JPH0828873B2 - Image transmission equipment - Google Patents

Description

Description: [Object of the Invention] (Industrial field of application) The present invention relates to an image transmission apparatus that divides an image signal into a plurality of blocks, and encodes and transmits each block.

(Prior Art) Orthogonal transform coding is known as one of the digital transmission methods of images used in TV telephones and video conferences.
This system divides one screen into a plurality of blocks, orthogonally transforms and encodes each block, and transmits it. Then, the receiving side performs inverse transformation to reproduce the screen for each block,
Furthermore, these are combined to reproduce the entire screen.

For example, one example is encoding using cosine transform (IEEE TRANS.ON COM.VOL.COM-25, No. 11, NOV. 19
77 “Adaptive Coding of Monochrome and Color Image
s "Wen-Hsiuns Chen and Harrison Smith).

In this encoding method, after the input image signal is A / D converted, the image is read using a memory, for example, 8 pixels in the horizontal direction and 8 pixels in the vertical direction.
Divide into blocks of 64 pixels of e. Then, the cosine transform is performed for each block, and the cosine transform coefficient is encoded and transmitted. The receiving side decodes the coded transform coefficient, performs inverse cosine transform, extracts an image signal for each block, and integrates the image for each block using a memory. Then, after scan conversion is performed to reproduce one screen of image information, D / A conversion is performed to reproduce the original image signal. According to this method, energy is concentrated in the low-order component of the cosine coefficient, so that there is an advantage that the coding speed can be lowered by allocating a small number of bits in the high-order component.

By the way, in such a system in which the screen is divided into a plurality of blocks and coding is performed for each block, block distortion occurs if the coding speed is lowered to perform efficient transmission. When the block distortion occurs, the boundary between the blocks is conspicuous, and the screen looks like a tile pattern.

Therefore, for example, by applying a two-dimensional filter to the entire screen, the block distortion can be reduced to some extent, but in this case, the block distortion is reduced and the content of the image is blurred.

In addition, in such a method of performing encoding for each block, when a pixel at a corresponding position on a continuous screen changes such as “white” “black” “white” “black”, There was a problem in that a phenomenon of flickering appears. This flicker was more prominent near the block boundary. Especially for moving images, due to the effect of noise around block boundaries called mosquito noise,
Flicker was noticeable.

(Problems to be Solved by the Invention) As described above, the conventional image transmission apparatus using block coding has a problem that image flicker and block distortion occur around block boundaries.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image transmission device that can reduce image flicker and block distortion without significantly lowering the screen sharpness.

[Structure of the Invention] (Means for Solving Problems) An image transmission apparatus according to the present invention encodes image information for each block obtained by this means and means for dividing a screen into a plurality of blocks. Means and means for receiving and decoding the image information for each block coded by this means through a transmission path, and the pixels of the block decoded by this means which are located around the boundary of the block On the other hand, the image information of the original image is reproduced by integrating the filter means for applying the filter processing using at least the pixel and the corresponding pixel of the subsequent screen and the image information for each block filtered by this means. And means for doing so.

Preferably, the filter means performs a three-dimensional filter process on the pixels of the block, using the pixels, pixels around a predetermined range of the pixels, and pixels of a subsequent screen corresponding thereto. If the pixel to be filtered is located around the boundary of the block, the three-dimensional low-pass filter process for suppressing the high-frequency component of the decoded image information is used as the three-dimensional filter process. May be

Further, preferably, the filter means may perform the three-dimensional low-pass filter processing only on pixels around a boundary of a moving block.

Also, preferably, the filter means increases attenuation in the vertical direction of the image signal when the horizontal frequency component of the image signal representing the image information of the block is high, and increases the attenuation of the image signal when the vertical frequency component is high. The filtering process may be performed so as to increase the attenuation of the signal in the horizontal direction.

(Operation) In the present invention, first, the screen is divided into a plurality of blocks, the image information of each block is encoded, and the encoded image information of each block is received via the transmission path and decoded. Then, before the image information of the original image is finally reproduced by integrating the image information of each block, at least the pixels that are located around the boundary of the block of the decoded block are processed. Filtering is performed using a pixel and a pixel at a position corresponding to the pixel in the subsequent screen.

Therefore, for example, even when the pixel at the corresponding position on the continuous screen changes such as “white”, “black”, “white”, and “black”, the degree of change in the pixel value is reduced by the filtering process, and the image flicker occurs. Is reduced.

Further, the decoded image information obtained by decoding and integrating the image information coded and transmitted for each block on the receiving side is an image in which the block boundary portion is noticeable because block distortion occurs. Of the decoded image information, the information around the block boundary is also used in the subsequent screen. 3
Since it is filtered by the dimensional low-pass filter, block distortion is reduced, and most of the image can be kept fine, so that a natural image can be reproduced.

(Example) Hereinafter, the Example of this invention is described based on drawing.

