MPEG2 moving picture encoding/decoding system

Details MPEG2 moving picture encoding/decoding system MPEG2 moving picture encoding/decoding system

2001-05-24

2003-09-16

Mehta, Bhavesh M. (Department: 2621)

Image analysis

Image compression or coding

Including details of decompression

C382S250000, C382S100000, C380S056000, C375S240230

Reexamination Certificate

active

06621933

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention can be used for a Moving Picture Experts Group (MPEG) moving picture compression/decompression application of motion prediction/compensation based on a discrete cosine transform and minimizes degradation of image quality.
This invention relates to a digital copy protection application of moving picture data which is seldom removed intentionally or unintentionally by a user except an author, and more particularly, relates to an MPEG2 moving picture encoder/decoder.
2. Description of the Related Art
The MPEG2 standard is a compression/decompression standard for video applications, and exploits temporal redundancy for motion compensated interpolated and predicted encoding. That is, the assumption is made that “locally” the current picture can be modeled as a translation of the picture at a previous and/or future time. “Locally” means that the amplitude and direction of the displacement are not the same everywhere in the picture.
The MPEG2 standard specifies predicted and interpolated interframe encoding and spatial domain intraframe encoding. It has block based motion compensation for the reduction of temporal redundancy, and a block based Discrete Cosine Transform based on compression for the reduction of spatial redundancy.
The information relative to motion is based on a 16×16 array of pixels and is transmitted with the spatial information. Motion information is compressed with variable length codes, such as Huffman codes.
Recently, audio/video information expressed as digital information is becoming more widely used and a method utilizing digital products also has been increasing accordingly as digitalization of the A/V application and popularity of the Internet have been increasing rapidly.
Specifically anybody who is able to use a PC can copy/edit digital products easily and accordingly a social issue of illegal copying has been raised. A watermark technique has become prominent as a solution to prevent this problem.
There are two exemplary methods for providing copy protection of digital A/V data to prevent illegal copying. The first method is encryption, i.e., a copy protection method by scrambling the digital information. The second method is a digital watermark method with the purpose of preventing the illegal use of digital information.
The first method is a technique for prevention of illegal copying of digital A/V data, by providing descramble information and password information capable of accessing and running the A/V product only in the case that the A/V product is bought legally.
The second method is a technique which utilizes self-restraint by a user to not produce an illegal copy of the A/V product by embedding ID information or a logo in a state of noise into A/V contents data of the A/V product for the purpose of forbidding the illegal or commercial use of digital information. The watermark technique is used on the original image and is invisible to a person who would copy it, but the author can prove that the copied image is his by virtue of an arbitrary reverse processing.
For example, in a case where a counterfeiter forges money using a color copier, a vignette on the original bill turns clear by copying the bill, and consequently, it makes it virtually impossible to copy a bank note. This is called a visible watermark.
Also, in the case where a spy writes a message onto paper with salt water, other people think this is ordinary paper, but this paper is a medium having important information for the spy. The spy can see the message anytime he wants to see by heating the paper. In this case, we call it an invisible watermark.
At present, the watermark technique is used for digital still images or audio, i.e., putting the message distinguishable from the original image into the image. Therefore, in case that an author's own image circulates illegally, the image can be proved to be that of author by performing an arbitrary reverse processing.
Thus, techniques for preventing the illegal copying of digital product are increasingly being studied these days.
FIGS. 1 and 2
illustrate a conventional MPEG2 moving picture encoder and decoder respectively.
FIG. 3
illustrates a structure of a video picture used in the MPEG2 moving picture encoder/decoder and,
FIG. 4
illustrates three types of pictures and their relationship under the MPEG2 standard. We will explain the conventional MPEG2 moving picture encoder and decoder by referring to these figures.
MPEG encoding is a hybrid type lossy coding technique wherein the redundancy information which the image signals have in the spatial domain and the temporal domain are removed and the data are compressed (refer to FIG.
3
). At this time, the compression technique of spatial domain is called intra-coding and the image data used in intra-coding are called intra-picture (in short, I picture). The compression technique of temporal domain is called inter-coding and in this case, the image data are classified by two according to two prediction methods. The first one is a predicted picture (P picture) wherein prediction error rate of a forward direction is encoded, and the second one is an interpolated picture or bi-directional picture (B picture) wherein a prediction error rate of the bi-directional direction is encoded.
In other words, the I picture is encoded independently of other near pictures (in this instance, the picture is a frame signal or a field signal). In the P picture, the difference signals of predicted/interpolated movement are encoded only after considering the correlation of the movement of the previous I picture or P picture. In the B picture, the difference signals of predicted/interpolated movement are coded only after considering the correlation of the movement of the previous I or P picture and the next I or P picture.
Among the three modes, that is, the forward direction mode, backward direction mode, and forward and backward direction mode, the mode having the smallest value of prediction error rate is selected in the prediction/interpolation method of the B picture.
The picture structure of the MPEG recommendation (called a main profile, main level; MP@ML) is I, B, B, P, B, B, P . . . pictures and needs a frame memory
110
which can store at least three pictures (the cycle of a picture).
A field/frame adaptive coding method is possible in an MPEG2 encoding method so as to increase coding efficiency. The unit of the picture can be defined as a field or frame according to the purpose of the encoding. A frame/field memory
112
stores the field data or frame data to be coded.
A subtractor
134
receives the field data or frame data from the frame/field memory
112
and interpolated predicted motion data from an adaptive estimator
130
, to perform a subtraction of local decoded I or P pictures, wherein the motion prediction is interpolated to encode the prediction error rate of the P and B pictures, and the pictures which are now input. A Discrete Cosine Transform (DCT)
114
performs an orthogonal transform which transforms image signals from the subtractor
134
spatially structured into image signals of the frequency domain. A quantizer
116
approximates signals to a typical value to map the DCT-transformed image signals to a code book which is defined in a variable length coder (VLC). Data loss occurs in the quantizer
116
.
A dequantizer
122
performs an inverse process of the quantizer
116
for encoding the prediction error rate of P and B pictures. An inverse DCT (IDCT)
124
performs an inverse process of the DCT
114
for encoding the prediction error rate of the P and B pictures. A subtractor
126
performs a subtraction operation on the output from the IDCT
124
and the interpolated predicted output from the adaptive estimator
130
. A frame memory
128
stores local decoding images output from the subtractor
126
according to the dequantizer
122
and the IDCT
124
.
A motion estimator
132
encodes the prediction error rate of the P and B pictures output from the frame/field memory
112

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