JP2606319B2 - Image data quantization circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantization circuit for image data applied to compression-encode image data.

According to the present invention, there is provided an image data quantization circuit for expressing image data with a quantization code having a predetermined number of bits, the true value of a target pixel and the true values of peripheral pixels of the target pixel. And a spatial representative amount is detected from the detected value.The representative representative value is obtained closest to the difference between the detected spatial value and the decoded value of the quantized code of the peripheral pixel. By selecting a code, occurrence of visually noticeable noise in the decoded image is prevented.

[Conventional technology]

Each pixel (sample) of digitized image data
Various encoding schemes have been proposed for reducing the number of bits of an image signal using correlation of image signals. The applicant of the present application filed Japanese Patent Application No. 59-
As described in the specification of US Pat. No. 266407, a high-efficiency coding that obtains a dynamic range that is a difference between the maximum value and the minimum value of a plurality of pixels included in a two-dimensional block, and performs encoding adapted to the dynamic range The device is proposed. Also, as described in Japanese Patent Application No. 60-232789, a high-efficiency coding that performs coding suitable for a dynamic range with respect to a three-dimensional block formed from pixels in a plurality of regions belonging to a plurality of frames. A device has been proposed. Further, as described in Japanese Patent Application No. 60-268817, a variable length encoding method in which the number of bits changes according to the dynamic range so that the maximum distortion generated when performing quantization is constant. Has been proposed.

Coding adapted to these dynamic ranges (hereinafter abbreviated as ADRC) is based on the fact that an image has a strong correlation in a divided small area (block) of one screen, so that bits per pixel are used. This is a highly efficient coding method that reduces the number. That is, the difference between the minimum value or the maximum value in the block and the level of each pixel becomes smaller than the original level, and this difference can be quantized with a smaller number of bits than the original number of bits.

The present invention is applicable to the above-described quantization of the level normalized by the maximum value or the minimum value in ADRC. However, the present invention is not limited to the ADRC, and can be applied in the same manner as the ADRC as long as the quantization circuit expresses a digital image signal with a predetermined number of bits.

As shown in FIG. 4, in ADRC that performs 2-bit quantization, the dynamic range DR of the block, which is the difference between the maximum value MAX and the minimum value MIN, is equally divided into four level ranges, and the minimum value MIN is The value of the pixel after the removal is represented by a 2-bit quantization code corresponding to each of the level ranges.
On the decoding side, one of the middle decoding representative levels I0 to I3 in each level range is decoded from the dynamic range DR and the quantization code, and the minimum value MIN is added to the decoded value.
The pixel data in the block is restored.

FIG. 5 shows an example of quantization in ADRC. FIG. 5 shows an example of a one-dimensional ADRC in which one block is composed of six pixels that are continuous in the horizontal direction. The data indicated by ○ is the true value of the pixels in the block, and is therefore indicated by a solid line 41. Has a horizontal change. In the case where encoding is performed by the above-described 2-bit ADRC, a restoration level indicated by x is obtained on the decoding side, and a signal change indicated by a broken line 42 occurs in the restored image.

[Problems to be solved by the invention]

In conventional quantization, the original pixel level is replaced with the closest decoded representative level in order to reduce the quantization error and improve the S / N. However, even if quantitatively good, visually noticeable degradation may occur in the restored image. In the example shown in FIG. 5, the original gentle horizontal change 41 becomes a sharp change 42 after restoration, and visually noticeable noise occurs in the restored image. This noise is similar to that obtained by reducing snow noise generated in a television reception image at the time of a weak electric field, and is a noise that has been generated. The occurrence of such a problem is based on the fact that humans are sensitive to the differential characteristics of an image when recognizing the image.

Therefore, an object of the present invention is to provide an image signal quantization circuit which can preserve a spatial change of an original image signal even if a quantitative error increases, and can visually improve the quality of a restored image. Is to provide.

