Video transcoder with spatial resolution reduction

Details Video transcoder with spatial resolution reduction Video transcoder with spatial resolution reduction

2001-05-11

2003-12-30

Le, Vu (Department: 2613)

Pulse or digital communications

Bandwidth reduction or expansion

Television or motion video signal

C375S240130, C375S240210

Reexamination Certificate

active

06671322

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of transcoding bitstreams, and more particularly to reducing spatial resolution while transcoding video bitstreams.
BACKGROUND OF THE INVENTION
Video compression enables the storing, transmitting, and processing of visual information with fewer storage, network, and processor resources. The most widely used video compression standards include MPEG-1 for storage and retrieval of moving pictures, MPEG-2 for digital television, and H.263 for video conferencing, see ISO/IEC 11172-2:1993, “Information Technology—Coding of Moving Pictures and Associated Audio for Digital Storage Media up to about 1.5 Mbit/s—Part 2: Video,” D. LeGall, “MPEG: A Video Compression Standard for Multimedia Applications,” Communications of the ACM, Vol. 34, No. 4, pp. 46-58, 1991, ISO/IEC 13818-2:1996, “Information Technology—Generic Coding of Moving Pictures and Associated Audio Information—Part 2: Video,” 1994, ITU-T SG XV, DRAFT H.263, “Video Coding for Low Bitrate Communication,” 1996, ITU-T SG XVI, DRAFT13 H.263+Q15-A-60 rev.0, “Video Coding for Low Bitrate Communication,” 1997.
These standards are relatively low-level specifications that primarily deal with a spatial compression of images or frames, and the spatial and temporal compression of sequences of frames. As a common feature, these standards perform compression on a per frame basis. With these standards, one can achieve high compression ratios for a wide range of applications.
Newer video coding standards, such as MPEG-4 for multimedia applications, see ISO/IEC 14496-2:1999, “Information technology—coding of audio/visual objects, Part 2: Visual,” allow arbitrary-shaped objects to be encoded and decoded as separate video object planes (VOP). The objects can be visual, audio, natural, synthetic, primitive, compound, or combinations thereof. Also, there is a significant amount of error resilience features built into this standard to allow for robust transmission across error-prone channels, such as wireless channels.
The emerging MPEG-4 standard is intended to enable multimedia applications, such as interactive video, where natural and synthetic materials are integrated, and where access is universal. In the context of video transmission, these compression standards are needed to reduce the amount of bandwidth on networks. The networks can be wireless or the Internet. In any case, the network has limited capacity, and contention for scarce resources should be minimized.
A great deal of effort has been placed on systems and methods that enable devices to transmit the content robustly and to adapt the quality of the content to the available network resources. When the content is encoded, it is sometimes necessary to further decode the bitstream before it can be transmitted through the network at a lower bit-rate or resolution.
As shown in
FIG. 1
, this can be accomplished by a transcoder
100
. In a simplest implementation, the transcoder
100
includes a cascaded decoder
110
and encoder
120
. A compressed input bitstream
101
is fully decoded at an input bit-rate R
in
, then encoded at an output bit-rate R
out
102
to produce the output bitstream
103
. Usually, the output rate is lower than the input rate. In practice, full decoding and full encoding in a transcoder is not done due to the high complexity of encoding the decoded bitstream.
Earlier work on MPEG-2 transcoding has been published by Sun et al., in “Architectures for MPEG compressed bitstream scaling,” IEEE Transactions on Circuits and Systems for Video Technology, April 1996. There, four methods of rate reduction, with varying complexity and architecture, were described.
FIG. 2
shows a first example method
200
, which is referred to as an open-loop architecture. In this architecture, the input bitstream
201
is only partially decoded. More specifically, macroblocks of the input bitstream are variable-length decoded (VLD)
210
and inverse quantized
220
with a fine quantizer Q
1
, to yield discrete cosine transform (DCT) coefficients. Given the desired output bit-rate
202
, the DCT blocks are a re-quantized by a coarser level quantizer Q
2
of the quantizer
230
. These re-quantized blocks are then variable-length coded (VLC)
240
, and a new output bitstream
203
at a lower rate is formed. This scheme is much simpler than the scheme shown in
FIG. 1
because the motion vectors are re-used and an inverse DCT operation is not needed. Note, here the choice of Q
1
and Q
2
strictly depend on rate characteristics of the bitstream. Other factors, such as possibly, spatial characteristics of the bitstream are not considered.
FIG. 3
shows a second example method
300
. This method is referred to as a closed-loop architecture. In this method, the input video bitstream is again partially decoded, i.e., macroblocks of the input bitstream are variable-length decoded (VLD)
310
, and inverse quantized
320
with Q
1
to yield discrete cosine transform (DCT) coefficients
321
. In contrast to the first example method described above, correction DCT coefficients
332
are added
330
to the incoming DCT coefficients
321
to compensate for the mismatch produced by re-quantization. This correction improves the quality of the reference frames that will eventually be used for decoding. After the correction has been added, the newly formed blocks are re-quantized
340
with Q
2
to satisfy a new rate, and variable-length coded
350
, as before. Note, again Q
1
and Q
2
are rate based.
To obtain the correction component
332
, the re-quantized DCT coefficients are inverse quantized
360
and subtracted
370
from the original partially decoded DCT coefficients. This difference is transformed to the spatial domain via an I inverse DCT (IDCT)
365
and stored into a frame memory
380
. The motion vectors
381
associated with each incoming block are then used to recall the corresponding difference blocks, such as in motion compensation
290
. The corresponding blocks are then transformed via the DCT
332
to yield the correction component. A derivation of the method shown in
FIG. 3
is described in “A frequency domain video transcoder for dynamic bit-rate reduction of MPEG-2 bitstreams,” by Assuncao et al., IEEE Transactions on Circuits and Systems for Video Technology, pp. 953-957, 1998.
Assuncao et al. also described an alternate method for the same task. In the alternative method, they used a motion compensation (MC) loop operating in the frequency domain for drift compensation. Approximate matrices were derived for fast computation of the MC blocks in the frequency domain. A Lagrangian optimization was used to calculate the best quantizer scales for transcoding. That alternative method removed the need for the IDCT/DCT components.
According to prior art compression standards, the number of bits allocated for encoding texture information is controlled by a quantization parameter (QP). The above methods are similar in that changing the QP based on information that is contained in the original bitstream reduces the rate of texture bits. For an efficient implementation, the information is usually extracted directly from the compressed domain and can include measures that relate to the motion of macroblocks or residual energy of DCT blocks. The methods describes above are only applicable for bit-rate reduction.
Besides bit-rate reduction, other types of transformation of the bitstream can also be performed. For example, object-based transformations have been described in U.S. patent application Ser. No. 09/504,323, “Object-Based Bitstream Transcoder,” filed on Feb. 14, 2000 by Vetro et al. Transformations on the spatial resolution have been described in “Heterogeneous video transcoding to lower spatio-temporal resolutions, and different encoding formats,” IEEE Transaction on Multimedia, June 2000, by Shanableh and Ghanbari.
It should be noted these methods produce bitstreams at a reduced spatial resolution reduction that lack quality, or are accomplished with high complexity. Also, proper consideration has not been

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