JPH10108197A - Image coder, image coding control method, and medium storing image coding control program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use a pre-filter adaptively depending on a characteristic of an input image while conducting coding in real time. SOLUTION: A coding difficulty calculation section 42 of a coding control section 15 calculates a coding difficulty denoting degree of difficulty of image coding. A pre-filter control section 51 controls a spatial filter 52 and a time filter 53 in a pre-filter section 50 depending on a ratio of the coding difficulty to a bit rate and on a ratio of the coding difficulty of a newest bidirectional coded picture (B picture) to the coding difficulty of a newest intra coded picture (I picture).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image encoding apparatus for compressing and encoding image data, an image encoding control method, and a medium recording an image encoding control program.

[0002]

2. Description of the Related Art A transmitting side compresses image data and transmits the compressed data, and a receiving side compresses and records the compressed image data.
In a compressed image recording / reproducing apparatus or the like which expands and outputs compressed image data at the time of reproduction, a method of compressing image data is, for example, MPEG (Moving Picture Experts).
Group) standard is a bidirectional predictive coding system. In this bidirectional prediction encoding method, encoding efficiency is improved by using bidirectional prediction. In the bidirectional predictive coding method, three types of coding are performed: intra-frame coding, inter-frame forward predictive coding, and bidirectional predictive coding.
Picture (intra coded picture), P picture (pred
ictive coded picture) and B picture (bidirectio)
nally predictive coded picture).

[0003] By the way, image data is compression-encoded by an image encoding device using a bidirectional predictive encoding system adopted in the above-mentioned MPEG standard, and the image data which has been compression-encoded is subjected to image decoding. In a system for decoding and decompressing by a device, the image quality after encoding and further decoding largely depends on the picture of the input video material. In other words, if the video contains many high-frequency components, or in the case of a rapidly moving pattern, the redundancy in the spatial and temporal directions is small, that is, the encoding difficulty is large.
When encoding at a certain bit rate and further decoding, encoding distortion is likely to occur.

[0004] In order to reduce the coding distortion when the video material having a large coding difficulty as described above is input to the image coding device, the input portion of the image coding device may be used in the spatial direction or the time direction. It is generally known to use a low-pass filter to reduce a spatial high-frequency component or a motion component of an input video material in advance.
A low-pass filter used for such a purpose is called a pre-filter or a pre-filter.

FIG. 7 shows an example of the configuration of an image coding apparatus provided with both a pre-filter in the spatial direction and a pre-filter in the time direction. This image coding apparatus includes a spatial filter 252 as a prefilter in a spatial direction and a temporal filter 253 as a prefilter in a temporal direction. The input image signal S ₁ It passes in order. The spatial filter 252 is formed of a low-pass filter in the spatial direction, and is specifically easily realized by a normal digital filter. The time filter 253 includes a low-pass filter in the time direction, and specifically, for example, a frame memory 254 that stores the output signal of the spatial filter 252 for one frame,
Output signal of spatial filter 252 and frame memory 254
, And an adder 255 that adds the signal of one frame before and stored with a predetermined weight K: (1−K) and outputs the result. Note that 0 ≦ K ≦ 1.

[0006] The image encoding apparatus further includes a temporal filter 2
53, an image rearrangement circuit 221 for rearranging the order of pictures (I picture, P picture, B picture) according to the encoding order, and input data of the image rearrangement circuit 221 for A scan conversion / macroblock circuit 222 for performing scan conversion and macroblocking of 16 × 16 pixels according to the result of the determination, and outputs data from the scan conversion / macroblock circuit 222. And a motion detection circuit 214 for detecting a motion vector based on the motion vector.

The image coding apparatus further includes a subtraction circuit 231 for obtaining a difference between output data of the scan conversion / macroblock conversion circuit 222 and predicted image data, and the subtraction circuit 231
DCT (Discrete Cosine Transform) is performed on the output data of
A T circuit 232, a quantization circuit 233 for quantizing output data of the DCT circuit 232, and a quantization circuit 233
Length encoding circuit 23 for variable length encoding of output data of
4, a buffer memory 2 for outputting the output data of the variable-length coding circuit 234 temporarily holds, as compressed image data S ₂ made from a bitstream of constant bit rate
35, an inverse quantization circuit 236 for inversely quantizing the output data of the quantization circuit 33, an inverse DCT circuit 237 for performing an inverse DCT on the output data of the inverse quantization circuit 236, and an inverse DCT circuit 237 for the inverse DCT circuit 237. An adding circuit 238 for adding the output data and the predicted image data and outputting the added data;
And a motion compensation circuit 239 that holds the output data of the P.38, performs motion compensation according to the motion vector sent from the motion detection circuit 214, and outputs the predicted image data to the subtraction circuit 231 and the addition circuit 238.

In the image coding apparatus shown in FIG. 7, a spatial filter 252 removes a spatial high-frequency component of an input image signal S ₁ , and a temporal filter 253 removes a motion component (or a frame) in an output signal of the spatial filter 252. Noise between them). Other operations in the image encoding device illustrated in FIG. 7 are the same as those of a general image encoding device, and thus description thereof is omitted.