[Means for solving the problem]

According to the present invention, there is provided an image data quantization circuit for expressing image data with a quantization code having a predetermined number of bits, comprising: a true value of a target pixel and a true value of a spatial peripheral pixel of the target pixel. A circuit 1 for detecting a spatial change amount; a decoding representative value obtained by determining a spatial change amount with respect to a decoded value of a quantized code of a peripheral pixel to be closest to the detected spatial change amount; And circuits 14 to 17, 18, 20 to 23, and 28 to 32 for selecting a quantization code corresponding to.

[Action]

The true value x1 of the pixel of interest and the true value x0 of the surrounding pixel, for example, the previous pixel
(The amount of change in the horizontal direction) Δr (= x1−x0) is detected by the subtraction circuit 11. The decoding level X0 of the pixel x0 is generated by the local decoder 32 and the delay circuit. In addition, four decoding representative levels (I0 + MIN, I1 + MIN, I2 + MIN, I3 + MI
N) are generated by the local decoders 28, 29, 30, 31.
In the subtraction circuits 20 to 23, the decoding representative level and the decoding level
Differences (predicted change amounts) Δ0, Δ1, Δ2, and Δ3 from X0 are calculated. That is, Δ0 = (I0 + MIN) −X0 Δ1 = (I1 + MIN) −X0 Δ2 = (I2 + MIN) −X0 Δ3 = (I3 + MIN) −X0 Among these four differences, the change that the original image data has The change amount closest to the amount Δr is detected. Therefore, in the subtraction circuits 14 to 17, (β0 = Δr−Δ0)
(Β1 = Δr−Δ1) (β2 = Δr−Δ2) (β3 = Δ
(r−Δ3), a minimum subtraction output is detected, and a quantization code corresponding to this detection is output to the code selection circuit 19.
Is selected. That is, when the difference from 0 is the minimum,
The quantization code of (00) is selected, and thereafter, the quantization codes of (01), (10), and (11) are similarly selected.

As described above, so that the restored image signal has a change amount closest to the spatial change amount of the original image data,
Since the quantization is performed, visual deterioration of the image quality of the restored image can be prevented.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

In FIG. 1, a digital video signal in which one pixel (one sample) is quantized to 8 bits is supplied to an input terminal indicated by 1, for example. The blocking circuit 2 converts the data order of the input digital video signal from the scanning order to the block order. For example, a screen of one frame is subdivided to form a block of (4 × 4 = 16 pixels) as shown in FIG. In FIG. 2, N-1
Indicates the previous block, and N indicates the block of interest to be coded. In the block, the data of the top leftmost pixel as viewed in the drawing is transmitted first,
Next, data of three pixels arranged in a horizontal direction is transmitted,
Further, in the second line, data is transmitted in the same manner, and finally, the data of the rightmost lowermost pixel is transmitted.

The output signal of the blocking circuit 2 is supplied to the maximum value and minimum value detection circuit 3, and the maximum value, MAX, and minimum value MIN of the pixels included in each block are detected. The maximum value MAX and the minimum value MIN are supplied to the subtraction circuit 4, and a dynamic range DR, which is a difference between the maximum value MAX and the minimum value MIN, is calculated. The dynamic range DR and the minimum value MIN are supplied to the framing circuit 5.
In the framing circuit 5, the dynamic range DR, the minimum value
MIN and a quantization code DT described later are converted into a signal form of a frame configuration, and error correction coding processing is performed as necessary. Transmission data is obtained at the output terminal 6 of the framing circuit 5.

The output signal of the blocking circuit 2 is supplied to the delay circuits 7, 8 and 9
Is supplied to one input terminal of the selector 10 via the. The output signal of the delay circuit 8 is supplied to the other input terminal of the selector 10. The output signal of the delay circuit 7 and the output signal of the selector 10 are supplied to the subtraction circuit 11, and the original pixel data (true value)
Is calculated in the horizontal direction. The true value of the pixel of interest
If x1 is set and the true value of the pixel one sampling cycle before is x0, it is expressed as (Δr = x1−x0).