FIG. 1 shows the configuration of an image transmission apparatus according to an embodiment. The A / D conversion circuit 1 converts the input image signal into A
/ D conversion is performed and 8-bit gradation image information is output.
The block division circuit 2 divides the above-mentioned image information into blocks each of which is composed of, for example, 8 × 8 total 64 pixels by using an internal frame memory. The cosine conversion circuit 3 inputs the image information for each block divided by the block division circuit 2 and performs cosine conversion. The encoding circuit 4 encodes the cosine coefficient obtained by the cosine transform circuit 3 to generate a transmission line 5
Output to. The decoding circuit 6 on the receiving side decodes the encoded image information for each block transmitted via the transmission path 5. The inverse cosine transform circuit 7 performs inverse cosine transform on the image information for each block decoded by the decoding circuit 6,
Obtain reproduction image information for each block. Block integrated circuit 8
Uses the internal memory to integrate the reproduced image information for each block output from the inverse cosine transform circuit 7 to generate decoded image information for one screen. Adaptive low-pass filter (adaptive
The LPF) 9 performs different filtering on the part around the boundary of the block and the part not on the part of the decoded image information reproduced by the block integration circuit 8. This adaptation LPF9
Switches the internal filter under the control of the control circuit 10 which detects the boundary between blocks. D / A conversion circuit 11
Reproduces an image signal by D / A converting the image information that has passed through the adaptive LPF 9. The timing generation circuit 12 generates horizontal clock CK, frame synchronization V, and line synchronization H signals for timing the respective circuits 6 to 11 on the receiving side.

FIG. 2 is a diagram showing a more detailed configuration of the control circuit 10 of the adaptive LPF 9. The decoded image information reproduced by being block-integrated is given to LPF ₁ 21 and LPF ₂ 22. These LPF ₁ 21, LPF ₂
22 have different cutoff frequencies, and are selectively output by selecting the changeover switch 23 connected to the output end thereof. The LPF ₁ 21 and LPF ₂ 22 are composed of, for example, three-dimensional digital filters as shown in FIG. 31 in the figure is
1 is a dot delay circuit, that is, a delay circuit that delays 1 pixel in the horizontal direction, 32 is a 1 line delay, that is, a delay circuit that delays 1 line in the vertical direction, and 33 is a delay circuit that delays 1 frame. By multiplying each of these delayed outputs by the coefficient aijk of the coefficient circuit 34 and adding them by the adder 35, L
PF can be configured. This example is an example in the case of horizontal 3 taps, vertical 3 taps, and time 2 taps.

Here, the coefficient circuit 34 is appropriately changed so that the cutoff frequency of the LPF ₁ 21 is lower than the cutoff frequency of the LPF ₂ 22. Incidentally, omitted LPF ₂ 22, may be through.

The control circuit 10 includes a CK counter 41 as shown in FIG.
It is composed of a line counter 42 and an OR gate 43. The CK counter 41 is reset by the left end of the line, that is, the line synchronizing signal H, counts the horizontal clock CK by 8 and outputs "1" in the "1" and "8" columns. This is a signal for detecting the position of the horizontal boundary of the block. Also,
The line counter 42 is reset by the frame synchronization signal V, counts the line synchronization signal H by 8 and outputs "1" on the "1" and "8" lines. This becomes a signal for detecting the position of the vertical boundary of the block. The OR gate 43 takes the OR of the outputs from these counters 41 and 42 and outputs it as a control signal to the changeover switch 23. Changeover switch 23
When the signal from the OR gate 43 is "1", selects the LPF ₁ 21 side.

Therefore, as shown in FIG. 4, the image around the boundary of the block indicated by ● is a blurred image by the LPF ₁ 21, and the image around the boundary of the block indicated by ○ is fine by the LPF ₂ 22. It becomes an image. Therefore, block distortion can be reduced without blurring most of the image on the screen.

Incidentally, in the above example, as shown in FIG. 5, LPF ₁
~ By using LPF ₄ and a plurality of LPFs with successively higher cutoff frequencies, the degree of blurring is lowered from the periphery of the block toward the center, which further enhances the naturalness of the image quality.

It also separates the horizontal and vertical boundaries,
If the horizontal frequency component of the image signal is high or the vertical frequency component is low, the attenuation in the vertical direction is increased, and if the horizontal frequency component is low or the vertical frequency component is high, the attenuation in the horizontal direction is increased. That is also effective.

It should be noted that the filtering of the block boundary portion does not necessarily have to be performed for all blocks. That is, normally, the block distortion becomes large when low-speed encoding is performed when an image of a block having motion between frames is transmitted. Therefore, there is no need to filter still image blocks.

FIG. 6 is an example of an image transmission device configured on the basis of such a point.

That is, in this device, the motion detection circuit 51 on the transmission side,
Further, motion information decoding circuits 52 are added to the receiving side.

As shown in FIG. 7, the motion detection circuit 51 guides the image information divided into blocks by the block division circuit 2 and a signal obtained by delaying the image information by one frame in the frame memory 61 to the comparison operation circuit 62, Add the difference of signals for one block,
If the addition result is a certain value or more, it is determined that the block is a motion block. Then, a motion block address is generated from this motion information and output to the encoding circuit 4.
The motion block address is encoded by the encoding circuit 4,
It is multiplexed with the image signal and transmitted.

