JP2008124937A - Image encoding apparatus - Google Patents

Abstract

An object of the present invention is to provide an image encoding apparatus that performs encoding with stable image quality by determining an appropriate target code amount for the first image immediately after the start of encoding.
A frame memory (101) for storing a plurality of frames of input moving image data, an encoder (102) for encoding moving image data in the frame memory, and an arithmetic unit for calculating screen complexity (103), and the encoder reads out a part of the moving image data to be encoded immediately after the start of encoding from the frame memory and temporarily encodes the arithmetic unit, Based on the result, the image complexity is calculated, and the encoder encodes the moving image data based on the image complexity to provide an image encoding device.
[Selection] Figure 1

Description

  The present invention relates to an image encoding device.

  The MPEG system is widely known as an image compression encoding system. The MPEG system is a high-efficiency encoding system that compresses and encodes an image by motion compensation predictive encoding and DCT. An image is composed of an I picture that performs intra-frame code, a P picture that performs forward prediction code, and a B picture that performs bidirectional prediction, and is grouped around 15 frames called GOP.

  In the MPEG system, the amount of code generated varies greatly depending on the picture pattern. A fine and complex picture image generates a large amount of code, and a flat and simple pattern image generates a small amount of code. Therefore, code amount control is performed to keep the generated code amount constant. There is TM (Test Model) as a typical method of code amount control. In this method, control is performed as follows so that the code amount is almost the same in units of 1 GOP.

  When the target bit rate is BitRate, the number of frames per second is PictureRate, and the number of GOP frames is N, the code amount R allocated to 1 GOP is given by the following equation.

    R = (BitRate / PictureRate) * N (Formula 1)

  The code amount R of 1 GOP obtained by Expression 1 is distributed for each picture in 1 GOP. Distribution to each picture is performed as follows for each frame to be encoded. First, the screen complexity of an I picture, a P picture, and a B picture is obtained from the product of the generated code amount of an already encoded picture of the same type and the average quantization scale, and is set as Xi, Xp, and Xb. Next, the number of unencoded P and B pictures in the GOP is set to Np and Nb, and the quantization scale setting ratios of the P and B pictures for the I picture are set to Kp and Kb, respectively. Target code amounts Ti, Tp, and Tb of each picture are obtained by the following equations. max [a, b] indicates a process of selecting a larger one of a and b.

Ti = max [R / [1 + Np * Xp / (Xi * Kp) + Nb * Xb / (Xi * Kb)], Bitrate / (8 * PictureRate)] (Formula 2)
Tp = max [R / [Np + Nb * Kp * Xb / (Xp * Kb)], Bitrate / (8 * PictureRate)] (Formula 3)
Tb = max [R / [Nb + Np * Kb * Xp / (Xb * Kp)], Bitrate / (8 * PictureRate)] (Formula 4)

  The code amount R is subtracted from the generated code amount actually generated in one frame every time encoding of one frame is completed, and the value of Expression 1 is added every time encoding of 1 GOP is started.

  When the target code amount of the picture is determined in this way, the code amount control is performed in units of macroblocks of 16 pixels × 16 pixels. Code amount control in units of macroblocks is performed so that the quantization scale is adjusted according to the code amount generated for each macroblock and approaches the target code amount. Such an encoding apparatus is disclosed in Patent Document 1.

Japanese Patent Laid-Open No. 10-164577

In the conventional method described above, the code amount generated in the past and the average quantization scale are used to calculate the screen complexity Xi, Xp, and Xb for each picture, but the first (first) image immediately after the start of encoding is used. Since there is no image encoded in the past, it is necessary to use the default screen complexity. For this reason, when the actual complexity is different from the default value, appropriate code amount allocation is not possible, and there is a problem that the image quality may be improved or deteriorated continuously for several frames depending on the frame. It was.

  The present invention has been made in view of such a situation, and an image encoding apparatus that performs encoding with stable image quality by determining an appropriate target code amount for the first image immediately after the start of encoding. The purpose is to provide.

  An image encoding device of the present invention includes a frame memory that stores a plurality of frames of input moving image data, an encoder that encodes moving image data in the frame memory, and an arithmetic unit that calculates screen complexity. The encoder reads a part of moving image data to be encoded immediately after the start of encoding from the frame memory and temporarily encodes the arithmetic unit, and the arithmetic unit is based on the result of the temporary encoding. Then, the screen complexity is calculated, and the encoder performs the main encoding of the moving image data based on the screen complexity.

  The screen complexity is calculated based on the result of provisional encoding of the image data, whereby the code amount allocation and the target code amount of the main encoding can be obtained. For this reason, encoding with stable image quality is possible immediately after the start of encoding.

  FIG. 1 shows a configuration example of an image encoding apparatus according to an embodiment of the present invention. In the figure, reference numeral 101 denotes a frame memory having an area capable of storing a plurality of frames of input moving image data, and is configured such that writing and reading can be performed separately for an arbitrary frame area. The frame memory 101 has a role of storing input images of a plurality of frames in order to rearrange the frames at the preceding stage of the encoding process.