[0009]

By the way, if the pre-filter as described above is always used, it is effective in reducing the information amount of a picture having a large coding difficulty, but a picture having a small coding difficulty, for example, Even in the case of still images, since the high-frequency components are dropped by the filter, there is a problem that the decoded image often gives a blurred impression as a whole.

On the other hand, non-compressed image data is preliminarily subjected to compression encoding (hereinafter, referred to as first-pass compression encoding) to estimate the data amount after compression encoding, and then to the estimated data amount. In the case of an encoding method that does not require real-time performance, that is, compression encoding (hereinafter referred to as second-pass compression encoding) is performed while controlling the amount of generated code based on the first-pass compression encoding. Based on the obtained data, a technique is also known in which a pre-filter is used in the second-pass compression encoding only for a location (temporal location) where the encoding difficulty is large. However,
This method has a problem that it is not possible to adaptively use a pre-filter according to the characteristics of an input image while encoding in real time.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to provide an image coding system capable of adaptively using a pre-filter in accordance with the characteristics of an input image while encoding in real time. It is an object of the present invention to provide a recording apparatus, an image encoding control method, and a medium recording an image encoding control program.

[0012]

According to a first aspect of the present invention, there is provided an image encoding apparatus comprising: an encoding unit for compressing and encoding input image data; and an encoding unit provided before the encoding unit to reduce an information amount of the input image data. A pre-filter for reducing, a coding difficulty calculating means for calculating a coding difficulty indicating a coding difficulty of the input image data, and a coding difficulty calculated by the coding difficulty calculating means. And a pre-filter control means for controlling the characteristics of the pre-filter accordingly.

According to a seventh aspect of the present invention, there is provided an image encoding control method comprising: encoding means for compressing and encoding input image data; and a pre-processing means provided before the encoding means for reducing the information amount of the input image data. An image encoding control method for controlling an image encoding device having a filter, comprising: an encoding difficulty calculation procedure for calculating an encoding difficulty indicating an encoding difficulty of input image data; And a pre-filter control procedure for controlling the characteristics of the pre-filter in accordance with the encoding difficulty calculated by the degree calculation procedure.

According to another aspect of the present invention, there is provided a medium recording an image encoding control program, comprising: encoding means for compressing and encoding input image data; and a medium provided before the encoding means for reducing an information amount of the input image data. A medium for recording an image encoding control program for controlling the image encoding device having a pre-filter for reducing by a computer, wherein the encoding difficulty represents the encoding difficulty of the input image data. For causing a computer to execute a coding difficulty calculation procedure for calculating the pre-filter characteristic, and a pre-filter control procedure for controlling characteristics of the pre-filter in accordance with the coding difficulty calculated by the coding difficulty calculation procedure It records an encoding control program.

In the image coding apparatus according to the present invention, the coding difficulty calculating means calculates the coding difficulty representing the coding difficulty of the input image data, and according to the coding difficulty, The characteristics of the prefilter are controlled by the prefilter control means.

According to a seventh aspect of the present invention, in the image encoding control method,
The encoding difficulty calculation procedure calculates the encoding difficulty indicating the encoding difficulty of the input image data, and the prefilter control procedure controls the characteristics of the prefilter according to the encoding difficulty. .

In a program recorded on a medium recording the image encoding control program according to the eleventh aspect, an encoding difficulty representing a difficulty of encoding input image data is calculated by an encoding difficulty calculating procedure. Then, the characteristics of the pre-filter are controlled by the pre-filter control procedure according to the encoding difficulty.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 2 is a block diagram showing a schematic configuration of the image coding apparatus according to the first embodiment of the present invention.
The image encoding apparatus receives an input image signal S ₁ , receives a pre-filter unit 50 for reducing the amount of information, and an output signal of the pre-filter unit 50, and performs pre-processing for compression encoding. And a FIFO (First-In-First-Out) memory 12 for outputting output data of the encoder control unit 11 with a delay of a predetermined time.
When inputs the output data of the FIFO memory 12, and compression coding by the coding method according to picture type for each picture, the encoder 13 as an encoding means for outputting the compressed image data S _2, the encoder control unit 11
Detecting a motion vector on the basis of the output data, a motion detecting circuit 14 sends to the encoder 13, the intra AC data S ₃ and ME residual data S ₄ that is output from the motion detection circuit 14 which is output from the encoder control unit 11 And an encoding control unit 15 for controlling the encoder 13 based on the generated bit amount data S ₅ output from the encoder 13 and a pre-filter control signal S ₆ based on the data from the encoding control unit 15. A pre-filter control section 51 for controlling the characteristics of the filter section 50; FIG.
Is configured to perform a feed-forward type rate control for controlling a generated code amount based on characteristics of image data before compression encoding.

As shown in FIG. 3, the encoding control unit 15 and the pre-filter control unit 51 include a CPU (central processing unit) 16, a ROM (read only memory) 17, and a ROM (read only memory) 17 connected to each other via a bus 19. The CPU 16 is configured by a computer having a RAM (random access memory) 18, and executes a program stored in the ROM 17 using the RAM 18 as a working area. Are realized. The ROM 17 stores the image encoding control program according to the present invention, and may be an IC (integrated circuit), a storage device using a magnetic disk such as a hard disk or a floppy disk as a recording medium, or a CD.
(Compact disk) A storage device using an optical disk such as a ROM as a recording medium or a storage device using another type of recording medium may be used. In any case, IC
And various recording media correspond to media on which the image encoding control program according to the present invention is recorded.

