JPS62172884A - Orthogonal transform coding method for picture data - Google Patents

Abstract

PURPOSE:To prevent the occurrence of block distortion by encoding a picture data that is used as the boundary of a block without applying an orthogonal transformation, and performing a smoothing between both picture data. CONSTITUTION:A picture data (x) on which a preceding process has been applied is extracted by every data x1 that constitutes a prescribed block, and every data x2 that is used as the boundary of the block. The picture data x1 is passed through an orthogonal transformation circuit 12, and receives a two-dimensional orthogonal transformation, and an encoded data f(y1) is recorded on a recording and reproducing device 14. The picture data x2 is recorded on the recording and reproducing device 14 as the data having the same code length. When a picture is reproduced, the picture data x2 and the picture data f(y1) are read out, and a restored picture data x1 and the picture data x2 are synthesized, and the peripheral part of the boundary is reproduced receiving a smoothing process.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to an image data encoding method for the purpose of data compression, and more particularly to an image data encoding method using orthogonal transformation.

(Technical Background of the Invention and Prior Art) Since an image signal carrying a halftone image, such as a TV signal, has a huge amount of information, a broadband transmission path is required for its transmission. Therefore, in view of the fact that such image signals have large redundancy, various attempts have been made to compress image data by suppressing this redundancy. Recently, it has become common practice to record halftone images on optical disks, magnetic disks, etc., and in this case, image data r [widely applied has been done.

One well-known method for compressing image data is one that utilizes orthogonal transformation of image data. This method divides digital two-dimensional image data into blocks with an appropriate number of samples, and orthogonally transforms the numerical sequence consisting of the sample values for each block.This transformation concentrates energy in a specific component, so the energy By assigning a long code length to a large component of
This reduces the number of codes per block. The above-mentioned orthogonal transformation includes Fourier transformation,
Cosine (CO8i ne) transformation, Hadamard (Had
amard>transformation, Karhun-Lebe
Although en-Loeve transform, Haar transform, etc. are often used, the above method will be briefly explained in detail using Hadamard transform as an example. First, as shown in FIG. 2, digital two-dimensional image data is divided into two blocks in a predetermined one-dimensional direction to form the blocks. Two sample values X (0) and × in this block
(1) in a rectangular coordinate system, they are highly correlated as mentioned above, so as shown in Figure 3, x (1) =
They are mostly distributed near the straight line x (0).

Therefore, this orthogonal target system is transformed by 45 degrees as shown in FIG. 3 to define a new y(0)-y(1) coordinate system. In this coordinate system, y(0) indicates the low frequency component of the original image data before conversion, and the V(0) is X(0)
, takes a slightly larger value (approximately V''2 times) than X(1), but on the other hand, y(1), which indicates the high frequency component of the original image data, is distributed only in a very narrow range near the y(O) axis. Therefore, for example, the above X (0), X ('1)
If a code length of 7 bits is required to encode each of As a result, the code length per block is reduced, and image data compression is put into practical use.

Above, we have explained the second-order orthogonal transformation that constitutes one block for each two pieces of image data, but as the order increases, energy tends to concentrate on a specific component, increasing the effect of reducing the number of bits. I can do it. In general, the above transformation can be performed using an orthogonal function matrix, and in the extreme, if the eigenfunction of the target image is selected as the orthogonal function matrix, the transformed image becomes its eigenvalue matrix, and the pair of matrices This means that the original image can be expressed using only the angular components. Also, in the above example, image data is grouped into a block in only one dimension, but this block may be composed of several pieces of image data spanning two dimensions, in which case one-dimensional orthogonal transformation is performed. A more remarkable effect of reducing the number of bits can be obtained than in the case where the number of bits is reduced.

The encoded data obtained by encoding each converted data with a unique code length as described above is transmitted as described above, or is recorded on a recording (storage) means such as an optical disk, and then is finally stored. is used to reproduce the original image. That is, the encoded data is decoded into the transformed data by a restoration circuit, and then the original image data is reconstructed by applying an inverse orthogonal transformation to this transformed data.

By the way, as mentioned above, when performing orthogonal transformation on image data, orthogonal transformation is performed for each block to shorten the transformation time and reduce the capacity of the buffer memory. In some cases, so-called block distortion may be noticeable in the reproduced image.

This block distortion occurs when, in a reproduced image reproduced based on a reconstructed original image signal, a step in the density pattern occurs at a portion corresponding to the boundary of the block, and this is visually recognized as a line. This block distortion becomes more noticeable as the compression rate of image data is increased, and it greatly impairs the quality of the reproduced image.

(Objective of the Invention) Therefore, it is an object of the present invention to provide an orthogonal transform encoding method for image data that can prevent the occurrence of block distortion.

(Structure of the Invention) The orthogonal transform encoding method of image data of the present invention involves converting transformed data obtained by performing orthogonal transform on a block-by-block basis as described above into codes with a unique code length. In an orthogonal transform encoding method for image data in which the original image data is reconstructed by decoding the converted data and applying an inverse orthogonal transform, the image data at the boundaries of the blocks are coded without orthogonal transform. When performing the above reconstruction, the image data obtained by inverse transformation is
The boundary portion is reconstructed by combining image data obtained by decoding the encoded data without orthogonal transformation, and then smoothing is performed between these two image data. .

(Tora O1! Aspect) Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings.

FIG. 1 schematically shows an apparatus for carrying out the orthogonal transform encoding method of image data according to the present invention.

Image data (original image data) X representing a halftone image is first passed through a preprocessing circuit 10, and undergoes preprocessing such as smoothing to remove noise to increase data compression efficiency. The image data X that has undergone this preprocessing is extracted in a signal extraction circuit 11 for each data forming a predetermined block and data serving as a boundary between these blocks. That is, as shown in FIG. 4, the image data X carrying the halftone image @F is divided into a predetermined number of two-dimensional blocks B, and these 70
The data constituting the block B8 is extracted as image data x 1, and on the other hand, data for each pixel column or the boundary between each block 8, for example, is extracted as image data x 2.