The motion information decoding circuit 52 separates and decodes the motion block address multiplexed in the image signal, and the motion block address is decoded by the control circuit 10.
Output to.

FIG. 8 shows a configuration example of the adaptive LPF 9 and the control circuit 10. The description of the adaptive LPF9 is omitted because it is the same as the previous example. When the control circuit 10 inputs the motion block address, the address decoding circuit 71 calculates the horizontal address Sh, Eh and the vertical address Sv, Ev of the boundary portion of the block,
Output to comparator 72.73. For example, if the motion block shown in FIG. 9 is given as an address, the Sh column, the Eh column, the Sv line, and the Ev line are designated. The horizontal clock signal CK is counted by the counter 74. The output Ch of the counter 74 of the comparator 72 is Ch = Sh or Ch =
Outputs "1" when Eh. On the other hand, the line synchronization signal H is counted by the counter 75. The comparator 73 outputs "1" when the output Cv of the counter 75 is Cv = Sv or Cv = Ev. Therefore, the OR gate 76 outputs "1" when scanning the boundary portion of the block of pixels in this motion block. Then, the changeover switch 23 is controlled by this signal.

Thus using the filter output of the LPF ₁ 21 at the boundary of the motion block, by using the output of the LPF ₂ 22 to other parts, large block distortion of the motion block portion can be prevented. Moreover, the fineness of other parts is not impaired.

In the above embodiment, the motion information is detected on the transmission side and multiplexed and transmitted to the image signal. However, when the encoding is performed by conditional pixel supplementation, that is, when the block of the motion image is transmitted in a frame period, the transmission is performed. Since the block address becomes the motion information as it is, the multiplex transmission of the image is unnecessary. The receiving side can also determine whether or not the image is a moving image.

Furthermore, in each of the above embodiments, the cosine transform is used as the orthogonal transform, but it is needless to say that it can be similarly applied to the case where other transform methods such as Fourier transform, Hadamard transform, slant transform, and KL transform are adopted. There is no.

[Advantages of the Invention] As described above, according to the present invention, it is possible to reduce image flicker.

Further, as described above, according to the present invention, since the filtering for suppressing the block distortion is applied only to the portion where the influence of the block distortion is particularly large,
The block distortion can be reduced without significantly impairing the screen definition, and an image transmission device with good image quality can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an image transmission apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing a more specific configuration of an adaptive LPF and a control circuit in the apparatus, and FIG. FIG. 4 is a block diagram showing a more specific configuration of the LPF constituting the LPF, FIG. 4 is a diagram for explaining the filtering using the same device in association with the screen, and FIG. 5 is another embodiment of the present invention. FIG. 6 is an explanatory diagram, and FIG. 6 is a block diagram showing a configuration of an image transmission device according to still another embodiment of the present invention.
FIG. 8 is a block diagram showing a more specific configuration of a motion detection circuit in the same device, FIG. 8 is a diagram showing a more specific structure of an adaptive LPF and a control circuit in the same device, and FIG. 9 is a motion block in the same device. It is a figure for demonstrating the detection procedure of the boundary. 1 ... A / D conversion circuit, 2 ... block division circuit, 3 ...
Cosine conversion circuit, 4 ... Encoding circuit, 5 ... Transmission line,
6 ... Decoding circuit, 7 ... Inverse cosine circuit, 8 ... Block integration circuit, 9 ... Adaptive LPF, 10 ... Control circuit, 11 ...
… D / A conversion circuit, 12 …… Timing generation circuit, 51 …… Motion detection circuit, 52 …… Motion information decoding circuit.

Claims (4)

[Claims] 1. A means for dividing a screen into a plurality of blocks, a means for encoding image information for each block obtained by this means, and a transmission path for transmitting the image information for each block encoded by this means. Means for receiving and decoding via the block, and a filter using at least the pixel and the corresponding pixel of the subsequent screen for pixels located around the boundary of the block among the pixels of the block decoded by this means An image transmission apparatus comprising: filter means for performing respective processing; and means for reproducing image information of an original image by integrating image information for each block filtered by this means. 2. The filter means performs a three-dimensional filtering process on the pixels of the block, using the pixels, pixels around a predetermined range of the pixels, and pixels of a subsequent screen corresponding thereto. If the pixel to be filtered is located around the boundary of the block, the three-dimensional low-pass filter processing for suppressing the high-frequency component of the decoded image information is performed as the three-dimensional filter processing. The image transmission device according to claim 1, which is used. 3. The image transmission device according to claim 2, wherein the filter means performs the three-dimensional low-pass filter processing only on pixels around a boundary of a moving block. 4. The filtering means increases attenuation in the vertical direction of the image signal when the horizontal frequency component of the image signal representing the image information of the block is high, and increases the vertical frequency component of the image signal when the vertical frequency component is high. The image transmission device according to claim 2, wherein the filtering process is performed so as to increase the attenuation of the signal in the horizontal direction.