FIG. 1 is a block diagram showing a detailed configuration of the image coding apparatus shown in FIG. As shown in this figure, the pre-filter unit 50 is provided with a spatial filter 52 as a pre-filter in the spatial direction, and a temporal filter 53 as a pre-filter for the time direction, the input image signals S _1, the space Filter 52, time filter 5
3 in order. Spatial filter 52
Consists of a low-pass filter in the spatial direction. Specifically,
For example, it is realized by a digital filter. The time filter 53 is composed of a low-pass filter in the time direction,
Specifically, for example, the output signal of the spatial filter 52 is set to 1
A frame memory 54 for storing the frames, an output signal of the spatial filter 52 and the one stored in the frame memory 54
This is realized by an adder 55 that adds the signal before the frame and adds a predetermined weight K: (1−K) and outputs the result. Note that K
Is a weighting coefficient, and 0 ≦ K ≦ 1. The spatial filter 52 can change the filter characteristics such as the pass bandwidth by changing the filter coefficient. Similarly, the time filter 53 can change the filter characteristics such as the pass bandwidth by changing the value of the weighting coefficient K.

The encoder control section 11 receives the output signal of the pre-filter section 50, and rearranges the order of pictures (I picture, P picture, B picture) in accordance with the coding order. A scan conversion / macroblock conversion circuit 22 which receives output data of the rearrangement circuit 21, determines whether the frame structure or the field structure, and performs scan conversion and macroblock formation of 16 × 16 pixels according to the determination result. The output data of the scan conversion / macroblocking circuit 22 is input, the intra AC in the I picture is calculated, the intra AC data S ₃ is sent to the encoding control unit 15, and the output data of the scan conversion / macroblocking circuit 22 is output. And an intra AC operation circuit 23 that sends the data to the FIFO memory 12 and the motion detection circuit 14. The intra AC is defined as the sum of the absolute values of the differences between the pixel value of each pixel in the DCT block of 8 × 8 pixels and the average value of the pixel values in the DCT block in the I-picture. It can be said to represent.

The encoder 13 includes a subtraction circuit 3 for calculating a difference between output data of the FIFO memory 12 and predicted image data.
1 and the output data of the subtraction circuit 31
DCT that performs DCT in block units and outputs DCT coefficients
A CT circuit 32, a quantization circuit 33 for quantizing output data of the DCT circuit 32, a variable length encoding circuit 34 for variable length encoding of the output data of the quantization circuit 33, and a variable length encoding circuit temporarily holding the 34 output data, the buffer memory 35 to output as compressed image data S ₂ made from a bitstream of constant bit rate, the inverse quantization circuit 3 for inversely quantizing the output data of the quantization circuit 33
6, and the inverse D with respect to the output data of the inverse quantization circuit 36.
An inverse DCT circuit 37 for performing CT,
An addition circuit 38 for adding and outputting the output data of the prediction image data and the prediction image data, and holding the output data of the addition circuit 38 and performing motion compensation in accordance with the motion vector sent from the motion detection circuit 14 to obtain the prediction image data. , And a motion compensation circuit 39 that outputs the result to a subtraction circuit 31 and an addition circuit 38. The buffer memory 35 sends generated bit amount data S ₅ representing the bit amount generated by the variable length coding circuit 34 to the coding control unit 15.

The motion detection circuit 14 includes the encoder control unit 1
1 is a macro that minimizes the sum of absolute values or the sum of squares of pixel value differences between the macroblock of interest of the picture to be compression-encoded and the macroblock of interest in the referenced picture based on the output data of A block is searched, a motion vector is detected, and sent to the motion compensation circuit 39. The motion detecting circuit 14 sends when obtaining the motion vector, the absolute value sum or square sum of differences of pixel values between the minimum and since macroblock, the encoding control unit 15 as the ME residual data S ₄ It has become.

The encoding control unit 15 calculates an ME residual value which is a value obtained by adding the ME residual data S ₄ from the motion detection circuit 14 for the entire picture.
Based on the ME residual calculated by the ME residual calculating unit 41 and the intra AC data S ₃ from the intra AC operation circuit 23, an encoding difficulty indicating the encoding difficulty of the picture is calculated. a coding difficulty calculating unit 42, based on the generated bit amount data S ₅ from the buffer memory 35, the image decoding apparatus for expanding image data compressed and encoded by the image encoding apparatus according to this embodiment V which is a virtual buffer corresponding to the input buffer
A VBV buffer occupancy calculator 43 for calculating the data occupancy (hereinafter simply referred to as occupancy) of a BV (Video Buffering Verifier) buffer is provided. The ME residual can be said to represent the speed of movement of the video and the complexity of the picture.

The encoding control section 15 further calculates a target based on the encoding difficulty calculated by the encoding difficulty calculating section 42 and the occupancy of the VBV buffer calculated by the VBV buffer occupancy calculating section 43. A target code amount determining unit 44 for determining the code amount, and a quantum corresponding to the quantization characteristic value in the quantization circuit 33 such that the generated code amount in the encoder 13 becomes the target code amount determined by the target code amount determining unit 44. The quantization index is determined, and the quantization
And a quantization index determination unit 45 for sending to the.

