Generalized scalability for video coder based on video objects

A video coding system that codes video objects as scalable video object layers. Data of each video object may be segregated into one or more layers. A base layer contains sufficient information to decode a basic representation of the video object. Enhancement layers contain supplementary data regarding the video object that, if decoded, enhance the basic representation obtained from the base layer. The present invention thus provides a coding scheme suitable for use with decoders of varying processing power. A simple decoder may decode only the base layer of video objects to obtain the basic representation. However, more powerful decoders may decode the base layer data of video objects and additional enhancement layer data to obtain improved decoded output. The coding scheme supports enhancement of both the spatial resolution and the temporal resolution of video objects.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a video coding system in which image data
 is organized into video objects and coded according to a scalable coding
 scheme. The coding scheme provides spatial scalability, temporal
 scalability or both.
 2. Related Art
 Video coding is a field that currently exhibits dynamic change. Video
 coding generally relates to any method that represents natural and/or
 synthetic visual information in an efficient manner. A variety of video
 coding standards currently are established and a number of other coding
 standards are being drafted. The present invention relates to an invention
 originally proposed for use in the Motion Pictures Experts Group standard
 MPEG-4.
 One earlier video standard, known as "MPEG-2," codes video information as
 video pictures or "frames." Consider a sequence of video information to be
 coded, the sequence represented by a series of frames. The MPEG-2 standard
 coded each frame according to one of three coding methods. A given image
 could be coded according to:
 Intra-coding where the frame was coded without reference to any other frame
 (known as "I-pictures"),
 Predictive-coding where the frame was coded with reference to one
 previously coded frame (known as "P-pictures"), or
 Bi-directionally predictive coding where the frame was coded with reference
 to as many as two previously coded frames (known as "B-pictures").
 Frames are not necessarily coded in the order in which they appear under
 MPEG-2. It is possible to code a first frame as an I-picture then code a
 fourth frame as a P-picture predicted from the I-picture. Second and third
 frames may be coded as B-pictures, each predicted with reference to the I-
 and P-pictures previously coded. A time index is provided to permit a
 decoder to reassemble the correct frame sequence when it decodes coded
 data.
 MPEG-4, currently being drafted, integrated the concept of "video objects"
 to I-, P- and B-coding. Video object based coders decompose a video
 sequence into video objects. An example is provided in FIGS. 1 (a)-(d).
 There, a frame includes image data including the head and shoulders of a
 narrator, a suspended logo and a background. An encoder may determine that
 the narrator, logo and background are three distinct video objects, each
 shown separately in FIGS. 1 (b)-(d). The video coder may code each
 separately.
 Video object-based coding schemes recognize that video objects may remain
 in a video sequence across many frames. The appearance of a video object
 on any given frame is a "video object plane" or "VOP". VOPs may be coded
 as I-VOPs using intra coding techniques, as P-VOPs using predictive coding
 techniques or B-VOPs using bi-directionally predictive coding techniques.
 For each VOP, additional administrative data is transmitted with the coded
 VOP data that provides information regarding, for example, the video
 objects location in the displayed image.
 Coding video information on a video object-basis may improve coding
 efficiency in certain applications. For example, if the logo were a static
 image, an encoder may code it as an initial I-VOP. However, for subsequent
 frames, coding the logo as a P- or B-VOP would yield almost no image data.
 The P- or B-coding essentially amounts to an "instruction" that the
 original image information should be redisplayed for successive frames.
 Such coding provides improved coding efficiency.
 One goal of the MPEG-4 standard is to provide a coding scheme that may be
 used with decoders of various processing power. Simple decoders should be
 able to decode coded video data for display. More powerful decoders should
 be able to decode the coded video data and obtain superior output such as
 improved image quality or attached functionalities. As of the priority
 date of this application, no known video object-based coding scheme
 provides such flexibility.
 MPEG-2 provides scalability for its video picture-based coder. However, the
 scalability protocol defined by MPEG-2 is tremendously complicated. Coding
 of spatial scalability, where additional data for VOPs is coded into an
 optional enhancement layer, is coded using a first protocol. Coding of
 temporal scalability, where data of additional VOPs is coded in the
 enhancement layer, is coded using a second protocol. Each protocol is
 separately defined from the other and requires highly context specific
 analysis and complicated lookup tables in a decoder. The scalability
 protocol of the MPEG-2 is disadvantageous because its complexity makes it
 difficult to implement. Accordingly, there is a further need in the art
 for a generalized scalability protocol.
 SUMMARY OF THE INVENTION
 The present invention provides a video coding system that codes video
 objects as video object layers. Data of each video object may be
 segregated into one or more layers. A base layer contains sufficient
 information to decode a basic representation of the video object.
