Image analysis – Image compression or coding – Transform coding
Reexamination Certificate
1998-07-16
2001-05-22
Couso, Jose L. (Department: 2721)
Image analysis
Image compression or coding
Transform coding
C382S236000, C382S238000, C375S240130, C375S240210, C386S349000
Reexamination Certificate
active
06236760
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a video coding method comprising:
a first coding step including in series at least an orthogonal transform sub-step, a quantization sub-step, and a variable length coding sub-step;
a second prediction step including in series, between the output of said quantization sub-step and the input of the coding step, at least an inverse quantization sub-step, an inverse orthogonal transform sub-step, and a prediction sub-step;
a third decision step including a selection sub-step between so-called intra and inter coding modes. It also relates to a corresponding video coding device, to a coded video signal such as available at the output of such a coding device, to a storage medium for memorizing said signal, and to a corresponding decoding device. This invention may be used in the field of videophony and video conferencing.
BACKGROUND OF THE INVENTION
The standard H.263 for very low bitrate video coding, described for instance in “ITU standardisation of very low bitrate video coding algorithms”, K. Rijkse, Signal Processing: Image Communication, 7(1995), pp.553-565, is based on a hybrid video coding method dealing with macroblock structured pictures and using techniques such as DCT (Discrete Cosine Transform), to reduce the spatial redundancy, motion estimation and interpicture prediction, to reduce spatial redundancy, and finally quantization variable length entropy coding (as also provided in the case of the MPEG-2 standard).
The maximum bitrate for this standard H.263 is about 20 kbits/s for videophone and an integer multiple of 64 kbits/s (such as 64, 128, 256, . . . ) for video conference. At these very low bitrates, various kinds of solution are often used in order to reduce the transmitted bitrate, for instance a temporal sub-sampling. These solutions must however be implemented without degrading the picture quality.
A block diagram of the standard H.263 encoder is shown in FIG.
1
. The input bitstream IB corresponding to the images to be coded is received by the first positive input of a subtracter
11
. This subtracter is followed in series by an orthogonal transform device such as a DCT circuit
12
, a quantizer
13
(Q), a variable length coding (VLC) circuit
14
, a video multiplexer
15
(MUX), and an output buffer
16
that yields an output bitstream OB. An interpicture prediction loop, provided between the output of the quantizer
13
and the second negative input of the subtracter
11
comprises in series an inverse quantizer
17
(Q
−1
), an inverse DCT circuit
18
(DCT
−1
), an adder
19
, a prediction circuit
20
, the output of which is also sent back to the second input of the adder
19
for the reconstitution of a complete image at the output of said adder, and the subtracter
11
.
The output of the adder
19
is sent to a motion estimator
21
that also receives the input bitstream IB and yields motion vectors MV. These vectors are then coded by a second VLC circuit
22
and sent to the multiplexer
15
for transmission (or storage). A decision circuit
23
provided between the output buffer
16
and the prediction circuit
20
allows to choose between an intra coding mode, concerning only the first picture of the video sequence, which is then coded without temporal prediction, and an inter coding mode, according to which all the remaining pictures are coded using prediction.
As the intra pictures are coded without any reference to any previous picture, each of them needs from 4 to 10 times (depending on the scene content and on the average quantization parameter) the amounts of bits necessary to code the subsequent pictures in inter mode. The following table (=Table 1) illustrates, for some well known test sequences in CIF format (288 lines of 352 pixels), the difference in terms of bits between intra and inter modes:
CIF sequences
intra mode
inter mode
Miss America
35568
3936
Claire
37224
3496
Renata
149984
34736
Flower Garden
180456
63512
Foreman
67736
13016
Teeny
67344
38968
Interview
106320
11272
The values of these amounts of bits necessary to code the first picture in intra mode and the second subsequent picture in inter mode lead to observe that the output buffer
16
, necessary to transmit the output stream OB at constant bitrate, is strongly used during the intra coding. A buffer with a proper capacity might be used in order to store an intra picture without any risk of overflow, but the delay of the encoder is directly proportional to the total bit number of the first intra picture: the larger the number of bits of this picture, the larger the delay to empty the output buffer at the concerned constant target bitrate.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to propose a coding method allowing to avoid any risk of overflow when coding according to the intra mode.
To this end the invention relates to a coding method as described in the preamble of the description and characterized in that said third decision step also includes, only in said intra coding mode, a down-conversion filtering sub-step, carried out at the beginning of the coding step, and a corresponding up-conversion filtering sub-step, carried out in the prediction step, just before the prediction sub-step itself, said down-conversion allowing to code pictures with a reduced picture format with respect to the format of the pictures coded according to the inter coding mode.
The introduction of these additional steps constitutes a very simple and effective solution to save bits, with an average computational reduction of about 30% and a decreased encoding delay because of the less bit occupation of the output buffer (and of said reduced computational effort). Although not standardized, this solution is compatible with the standard H.263. Tests have been performed in the range of 64-256 kbits/sec. target bitrates, with 5 Hz picture rate and CIF format, and with either a very simple buffer control strategy or a very complex one (based on a pre-analysis), and show that the solution is efficient, without noticeable degradation of the final quality. Moreover, said solution is totally scalable, even if particularly adapted to low bitrate applications (less than or equal to 256 kbits/sec.) for which the requirements of high compression ratios and acceptable quality are more difficult to achieve.
REFERENCES:
patent: 5717816 (1998-02-01), Boyce et al.
patent: 5777677 (1998-07-01), Linzer
patent: 6028635 (2000-02-01), Owen et al.
patent: 6104434 (2000-08-01), Nakagawa et al.
patent: 0631441A2 (1994-12-01), None
patent: 9613128A1 (1996-05-01), None
“ITU Standardisation of Very Low Bitrate Video Coding Algorithms”, by K. Rijkse, Signal Processing: Image Communication, No. 7, 1995, pp. 553-565.
Bagni Daniele
Tono Stefano
Couso Jose L.
Gross Russell
Sherali Ishrat
U.S. Philips Corporation
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