The pre-filter control section 51 controls the pre-filter section 50 based on the coding difficulty calculated by the coding difficulty calculation section 42 in the coding control section 15.

Here, the encoding difficulty will be described.
The encoding difficulty represents the difficulty of encoding a picture, which can be rephrased as a ratio of the amount of data required to maintain the same image quality. There are various methods for digitizing the encoding difficulty, but in the present embodiment,
For I-pictures, the coding difficulty is obtained using intra AC, and for P-pictures and B-pictures, ME
The encoding difficulty is determined using the residual. As described above, the intra AC represents the complexity of the picture, and the ME
The residual indicates the speed of movement of the video and the complexity of the picture, and since these have a strong correlation with the difficulty of encoding, the intra AC and the linear function using the ME residual as a variable, etc. It is possible to calculate the encoding difficulty from the ME residual.

Next, a method of controlling the pre-filter unit 50 based on the encoding difficulty will be conceptually described. In the present embodiment, a first step of controlling the characteristics of the pre-filter unit 50 based on the ratio of the encoding difficulty to the bit rate, and a second step of controlling the characteristics of the pre-filter unit 50 based on the motion component Is performed in two stages.

First, the first stage will be described. The reason why the amount of information is desired to be reduced by the pre-filter unit 50 is when the pattern is complicated and contains many spatial high-frequency components, or when the motion amount is large or the motion is random, that is, when the encoding difficulty is large. It is. If a pre-filter is used for other pictures, only side effects such as blurring due to a spatial filter and jerkiness (movement is awkward) due to a temporal filter are conspicuous, which is not good.

On the other hand, even in the case of an image having a large encoding difficulty as described above, when the bit rate in the image encoding device is sufficiently high, that is, when the compression ratio is low, the image quality is hardly degraded. There is no need to use a pre-filter. As described above, whether it is better to use the pre-filter depends on not only the degree of difficulty of encoding but also the bit rate.

Therefore, in the present embodiment, on / off of the pre-filter unit 50 and a filter of a filter such as a pass band width are determined based on information indicating how large the encoding difficulty of the input image signal is relative to the bit rate. The characteristics are controlled. Further, in order to control the pre-filter unit 50 based on the encoding difficulty, it is preferable to look at the ratio of the encoding difficulty to the bit rate in a plurality of pictures. This is because the encoding difficulty varies depending on the picture type. Therefore, in the present embodiment, the pre-filter unit 5
As a parameter for controlling 0, a parameter x defined by the following equation is used.

X = ΣD _k / G

In the above equation, D _k represents the encoding difficulty of picture k, k represents the encoding order of the pictures,
Σ means the sum total from k = 1 to N (N is the number of pictures for one GOP (group of pictures)).
G is defined by the following equation.

G = ([bit rate] × N) / [picture rate]

In the above equation, [bit rate]
Represents the amount of data (bit amount) per second determined based on the transmission capacity of the communication line and the recording capacity of the recording medium, and [picture rate] represents the number of pictures per second (for example, 30 in the NTSC area). , 25) in the PAL area. Therefore, G represents the data amount (bit amount) allocated to the time corresponding to N pictures, and x is
It represents the ratio of the sum of the encoding difficulty _Dk of the N pictures to the data quantity (bit quantity) allocated to the time corresponding to the N pictures. Note that it is not always necessary to obtain x using N pictures, and x may be obtained using pictures larger or smaller than N. However, it should be noted that if the number is much larger than N, the precision of the parameter x decreases, and if the number is too small, the variation of the parameter x becomes large.

In the present embodiment, as a first step, the characteristics of the pre-filter unit 50 are controlled according to, for example, the magnitude of the parameter x defined as described above. Specifically, for example, by changing the tap coefficient (filter coefficient) in the digital filter that realizes the spatial filter 52 according to the magnitude of the parameter x, the pass bandwidth of the spatial filter 52 is changed, and By changing the value of the weighting coefficient K in the time filter 53 according to the magnitude of x, the pass band width of the time filter 53 is changed.

FIG. 4 shows an example of the relationship between the magnitude of the parameter x and the pass bandwidth of the spatial filter 52. In this figure, the horizontal axis represents the value of x, and the vertical axis represents the pass bandwidth of the spatial filter 52. In this example, the pass band width of the spatial filter 52 is set to through (pass as it is; filter is off), 5.0 MHz, 4.5 MHz, and 4.0.
5 and 3.5 MHz are prepared. As shown by reference numeral 61 (solid line) in FIG. 4, through is performed when 0 ≦ x <δ ₁ , 5.0 MHz when δ ₁ ≦ x <δ ₂ , δ ₂ ≦
When x <δ ₃ , 4.5 MHz is selected, when δ ₃ ≦ x <δ ₄ , 4.0 MHz, and when δ ₄ ≦ x, 3.5 MHz is selected. Note that δ ₁ <δ ₂ <δ ₃ <δ ₄ .

Next, the second stage will be described. When the characteristics of the pre-filter unit 50 are determined only from the value of the parameter x as in the first stage, the following problems occur due to human visual characteristics. That is, for example, in a pattern in which a finely patterned object moves slowly, since the spatial high-frequency component is large, the encoding difficulty becomes large,
The value of x also increases. However, here, the pre-filter is tightened (pass band width is narrowed).
Then, fine patterns are lost. Since the human eye follows the slow motion well, such a blur is immediately perceived, and the visual image quality after decoding is rather deteriorated.