  Reference numeral 102 denotes an MPEG (Moving Picture Experts Group) encoder, which encodes moving image data with an arbitrary picture type for each picture. An MPEG encoder (encoder) 102 sequentially encodes moving image data in the frame memory 101 in units of frames by the MPEG method.

  A controller 103 controls the frame memory 101 and the MPEG encoder 102. The controller 103 instructs a frame number for writing to the frame memory 101 for each frame and controls in which frame area the inputted image data is written. The controller (calculator) 103 calculates the screen complexity.

  In addition, the controller 103 instructs the frame memory 101 for a frame number for each frame, and controls which frame area the image data is read from the MPEG encoder 102 from.

  Further, the controller 103 sets parameters necessary for encoding such as a picture type and a target code amount for the MPEG encoder 102 for each frame, and further instructs start and stop of encoding.

  The processing of the image coding method by the image coding apparatus configured as described above will be described with reference to the timing chart of FIG. 2 and the flowchart of FIG. FIG. 2 can also be expressed as showing an encoding sequence of the MPEG encoder 102. In FIG. 2, the horizontal axis is a time axis, one section represents one frame period of an image frame, and T0 to T11 represent respective frame periods. F0 to F8 represent frames of input moving image data. F0 is the first first image frame at the start of encoding, and F1 is the second image frame. In the present embodiment, 1 GOP is 9 frames, the encoded picture type is “BBIBBPBBP” in display order, and the encoding order is “IBBPBBPBB”. Note that the prediction direction of the first two B pictures in the display order is only backward. The first two B pictures in the display order correspond to F0 and F1. In the figure, the arrows are written so as to connect the period in which the image data is written and the period in which the data is read, and indicate the flow of the image frame. A broken line with an arrow indicates a data flow at the time of temporary encoding, and a solid line indicates a data flow at the time of main encoding. For convenience of explanation, 1 GOP is set to 9 frames, but the same can be realized even if 1 GOP is 15 frames or the number of other frames.

  When the controller 103 commands the start of encoding, the MPEG encoder 102 performs provisional encoding in the following manner to obtain the screen complexity Xi, Xp, and Xb in the period from T0 to T2 ( S301). That is, the MPEG encoder 102 reads out image data for two pictures (two frames) immediately after the start of encoding from the frame memory 101, and temporarily encodes the data with an I picture and a P picture. Here, the I picture is a picture generated by intraframe encoding, and the P picture is a picture generated by forward predictive encoding, which is one of interframe predictive encoding.

  First, in the period T0, the controller 103 writes the first image frame F0 immediately after the start of encoding in the moving image data input as the encoding target in the frame area 0 of the frame memory 101. Here, the MPEG encoder 102 does not operate.

  Next, in the period T1, the controller 103 writes the second image frame F1 immediately after the start of encoding among the moving image data input as the encoding target in the frame area 1 of the frame memory 101. At the same time, the controller 103 reads the image frame F0 from the frame area 0 of the frame memory 101, and the MPEG encoder 102 performs provisional encoding with the I picture for the image data of the image frame F0 (S301). A default value is used as the target code amount in the provisional encoding. The encoded data at the time of provisional encoding is not actually output, and the MPEG encoder 102 outputs the generated code amount and the average quantization scale value to the controller 103. The controller 103 calculates the screen complexity Xi of the I picture by the product of the generated code amount and the average quantization scale value (S302).

  Next, in the period T2, the controller 103 writes the image frame F2 in the frame area 2 of the frame memory 101. At the same time, the controller 103 reads the image frame F1 from the frame area 1 of the frame memory 101, and the MPEG encoder 102 performs provisional encoding of a P picture on the image data of the image frame F1 (S301). A default value is used as the target code amount here. In addition, as a reference image for inter-frame prediction, a local decoded image of the I picture of the frame F0 previously provisionally encoded is used. The encoded data at the time of provisional encoding is not actually output, and the MPEG encoder 102 outputs the generated code amount value and the average quantization scale value to the controller 103.

  The controller 103 calculates the screen complexity Xp of the P picture by the product of the generated code amount and the average quantization scale value (S302). The screen complexity Xb of a B picture (bidirectional predictive coding) is calculated by multiplying the screen complexity Xp of a P picture by a predetermined coefficient Kpb. Since the screen complexity Xb of the B picture is about 0.4 to 0.7 times the screen complexity Xp of the P picture, the coefficient Kpb is preferably about 0.5.

  As described above, the controller 103 calculates the screen complexity Xi of the I picture based on the result of provisional encoding with the I picture, and the screen complexity Xp and B of the P picture based on the result of provisional encoding with the P picture. Calculate the screen complexity Xb of the picture.

  Through the processing so far, the screen complexity Xi, Xp, and Xb are obtained from the image frames F0 and F1, and are used for calculation of the target code amount of the main encoding performed thereafter.

