zero for an input) to a stationary area. However, in this method, the size of the dead zone changes abruptly due to the difference between the moving area and the still area, so that a tail-like screen stain remains after the object moves, and this area becomes the still area in particular. Since the dead zone is enlarged, there is a drawback that dirt remains forever. Further, since the selection of the quantization characteristic is usually made in units of a plurality of lines, in the still region after the scene change, a slight gradation difference may be noticeable at the boundary portion where the characteristic is changed.

(Object of the Invention) It is an object of the present invention to provide an encoding system that can obtain a high compression rate with little deterioration in image quality of a decoded image, particularly in a still region.

(Structure of the Invention) The present invention separates a motion region and a still region of a moving image signal from each other in predicting a frame of a moving image signal such as a television signal, and predicts a prediction error for a pixel included in the still region. Is characterized by performing coarse quantization and performing dense quantization for one frame in a plurality of frames (for example, a period of 10 frames or more) when the generated information amount is equal to or less than a predetermined value. It is an encoding method.

(Principle of the Invention) In the present invention, a moving image signal is first separated into a moving region and a still region. Several methods for this separation are already known. For example, as disclosed in Japanese Patent Application No. 59-194110 "Motion Image Separation Device", the screen is divided into blocks of a certain size, and the absolute value of the frame difference value of each pixel in the block is calculated within the block. There is a method of performing the addition and comparing the result of the addition with a predetermined threshold value to determine the motion of the block. Alternatively, in the method called the gradient method, from the brightness gradient in the frame and the frame difference value,
A motion vector for each pixel is obtained, and a motion region can be defined as a set of pixels whose motion vector is not zero. In the present invention, any of the dynamic and static separation methods may be used. Next, a method of performing dense quantization in an arbitrary frame only for pixels included in the still area will be described with reference to FIGS. FIG. 1 shows the result of performing dynamic-separation by any of the above-mentioned methods. It is assumed that the shaded portion in each frame in FIG. 1 is determined to be a static region by the motion / static separation. Information that is generated by noise in the still region can be suppressed by performing coarse quantization in the still region of the diagonal line of each frame. Next, dense quantization is performed on the entire static region of an arbitrary frame, for example, the diagonal line of the nth frame. A method of performing dense quantization will be described with reference to FIG. Usually, coarse quantization is performed on a portion determined to be a still region by the motion determination, for example, dense quantization is performed on a still region in the nth frame,
From the next frame to the P frame, coarse quantization is performed in the still region. Then, dense quantization is performed on the still area of the Qth frame. In this way, dense quantization is performed on the stationary region at an appropriate period with a certain interval. When the quantization of the still area is rough, the movement of the image or the scene change sometimes leaves a stain on the screen between the moving area and the still area or the still area. By doing so, it is possible to suppress the information generated by the noise in the still area and remove the boundary between the moving area and the still area and the stain on the screen in the still area after the scene change. Moreover, once the quantization is made dense, even if coarse quantization is performed thereafter, the screen will not become dirty in the still area as long as there is no movement or scene change. In this way, a coding method with a high compression rate can be obtained.

(Embodiment) An embodiment of the present invention will be described in detail with reference to FIG. Third
The figure is an embodiment of an encoder using the present invention. Signal line
The digitized moving image signal input from 101 is
It is supplied to the delay circuit 1 and the motion / static separation circuit 2. The motion determination signal calculated by the motion separation circuit 2 is supplied to the encoding control circuit 7 via the signal line 201. The moving image signal whose delay has been adjusted by the delay circuit 1 enters the subtractor 3. The subtractor 3 calculates a frame difference from the prediction signal delayed by one frame from the frame memory, and sends this to the quantization circuit 4 as a prediction error signal. When quantizing the prediction error signal, the quantizing circuit 4 uses a quantization switching signal from the coding control circuit 7 to switch between coarse and fine quantization with respect to the still region as shown in FIG. Take control. The moving area is quantized, for example, based on the occupied state of the buffer memory 11, which will be described later, as in the conventional case. Next, the prediction error signal quantized by the quantization circuit 4 is supplied to the adder 5 and the code converter 8.