On the other hand, for an image that moves very quickly,
Human visual characteristics make it hard to notice blur. Therefore, if you know that the video is fast moving,
The prefilter should be tight. This is because when trying to express high-frequency components in fast-moving video,
This is because remarkable image quality deterioration such as block distortion is caused.

Based on such a concept, in the present embodiment, the pre-filter is tightened when the motion component representing the magnitude of the video motion is large, and the pre-filter is loosened when the motion component is small. Consider realizing the process of applying in real time. The parameters that determine the motion component of the image, for example, the latest encoding difficulty D _B of the B picture which is calculated on the basis of the ME residual and the like, the coding of the most recent I-picture which is calculated on the basis of the intra AC, etc. using the ratio y = D _I / D _B with difficulty D _I. The motion is large the picture, to the motion prediction error increases, because also increases the amount of codes of the motion vectors themselves, coding of B-pictures is increased, as a result, has y = D _I / D _B decreases. Therefore, in the present embodiment, as the second stage, it is determined that the motion component is larger as the value of y is smaller, and the pre-filter unit 50 is controlled so that the pre-filter is tighter.
Such control can be realized, for example, by changing the filter characteristic determined in the first stage according to the value of y.

FIG. 4 shows an example in which the pass band width of the spatial filter 52 determined in the first stage is changed to three stages according to the value of y. In this example, when y ≧ 4, the pass bandwidth of the spatial filter 52 determined in the first step is not changed, and when 3 ≦ y <4, the first step Is narrowed by 0.5 MHz, and when 2 ≦ y <3, the pass bandwidth of the spatial filter 52 determined in the first stage is determined as indicated by reference numeral 63. Is narrowed by 1.0 MHz, and y <
In the case of 2, as indicated by reference numeral 64, the pass band width of the spatial filter 52 determined in the first stage is narrowed by 1.5 MHz.

Next, the operation of the image coding apparatus according to this embodiment will be described. The input image signal S ₁ is input to the encoder control unit 11 after passing through the spatial filter 52 and the time filter 53 of the pre-filter unit 50 in order.
In the encoder control unit 11, first, the image rearranging circuit 2
1, the order of the pictures (I picture, P picture, B picture) is rearranged in accordance with the coding order, and then the scan conversion / macroblocking circuit 22 determines whether the frame structure or the field structure, Is performed according to the scan conversion and the macroblock conversion.
In the case of a picture, an intra AC is calculated by the intra AC operation circuit 23 and the intra AC data S ₃ is sent to the encoding control unit 15. The output data of the scan conversion / macroblock conversion circuit 22 is output from the intra AC operation circuit 2.
3, the FIFO memory 12 and the motion detection circuit 14
Sent to

The FIFO memory 12 stores the input image data for the time necessary for the encoding difficulty calculator 42 to calculate the encoding difficulty of N pictures subsequent to the picture for which encoding has been completed. The signal is output to the encoder 13 with a delay. Motion detection circuit 14, and sends to detect the motion vector to the motion compensation circuit 39, sends the ME residue data S ₄ to the ME residual calculating portion 41.

In the case of an I picture, the encoder 13 inputs the output data of the FIFO memory 12 as it is to the DCT circuit 32 without performing a difference from the predicted image data in the subtraction circuit 31, performs DCT, and performs quantization. The DCT coefficient is quantized by the circuit 33, the output data of the quantization circuit 33 is variable-length encoded by the variable-length encoding circuit 34, and the variable-length encoding circuit 3 is encoded by the buffer memory 35.
Temporarily holding the fourth output data, and outputs it as compressed image data S ₂ made from a bitstream of constant bit rate. Also, the quantization circuit 33 is generated by the inverse quantization circuit 36.
Is inversely quantized by the inverse DCT circuit 37, inverse DCT is performed on the output data of the inverse quantization circuit 36, and the output image data of the inverse DCT circuit 37 is added to the addition circuit 38.
Is input to the motion compensation circuit 39 via the.

In the case of a P picture, the encoder 13 in the encoder 13 stores the past I
It generates predicted image data based on the image data corresponding to the picture or the P picture and the motion vector from the motion detection circuit, and outputs the predicted image data to the subtraction circuit 31 and the addition circuit. The output data of the FIFO memory 12 and the motion compensation circuit 3 are output by the subtraction circuit 31.
9 and the difference between the predicted image data and the DCT circuit 3
2 performs DCT, and the quantization circuit 33 performs DCT.
The T coefficients quantizes the output data of the quantization circuit 33 by the variable-length coding circuit 34 variable-length coding, as to temporarily holds the output data of the variable length coding circuit 34 by the buffer memory 35 the compressed image data S ₂ I do. Also,
The output data of the quantization circuit 33 is inversely quantized by the inverse quantization circuit 36, the inverse DCT circuit 37 performs inverse DCT on the output data of the inverse quantization circuit 36, and the addition circuit 3
8, the output data of the inverse DCT circuit 37 and the predicted image data are added and input to the motion compensation circuit 39 to be held.