The above image data for each block B x 1 is orthogonal transform circuit 1
2 and undergoes a two-dimensional orthogonal transformation. Note that as this orthogonal transformation, for example, the aforementioned Hadamard transformation is used. Since the Hadamard transform has a transform matrix consisting of only +1 and -1, it can be executed by a simpler transform circuit compared to other orthogonal transforms. Also, as is well known, 2
A dimensional orthogonal transformation can be reduced to a one-dimensional orthogonal transformation. In other words, one-dimensional orthogonal transformation is applied to the image data regarding MXN pixels in the two-dimensional block B in the vertical direction,
Roughly speaking, a two-dimensional orthogonal transformation is performed by applying a one-dimensional orthogonal transformation in the horizontal direction to the obtained MXN transformation data. Note that the order of vertical and horizontal conversion may be reversed.

Transformed data y! obtained by the above two-dimensional orthogonal transformation!
is sent to a known encoding circuit 13 and encoded (quantized)
However, as mentioned above, this conversion data y1 has a specific component (
Since the energy is concentrated in the low-frequency components), a relatively long code length is given to the low-frequency components with high energy, while a relatively short code length is given to the high-frequency components with low energy (or the image reproduction device The number of bits required per block B is reduced, and image data compression is achieved.

The image data f(yl) encoded as described above is recorded on a recording medium (image file) such as an optical disk or a magnetic disk by the recording/reproducing device 14. As described above, since this image data f(yl) is compressed with respect to the original image data X, a large amount of images can be recorded on a recording medium such as an optical disk.

On the other hand, image data carrying the boundary of the block B x 2
are recorded on an optical disk or the like together with the image data f(Vt) in the recording/reproducing device 14 while maintaining the same code length.

When playing an image, the above image data x 2 and fN/
l) are both read from the recording medium and compared.

The image data f(yt) is then decoded into the converted data y1 in the decoding circuit 16. This transformed data y1 is sent to the inverse transform circuit 17 and undergoes the inverse transform to the two-dimensional orthogonal transform. As a result, image data x1 is restored.

This image data x 1 is sent to the signal synthesis circuit 19 together with image data x 2 read from the recording medium, and is synthesized under the relationship given to the original image data X. That is, as shown in FIG. 4, image data x2 is incorporated at the boundary of each block B indicated by image data x1. The composite image data Xo obtained in this manner is sent to the smoothing circuit 20, where the peripheral portions of the boundary portion (that is, the image data x 2 and the image data x 1 surrounding them) are smoothed. This smoothing is performed by a known method such as a mean value replacement method or an interpolation method.

The image data X that has been smoothed is sent to an image reproducing device 21 such as a CRT or an optical scanning recording device, where the image carried by the image data X is reproduced. Here, as described above, if the image ri:f5 tb is increased by orthogonal transformation, the signal value may change rapidly at the boundary between adjacent blocks B in the image data x 1 that has been 1q transformed by the inverse transformation. arise.

If an image is reproduced using such image data as is, the aforementioned block distortion will occur. However, in the method of the present invention, since the above-described smoothing process is performed, it is possible to prevent the block distortion from occurring in the reproduced image. Furthermore, since this smoothing process is performed using the image data x2, which is the original image data that has not undergone orthogonal transformation, the image density at this boundary portion is faithfully reproduced with respect to that in the original image.

In the embodiment described above, as shown in FIG.
As shown in the figure, each block B is set to be close to each other, and all the image data in this block B is orthogonally transformed, and the data before orthogonal transformation of, for example, the rightmost column and bottommost pixels of this block B is Alternatively, the image data may be extracted as boundary value data, and after inverse transformation, the image data for the pixel may be replaced with the extracted boundary value data, and then smoothing processing may be performed.

Furthermore, in the embodiment described above, the image data x 2 is recorded without data compression at all, but as shown in FIG. 6, an intelligent predictive coding circuit 15 is provided to predict the image data x 2. The data may be encoded, compressed to some extent, and recorded on a recording medium. In this case, when reproducing an image, the predicted code data f (Xz ) read from the recording medium may be passed through the predicted value decoding circuit 18 to restore image data x2.

(Effects of the Invention) As explained in detail above, according to the orthogonal transform encoding method for image data of the present invention, it is possible to reliably prevent block distortion from occurring in a reproduced image, thereby preventing deterioration of the image quality of the reproduced image. This makes it possible to increase the data compression rate without any problems.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an apparatus for carrying out the first embodiment method of the present invention, FIGS. 2 and 3 are explanatory diagrams illustrating orthogonal transformation according to the present invention, and FIGS. 4 and 5 FIG. 6 is an explanatory diagram illustrating division of image data into blocks according to the present invention, and FIG. 6 is a block diagram showing a schematic configuration of an apparatus that implements a second embodiment method of the present invention.

Claims (1)

[Claims] Two-dimensional image data is orthogonally transformed for each block consisting of a predetermined number of data to obtain transformed data, each of these transformed data is encoded with a unique code length, and then this In the orthogonal transform encoding method for image data, the image data is reconstructed by decoding encoded data to obtain the transform data and subjecting the transform data to an inverse transform of the orthogonal transform, wherein the boundaries of the blocks The image data resulting in is encoded without applying the orthogonal transformation, and when performing the reconstruction, the image data obtained by decoding the encoded data without applying the orthogonal transformation to the image data resulting from the inverse transformation. An orthogonal transform encoding method for image data, characterized in that the boundary portion is reconstructed by combining the data, and smoothing is performed between the two image data.