The delay amount DL1 of the delay circuit 7 corresponds to the time required for detecting the maximum value and the minimum value. The delay amount DL2 of the delay circuit 8 corresponds to a horizontal interval between pixels, that is, one sample period. Accordingly, the subtraction circuit 11 generates a horizontal difference Δr between the data of the pixel one sample before and the data of the pixel of interest to be encoded.

However, in the case of the pixel in the leftmost column of the block, since the data of the previous pixel does not exist in the block, the previous block N
It is necessary to form a difference using the data of the rightmost pixel of -1. When the leftmost pixel in the block is supplied to the subtraction circuit 11, the selector 10 selects the data of the rightmost pixel of the previous block from the delay circuit 9. The delay amount DL3 of the delay circuit 9 is set to (1 block period−3 sample periods). The selector 10 is controlled by a select signal from a select signal generation circuit 12. Select signal generation circuit
The clock signal (sampling clock and block clock) from the terminal 13 is supplied to the terminal 12, and the select signal for controlling the selector 10 is formed as described above.

The difference Δr between the true values of the image data from the subtraction circuit 11 is supplied to the subtraction circuits 14, 15, 16, and 17. These subtraction circuits 14
The output signals .beta.0, .beta.1, .beta.2, and .beta.3 of .about.17 are supplied to the minimum value detection circuit 18, and the minimum output signal is detected. The detection signal of the minimum value detection circuit 18 is supplied to the code selection circuit 19,
A 2-bit quantization code DT is generated from the code selection circuit 19. The quantization code DT is transmitted via the framing circuit 5. In the code selection circuit 19, the decoding representative level I0,
2-bit quantization codes (0, 1) corresponding to I1, I2, and I3, respectively.
One of (0), (01), (10) and (11) is selected.

 The selection operation of the code selection circuit 19 is as follows.

When β0 is the minimum, (00) is selected as the quantization code DT.

When β1 is the minimum, (01) is selected as the quantization code DT.

When β2 is the minimum, (10) is selected as the quantization code DT.

When β3 is the minimum, (11) is selected as the quantization code DT.

Signals Δ0, Δ1, Δ2, and Δ3 are supplied from the subtraction circuits 20, 21, 22, and 23 to the subtraction circuits 14 to 17, respectively. These signals Δ0 to Δ3 are the respective differences between the decoding level (X0) of the pixel before the pixel of interest and the four decoding representative levels, and are prediction change amounts. Subtraction circuits 14 to 17 and minimum value detection circuit 18
Detects the signal closest to the horizontal difference Δr of the true value of the image data among the signals Δ0 to Δ3. That is, a quantization code DT corresponding to a decoded representative level that causes a change closest to the horizontal signal change of the original image signal is selected for the target pixel.

In the subtraction circuits 20 to 23, local decoders 28, 29, 30,
Decoding representative level formed by 31 (MIN + I0) (MIN + I1)
(MIN + I2) and (MIN + I3) are supplied. To generate these decoded representative levels, the local decoders 28-31
Is supplied with the dynamic range DR and the minimum value MIN,
Also, 2-bit quantization codes (00) (01) (10) (11) are supplied from terminals 24, 25, 26 and 27, respectively. Each of the local decoders 28 to 31 and 32 is composed of a ROM to which a dynamic range DR and a quantization code DT are supplied as addresses, and adds a minimum value MIN to data read from the ROM.

The decoding level X0 of the pixel before the pixel of interest is formed by the local decoder 32, the delay circuits 33, 34, 35, and the selector 35. The local decoder 32 is supplied with the quantized code DT from the code selection circuit 19, and the local decoder 32 generates a decoding level of the pixel of interest. This decoding level is 1
The signal is supplied to one input terminal of the selector 35 via a delay circuit 33 having a delay amount DL2 of the sampling period. Delay circuit
The output signal of the selector 33 is supplied to the selector 35 via the delay circuit 34 having the delay amount DL3 of (1 block period-3 sampling period).
Is supplied to the other input terminal. The selector 35 is controlled by a select signal from the select signal generation circuit 12, similarly to the selector 10 described above.