  Thereafter, the same encoding as normal is performed as the main encoding (S303). That is, the MPEG encoder 102 performs the main encoding by setting a target code amount based on the screen complexity Xi, Xp, and Xb. In the period T3, the MPEG encoder 102 encodes the image data of the image frame F2 stored in the frame memory 101 with an I picture. In the period T4, the MPEG encoder 102 encodes the image data of the image frame F0 stored in the frame memory 101 with a B picture. In the period T5, the MPEG encoder 102 encodes the image data of the image frame F1 stored in the frame memory 101 with a B picture. In period T6, the MPEG encoder 102 encodes the image data of the image frame F5 stored in the frame memory 101 with a P picture. As described above, encoding is sequentially performed as shown in FIG. At the time of encoding at least the first picture in the main encoding process or the target code amount calculation at the time of encoding the first GOP (Group Of Pictures), the screen complexity Xi, Xp, Use Xb information.

  As described above, in FIG. 3, in step S301, the MPEG encoder 102 reads part of the moving image data to be encoded immediately after the start of encoding from the frame memory 101 and temporarily encodes it (temporary encoding step). ). For example, the MPEG encoder 102 reads out image data for two frames immediately after the start of encoding from the frame memory 101, temporarily encodes one frame by intra-frame predictive encoding, and performs the other frame by inter-frame predictive encoding. Temporary encoding. Next, in step S302, the controller 103 calculates the screen complexity based on the result of the provisional encoding (calculation step). Next, in step S303, the MPEG encoder 102 performs main encoding of the moving image data based on the screen complexity (main encoding step).

  Therefore, the target code amount and the code amount allocation immediately after the start of the main encoding can be adjusted according to the information obtained by the temporary encoding, and the encoding with stable image quality can be performed immediately after the start of the encoding.

  In this embodiment, in order to stabilize the image quality immediately after the start of encoding and the amount of generated code, the first two frames are temporarily set as an I picture and a P picture using the free time for two frames for frame order change. By encoding, screen complexity is obtained and appropriate code amount allocation is enabled.

  According to the image encoding device of the present embodiment, the screen complexity Xi, Xp is determined by the generated code amount obtained by provisionally encoding the image frames F0 and F1 that are actual encoding targets, and the average quantization scale value. , Xb is calculated. As a result, an appropriate code amount allocation and a target code amount can be obtained, so that encoding with stable image quality can be performed immediately after the start of code.

  In addition, the periods T1 and T2 in which temporary encoding is performed are empty periods for changing the frame order, and it is not necessary to increase the processing time or the frame memory area when implementing this embodiment.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

It is a block diagram which shows the structural example of the image coding apparatus by embodiment of this invention. It is a timing diagram which shows operation | movement of the image coding apparatus by embodiment of this invention. It is a flowchart which shows operation | movement of the image coding apparatus by embodiment of this invention.

Explanation of symbols

101 frame memory 102 MPEG encoder 103 controller

Claims (10)

A frame memory for storing multiple frames of input moving image data;
An encoder for encoding moving image data in the frame memory;
An arithmetic unit for calculating the screen complexity,
The encoder reads a part of moving image data to be encoded immediately after the start of encoding from the frame memory and temporarily encodes it,
The computing unit computes screen complexity based on the result of the provisional encoding,
The image encoding device, wherein the encoder performs main encoding of the moving image data based on the screen complexity.   2. The image encoding apparatus according to claim 1, wherein the encoder reads out image data for two frames immediately after the start of encoding from the frame memory and performs temporary encoding.   The encoder temporarily encodes image data of the first frame immediately after the start of encoding by intra-frame encoding, and temporarily encodes image data of the second frame immediately after the start of encoding by forward predictive encoding. The image coding apparatus according to claim 2.   The image encoder according to claim 1, wherein the encoder performs the provisional encoding using a default value as a target code amount.   5. The image coding according to claim 1, wherein the computing unit computes the screen complexity based on a generated code amount and an average quantization scale value obtained by the provisional coding. apparatus.   6. The image coding apparatus according to claim 5, wherein the computing unit calculates a screen complexity of intra-frame coding, a screen complexity of forward predictive coding, and a screen complexity of bidirectional predictive coding.   The encoder reads image data for two frames immediately after the start of encoding from the frame memory, temporarily encodes one frame by intraframe encoding, and temporarily encodes the other frame by interframe predictive encoding. The image encoding apparatus according to claim 1, wherein   The computing unit calculates a screen complexity of intra-frame coding based on a result of provisional coding by the intra-frame coding, and forward predictive coding based on a result of provisional coding by the inter-frame prediction coding. The image encoding apparatus according to claim 7, wherein the image complexity and the screen complexity of bidirectional predictive encoding are calculated.   The image encoding apparatus according to claim 1, wherein the encoder performs a main encoding by setting a target code amount based on the screen complexity.   The image encoding apparatus according to claim 1, wherein the encoder performs encoding according to an MPEG system.

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