The adder 5 adds the quantized prediction error signal and the prediction signal from the frame memory 6 and supplies the result to the frame memory 6 as a local decoded signal. The code converter 8 performs variable length coding and compresses the prediction error signal supplied from the quantization circuit 4. When performing variable-length coding, the code converter 8 performs variable-length coding corresponding to which quantization characteristic is selected by the quantization switching signal from the coding control circuit 7, and outputs the variable-length code. Output to the multiplexing circuit. The code converter 9 encodes which quantization characteristic is selected by the quantization switching signal from the encoding control circuit 7 as a mode code and outputs it to the multiplexing circuit. The multiplexing circuit 10 multiplexes the mode code and the variable length code and outputs them to the buffer memory 11. The buffer memory 11 is a transmission line
The encoding speed and the transmission path speed are matched so that the transmission speed of 1101 is kept constant. Further, the buffer occupancy indicating the occupied state of the buffer memory 11 is supplied to the encoding control circuit 7 via 1102.

Next, the encoding control circuit 7 will be described with reference to FIG. The encoding control circuit 7 is composed of a quantization switching unit 701 and a timer 702 as shown in FIG. Timer 70
The reference numeral 2 supplies a timing signal to the quantizing switch in an arbitrary cycle, for example, in units of several frames. See also timer 702
Monitors the amount of generated information by the buffer occupancy supplied from the buffer memory 11 via the signal line 1102, and does not output the timing signal when the generated information is large. The quantization switching unit 701 outputs a quantization switching signal that normally selects coarse quantization to a portion determined to be a still region by the motion determination signal supplied from the motion separation circuit 2 via the signal line 201, and the timer 702. Only when the timing signal is sent from the output terminal, the quantization switching signal for selecting the dense quantization over the entire stationary area is output. In this way, the motion determination signal and the buffer occupancy usually perform coarse quantization, and sometimes dense quantization is performed on the entire still region. At this time, if the generated information is large (that is, the signal line
When 1102 indicates that the buffer occupancy is large) coarse quantization is performed without dense quantization. Further, instead of supplying the signal 1102 indicating the buffer occupancy to the timer 702, the signal 1102 is supplied to the quantization switching unit 701, and when the buffer occupancy becomes large, the timing signal supplied from the timer 702 is changed. Even if it is configured to be ignored, almost the same effect can be obtained.

Next, the decoder will be described with reference to FIG. In the buffer memory 12, the compression-coded signal sent from the encoder at a constant speed through the transmission path 1201 is matched with the decoding speed, and the result is output to the code reverse exchangers 13 and 14. The code reverse exchange 13 extracts only the mode code from the multiplexed compression code and decodes it. The decoded signal (representing the quantization characteristic) is supplied to the code inverse switch 14 by 1301 and used for controlling the quantization switching. The code inverse exchange unit 14 performs decoding of the variable length code excluding the mode code of the multiplexed compression code based on this quantization characteristic, and outputs the decoded prediction error signal to the adder 15. . The adder 15 is
The decoded prediction error signal from the code inverse exchanger 14 and the prediction signal from the frame memory 16 are added and the decoded signal is supplied to the frame memory 16 and is also output from the decoder via the line 1501.

(Effects of the Invention) As described in detail above, while coarsely quantizing a still region to suppress information generated by noise in the still region, the image becomes finer and denser in the still region on a frame-by-frame basis. By performing the quantization, it is possible to remove the stain on the screen in the still area, which has been a problem in the past, by adding a small amount of information, and an interframe coding method with a high compression rate is realized. Thus, when the present invention is put to practical use, its effect is extremely large.

[Brief description of drawings]

1 and 2 are diagrams for explaining the principle of the present invention, FIG. 3 is a diagram for explaining an embodiment of an encoder according to the present invention, and FIG. 4 is a diagram for showing a configuration example of an encoding control circuit, FIG. 5 is a diagram for explaining an embodiment of the decoder according to the present invention. In the figure, 1 ... Delay circuit, 2 ... Motion / separation separation circuit, 3 ... Subtractor, 4 ... Quantization circuit, 5 ... Adder, 6 ... Frame memory, 7 ... Encoding control circuit, 8 ... Code conversion Device 9 ... Code converter, 10 ... Multiplexing circuit 11 ... Buffer memory 12 ... Buffer memory 13 ... Sign inverse converter, 14 ... Sign inverse converter 15 ... Adder, 16 ... Frame memory Is.

Claims (1)

[Claims] 1. In inter-frame predictive coding of a moving picture signal such as a television signal, a moving area and a still area of the moving picture signal are separated, and a prediction error for a pixel included in the still area is rough quantum. The inter-frame coding method is characterized in that, when the generated information amount is equal to or less than a predetermined value, one frame in a plurality of frames is densely quantized.