The delay circuits 33 and 34 and the selector 35 generate the decoding level X0 of the pixel x before the pixel of interest x1 in the same manner as the delay circuits 8 and 9 and the selector 10 described above. This decoding level is supplied to subtraction circuits 20 to 23. Therefore, the signals Δ0 to Δ3 generated from the subtraction circuits 20 to 23 respectively are the predicted differences between the four decoded representative levels and the decoded level X0 of the previous pixel as described below.

Δ0 = (I0 + MIN) −X0 Δ1 = (I1 + MIN) −X0 Δ2 = (I2 + MIN) −X0 Δ3 = (I3 + MIN) −X0 In the subtraction circuits 14 to 17, β0 = Δr−Δ0 β1 = Δr−Δ1 β2 = Δr−Δ2 An output signal of β3 = Δr−Δ3 is formed. By the minimum value detection circuit 18, β
Since the smallest one among 0 to β3 is detected, the quantization code whose predicted change amount is closest to the change amount Δr of the true value is selected by the code selection circuit 19.

It is also effective to perform quantization so as to faithfully represent not only the horizontal difference in the above-described embodiment but also two-dimensional changes in the horizontal, vertical, and oblique directions.
For example, as shown in FIG. 3, the level of the pixel of interest is x, and the levels of peripheral pixels above, left, and diagonally above are a,
In the case of b and c, the change amount Δr of the true value of the target pixel and the predicted change amount Δi (i = 0, 1, 2, 3) are calculated as the difference between the average value of the levels of the peripheral pixels and the level of the target pixel. Desired. That is, Δr = (3x-abc) Δr = (3Ii-ABC) (However, A, B and C are obtained by decoding the quantization codes obtained for a, b and c, respectively. The quantization code signal that produces Δi closest to Δr is selected.

In order to determine the amount of change in space, not only the average value,
A prediction value obtained by spatial prediction may be used. That is, assuming that the predicted value is x ', x' = b + 1/2 (ca) [Delta] r = xx '= xb-1 / 2 (ca) [Delta] i = Ii-B-1 / 2 ( CA) Then, in the same manner as described above, the quantization code signal that generates Δi closest to Δr is selected.

Also, the present invention provides a variable-length ADRC and three-dimensional block.
The present invention can be applied to a quantization circuit for other high efficiency codes such as ADRC.

〔The invention's effect〕

When the present invention is applied to an image signal having a change indicated by a solid line 41 shown in FIG. 5, the data of each pixel is quantized so as to have a decoded representative level indicated by □.
The change in the restored image has a gradual change, similar to the original image signal, as indicated by the dashed line 43. As described above, according to the present invention, the spatial change of the original image signal can be preserved, so that visually noticeable noise can be prevented from being generated in the restored image.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a schematic diagram of an example of a block, FIG. 3 is a schematic diagram used to explain detection of a change in space, and FIG. FIG. 5 is a schematic diagram illustrating an example of quantization. Explanation of main symbols in the drawings 1: Input terminal of digital video signal, 11: Subtraction circuit that generates a change in true value, 18: Minimum value detection circuit, 19: Code selection circuit, 20, 21, 22, 23: Subtraction circuits for generating predicted change amounts, 28, 29, 30, 31, 32: local decoders.

Claims (1)

(57) [Claims] An image data quantization circuit for expressing image data with a quantization code of a predetermined number of bits, wherein a space between a true value of a target pixel and true values of peripheral pixels of the target pixel is provided. Means for detecting a temporal change amount; and determining a decoding representative value whose spatial variation amount relative to the decoded value of the quantized code of the peripheral pixel is closest to the detected spatial change amount. Means for selecting a quantization code corresponding to the image data.