In the case of a B picture, the encoder 13 in the encoder 13 uses the motion compensation circuit 39 to hold two pieces of image data corresponding to the past and future I or P pictures and the two motions from the motion detection circuit 14. It generates predicted image data based on the vector and outputs the predicted image data to the subtraction circuit 31 and the addition circuit. Further, the difference between the output data of the FIFO memory 12 and the predicted image data from the motion compensation circuit 39 is calculated by the subtraction circuit 31, the DCT is performed by the DCT circuit 32, the DCT coefficient is quantized by the quantization circuit 33, and the variable length The output data of the quantization circuit 33 is subjected to variable-length encoding by the encoding circuit 34, and the output data of the variable-length encoding circuit 34 is temporarily held by the buffer memory 35, and the compressed image data S ₂
Output as Note that the B picture is stored in the motion compensation circuit 39.
Not to be held.

The buffer memory 35 sends the generated bit amount data S ₅ representing the bit amount generated by the variable length coding circuit 34 to the coding control unit 15.

In the coding control unit 15, the coding difficulty calculating unit 42 calculates the intra AC data S ₃ from the intra AC calculating circuit 23 and the M calculated by the ME residual calculating unit 41.
The encoding difficulty is calculated from the E residual, the parameter x is obtained, and the VBV buffer occupancy calculator 43 calculates the occupancy of the VBV buffer. Then, the target code amount determining unit 44 calculates a target code amount based on the parameter x and the VBV buffer occupancy calculated by the VBV buffer occupancy calculating unit 43, and sends the target code amount to the quantization index determining unit 45. The quantization index determination unit 45 determines a quantization index corresponding to the quantization characteristic value in the quantization circuit 33 so that the generated code amount in the encoder 13 becomes the target code amount determined by the target code amount determination unit 44. , To the quantization circuit 33.

Next, the operation related to the control of the pre-filter unit 50 will be described with reference to the flowchart of FIG. Note that FIG. 5 shows only control of the spatial filter 52 for simplicity. In this operation, first, the encoding difficulty calculator 42 calculates the encoding difficulty and obtains the parameter x (step S101). Next, the pre-filter control unit 51 determines the pass bandwidth of the pre-filter (in this case, the spatial filter 52) as the first step based on the value of the parameter x, for example, based on the relationship shown in FIG. Step S102). Next, the pre-filter control unit 51 calculates the ratio y of the encoding difficulty of the I picture and the B picture (Step S103), determines the magnitude of y (Step S104), and determines the magnitude of y according to the magnitude of y. The following processing is performed. That is, in the example shown in FIG. 4, if y ≧ 4, the pass bandwidth determined in the first stage is not changed,
When 3 ≦ y <4, the pass bandwidth determined in the first stage is narrowed by 0.5 MHz (step S105), and 2 ≦
If y <3, the pass bandwidth determined in the first stage is narrowed by 1.0 MHz (step S106), and y <2
, The pass bandwidth of the spatial filter 52 determined in the first stage is narrowed by 1.5 MHz (step S10).
7). Next, the pre-filter control unit 51 sets the coefficients of the pre-filter (here, the spatial filter 52) so as to have the pass bandwidth determined as described above (step S108), and returns to the main routine. . The above operation is performed for each picture.

The control of the spatial filter 52 has been described so far. The control of the temporal filter 53 is the same as the control of the spatial filter 52 except that the control of the temporal filter 53 is performed using the weighting coefficient K instead of the tap coefficient. When representing the characteristics of the time filter, a weighting coefficient K is taken on the vertical axis instead of the pass bandwidth in FIG. The smaller the value of K, the tighter the filter.

As described above, according to the image encoding apparatus of the present embodiment, the characteristics of the pre-filter unit 50 are adapted according to the degree of difficulty of encoding the input image while encoding the input image signal in real time. When the coding difficulty of the input image is large, the amount of information can be reduced to reduce noticeable distortion such as block distortion, and the coding difficulty of the input image is small. At times, it is possible to prevent the image quality from deteriorating due to the reduction in the amount of information. Furthermore, according to the image coding apparatus according to the present embodiment, when the motion component is large, the pre-filter can be applied tightly, and when the motion component is small, the pre-filter can be applied loosely. It is possible to improve the visual impression of the image quality after decoding while making it difficult.

FIG. 6 is a block diagram showing a configuration of an image coding apparatus according to the second embodiment of the present invention. The image encoding apparatus according to the present embodiment uses a well-known TM5 (test model 5; ISO / IEC
JTC / SC29 (1993)) is an example of a configuration for performing feedback type rate control for controlling the generated code amount based on the generated code amount obtained by compression encoding in the past.

This image coding apparatus uses an input image signal S ₁
And a pre-filter unit 50 for reducing the amount of information, and an output signal of the pre-filter unit 50,
Encoder control unit 1 that performs pre-processing and the like for compression encoding
1 and inputs the output data of the encoder control unit 11, and compression coding by the coding method according to picture type for each picture, the encoder 13 as an encoding means for outputting the compressed image data S _2, encoder Detecting a motion vector based on output data of the control unit 11;
A motion detecting circuit 14 sends to the encoder 13, the coding control unit 75 for controlling the encoder 13 based on the generated bit amount data S ₇ output from the encoder 13, based on data from the encoding control unit 75, A pre-filter control section 51 for controlling the characteristics of the pre-filter section 50 by the pre-filter control signal S ₆ . In this image encoding device, the configuration other than the encoding control unit 75 is as follows.
This is the same as in the first embodiment.

The encoding control unit 75 includes an encoding difficulty calculating unit 46 for calculating the encoding difficulty based on the generated bit amount data S ₇ output from the variable length encoding circuit 34 of the encoder 13, A target code amount determining unit 44 for determining a target code amount based on the encoding difficulty calculated by the encoding difficulty calculating unit 46, and a target code amount generated in the encoder 13 by the target code amount determining unit 44. A quantization index determining unit 45 that determines a quantization index corresponding to a quantization characteristic value in the quantization circuit 33 so as to obtain a code amount and sends the quantization index to the quantization circuit 33 is provided.

The pre-filter control section 51 controls the pre-filter section 50 based on the coding difficulty calculated by the coding difficulty calculation section 46 in the coding control section 15.

In the present embodiment, the encoding difficulty calculating section 4
As the encoding difficulty calculated in step 6, the global complexity (Global Compl.
exity). The global complexity is a parameter indicating the complexity of a screen. Specifically, the amount of generated code at the time of compression encoding of a picture and the average quantization scale code (quantization at the time of compression encoding of a picture) (E.g., "Television Society Multimedia Selection MPEG" issued by Ohmsha)
See page 111. ).

Other operations and effects of the present embodiment are the same as those of the first embodiment.

The present invention is not limited to the above embodiments. For example, when controlling the pre-filter unit 50 based on the degree of difficulty of encoding, the parameters x and ratio y, Alternatively, in addition to the global complexity, information of the changing tendency of whether the encoding difficulty is increasing or decreasing is also determined,
It is possible to further improve the accuracy of predicting the difficulty of the pattern of the input image in the future, and more appropriately control the pre-filter unit 50. The information on the tendency of the change of the encoding difficulty can be obtained by, for example, linearly approximating the encoding difficulty obtained in time series by the least square method or the like, and obtaining the inclination. In this case, information on the change tendency of the encoding difficulty is obtained by the encoding difficulty calculators 42 and 46, for example.

In the first embodiment, as shown in FIG. 2, the amount of generated code is based on the characteristics of image data before compression encoding without providing another encoder in addition to the encoder 13. However, the present invention provides a first-pass encoder separate from the encoder 13 in order to estimate the amount of data after compression encoding. The present invention can also be applied to a configuration in which a feedforward rate control for controlling the amount of generated codes in the second-pass encoder 13 based on the amount of data estimated by compression-encoding by the second encoder. In this case, it is possible to determine the encoding difficulty based on the generated code amount obtained by performing the compression encoding by the encoder of the first pass.

Further, in a configuration in which the feedback type rate control is performed as in the second embodiment, several future pictures may be obtained by linear approximation or the like based on the coding difficulty of the pictures already compressed and coded. May be predicted, and the pre-filter unit 50 may be controlled based on the predicted coding difficulty.

Further, in a configuration in which feed-forward type rate control is performed, a tendency of a change in encoding difficulty is obtained by linear approximation or the like. When predicting the encoding difficulty of a picture, since the continuity of the encoding difficulty is lost at the time of a scene change, a process of calculating a change tendency of the encoding difficulty or a process of predicting the encoding difficulty at the time of a scene change is performed. If the processing is completed before the scene change and a new process is performed after the scene change, the accuracy is further improved.

Further, needless to say, FIG. 4 shows an example of the characteristics of the pre-filter, and the characteristics of the pre-filter can be appropriately set.

The encoding difficulty is not limited to the one using the intra AC, the ME residual, the global complexity, etc. described in the embodiment, but may be any one representing the encoding difficulty of the picture. Other parameters may be used. The parameter representing the motion component is not limited to the ratio y = D _I / D _B mentioned in the embodiments, as long as it represents the magnitude of the motion in the input image data, or other parameters. For example, the ratio D _I / D of the coding difficulty D _I of the latest encoding difficulty D _P, the latest I-picture of P picture
_P may be used.

In the embodiment, the pre-filter unit 50
Has been described as having both the spatial filter 52 and the temporal filter 53, but the present invention can also be applied to an image coding apparatus having only one of a spatial filter and a temporal filter as a pre-filter.

[0066]

As described above, claims 1 to 6
An image encoding device according to any one of claims 1 to 7,
According to the image encoding control method described in any one of claims 0 to 15 or the medium storing the image encoding control program according to any one of claims 11 to 14, a code representing the difficulty of encoding the input image data. The encoding difficulty is calculated, and the characteristics of the pre-filter are controlled according to the encoding difficulty, so that the pre-filter is adaptively used according to the characteristics of the input image while encoding in real time. When the encoding difficulty of the input image is large, it is possible to reduce the information amount by reducing the amount of information, and to reduce the deterioration of the image quality due to the encoding distortion. When the encoding difficulty of the input image is small, the image quality is reduced by the information amount. This has the effect of preventing the deterioration of the device.

Further, according to the image encoding apparatus of the fifth or sixth aspect, the image encoding control method of the ninth or tenth aspect, or the medium recording the image encoding control program of the thirteenth or fourteenth aspect. Since the motion component representing the magnitude of the motion in the input image data is calculated and the characteristics of the pre-filter are controlled according to the encoding difficulty and the motion component, in addition to the above effects, This has the effect of improving the impression.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a detailed configuration of an image encoding device according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a schematic configuration of an image encoding device according to the first embodiment of the present invention.

FIG. 3 is a block diagram illustrating a configuration of a computer that implements an encoding control unit and a prefilter control unit in FIG. 1;

FIG. 4 is a characteristic diagram for explaining a function of a pre-filter control unit in FIG. 1;

FIG. 5 is a flowchart showing an operation related to control of a pre-filter unit in the image encoding device according to the first embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration of an image encoding device according to a second embodiment of the present invention.

FIG. 7 is a block diagram illustrating an example of a configuration of an image encoding device having a prefilter.

[Explanation of symbols]

11: Encoder control unit, 12: FIFO memory, 13
... Encoder, 14: Motion detection circuit, 15: Encoding control unit, 16: CPU, 17: ROM, 18: RAM, 23
... Intra AC operation circuit, 31 ... Subtraction circuit, 32 ... DC
T circuit, 33: quantization circuit, 34: variable length coding circuit,
35, a buffer memory, 36, an inverse quantization circuit, 37, an inverse DCT circuit, 39, a motion compensation circuit, 41, an ME residual calculation unit, 42, an encoding difficulty calculation unit, 43, a VBV buffer occupancy calculation unit, 44: target code amount determining unit, 45: quantization index determining unit, 50: prefilter, 51: prefilter control unit, 52: spatial filter, 53: temporal filter

Claims (14)

[Claims] An encoding unit for compressing and encoding the input image data; a pre-filter provided before the encoding unit for reducing an information amount of the input image data; Encoding difficulty calculating means for calculating the encoding difficulty representing the difficulty; and prefilter control means for controlling the characteristics of the prefilter in accordance with the encoding difficulty calculated by the encoding difficulty calculating means. An image encoding device comprising: 2. The image coding apparatus according to claim 1, wherein said pre-filter includes a spatial low-pass filter. 3. The image encoding apparatus according to claim 1, wherein the pre-filter includes a temporal low-pass filter. 4. The pre-filter control means controls characteristics of the pre-filter based on a ratio of a degree of difficulty of encoding to an amount of data per unit time allocated to output data of the encoding means. The image encoding device according to claim 1, wherein: 5. A motion component calculating means for calculating a motion component representing a magnitude of a motion in the input image data,
2. The image coding apparatus according to claim 1, wherein said pre-filter control means controls characteristics of said pre-filter in accordance with said encoding difficulty and a motion component calculated by said motion component calculation means. apparatus. 6. The motion component calculating means calculates, as a motion component, a ratio between the coding difficulty of the latest predicted coded image and the coding difficulty of the latest intra-frame coded image. The image coding apparatus according to claim 5, wherein 7. An image encoding apparatus comprising: an encoding unit for compressing and encoding input image data; and a prefilter provided before the encoding unit and configured to reduce an information amount of the input image data. A coding difficulty calculating step for calculating a coding difficulty indicating a coding difficulty of input image data; and a coding difficulty calculated by the coding difficulty calculating procedure. A pre-filter control procedure for controlling characteristics of the pre-filter according to the degree. 8. The pre-filter control step includes controlling characteristics of the pre-filter based on a ratio of encoding difficulty to a data amount per unit time allocated to output data of the encoding unit. The image encoding control method according to claim 7, wherein: 9. A motion component calculation procedure for calculating a motion component representing a magnitude of a motion in the input image data,
8. The image encoding apparatus according to claim 7, wherein the prefilter control step controls the characteristics of the prefilter in accordance with the encoding difficulty and a motion component calculated by the motion component calculation step. Control method. 10. The motion component calculating step includes calculating, as a motion component, a ratio between the coding difficulty of the latest predictive coded image and the coding difficulty of the latest intra-frame coded image. The image encoding control method according to claim 9. 11. An image encoding apparatus comprising: an encoding unit for compressing and encoding input image data; and a pre-filter provided before the encoding unit and configured to reduce the information amount of the input image data. A medium storing an image encoding control program to be controlled by a computer, comprising: an encoding difficulty calculating step for calculating an encoding difficulty indicating an encoding difficulty of input image data; A medium for recording an image encoding control program for causing a computer to execute a prefilter control procedure for controlling characteristics of the prefilter in accordance with the encoding difficulty calculated by the degree calculation procedure. 12. The pre-filter control step includes controlling a characteristic of the pre-filter based on a ratio of encoding difficulty to a data amount per unit time allocated to output data of the encoding unit. 12. A medium recording the image encoding control program according to claim 11, wherein: 13. The image encoding control program according to claim 1,
Further, the computer is caused to execute a motion component calculation procedure for calculating a motion component representing the magnitude of the motion in the input image data, and the pre-filter control procedure includes the coding difficulty and the motion calculated by the motion component calculation procedure. 12. The medium according to claim 11, wherein a characteristic of the pre-filter is controlled according to a component. 14. The motion component calculating step includes calculating, as a motion component, a ratio between the coding difficulty of the latest predicted coded image and the coding difficulty of the latest intra-frame coded image. 14. A medium recording the image encoding control program according to claim 13.