 Enhancement layers contain supplementary data regarding the video object
 that, if decoded, enhance the basic representation obtained from the base
 layer. The present invention thus provides a coding scheme suitable for
 use with decoders of varying processing power. A simple decoder may decode
 only the base layer to obtain the basic representation. However, more
 powerful decoders may decode the base layer data and additional
 enhancement layer data to obtain improved decoded output.

DETAILED DESCRIPTION
 The present invention introduces a concept of "video object layers" to the
 video object-based coding scheme. Data of each video object may be
 assigned to one or more layers of the video object and coded. A base layer
 contains sufficient information to represent the video object at a first
 level of image quality. Enhancement layers contain supplementary data
 regarding the video object that, if decoded, improve the image quality of
 the base layer. The present invention thus provides an object based a
 coding scheme suitable for use with decoders of varying processing power.
 A simple decoder may decode only the base layer of objects to obtain the
 basic representation. More powerful decoders may decode the base layer
 data and additional enhancement layer data of objects to obtain improved
 decoded output.
 FIG. 2 illustrates an organizational scheme established by the present
 invention. An image sequence to be coded is a video session. The video
 session may be populated by a number of video objects. Each video object
 may be populated by one or more video object layers. A video object layer
 is an organizational artifact that represents which part of the coded
 bitstream output by the video coder carries certain image information
 related to the video object. For example, base layer data may be assigned
 to a first video object layer (layers VOL1 for each video object VO.O
 slashed., VO1 and VO2 in FIG. 2). Enhancement layer data may be assigned
 to a second video object layer, such as VOL2 in each of VO1 and VO2. The
 video object layers are themselves populated by video object planes.
 Enhancement layers need not be provided for every video object. For
 example, FIG. 2 illustrates a video session that provides only a single
 video object layer for video object VO.O slashed..
 There is no limit to the number of video object layers that may be provided
 for a single video object. However, each video object layer added to a
 video object will be associated with a certain amount of administrative
 information required to code the video object layer. The overhead
 administrative data can impair coding efficiency.
 FIG. 3 illustrates a video coding system constructed in accordance with an
 embodiment of the present invention. The coding system includes an encoder
 100 and a decoder 200 separated by a channel 300. The encoder 100 receives
 input video objects data and codes the video objects data according to the
 coding scheme described above with respect to FIG. 2. The encoder 100
 outputs coded data to the channel 300. The decoder 200 receives the coded
 data from the channel 300 and decodes it using techniques complementary to
 those used at the encoder 100. The decoder outputs decoded video data for
 display, storage or other use.
 The channel 300 may be a real time data medium in which coded data output
 from the encoder 100 is routed directly to the decoder 200. As such, the
 channel 300 may be represented by a data communication channel provided by
 the Internet, a computer network, a wireless data network or a
 telecommunication network. The channel 300 may also be a storage medium,
 such as a magnetic, optical or electrical memory. In these applications,
 the encoder 100 and decoder 200 need not work contemporaneously. The
 encoder 100 may store coded data in the channel 300 where the coded data
 may reside until retrieved by the decoder 200.
 The encoder 100 includes a video object segmenter/formatter 400, plurality
 of video object encoders 500a-n and a systems multiplexer ("MUX") 600. In
 a typical application, the encoder 100 may be a microprocessor or digital
 signal processor that is logically divided into these components 400-600
 by program instructions. Alternatively, the components 400-600 may be
 populated by hardware components adapted to perform these functions.
 The video objects segmenter/formatter 400 receives input video data and
 identifies video objects therefrom. The process of decomposing an image
 sequence into video objects is well known and described in "Coding of
 Moving Pictures and Video," ISO/IEC 14496-2 (July 1997). The video object
 segmenter/formatter 400 outputs VOP data to each of the video object
 encoders 500a-n.
 The video object encoders 500a-n receive the VOP data of their respective
 video objects and code the VOP data according to the structure shown in
 FIG. 2. That is, the video object encoder (say, 500a) determines how many
 video object layers to use in coding the video object data. It determines
 what part of the input VOP data is coded as base layer data and what part
 is coded as enhancement layer data. The video object encoder codes the
 base layer data and any enhancement layer data as coded VOPs of each video
 object layer. It outputs coded video object data to the MUX 600.
 The MUX 600 organizes the coded video object data received from each of the
 video object encoders 500 into a data stream and outputs the data stream
 to the channel 300. The MUX 600 may merge data from other sources, such as
 audio coders (not shown), graphics coder (not shown), into the unitary
 signal stream.
 The decoder 200 includes a systems demultiplexer ("DEMUX") 700, a plurality
 of video object decoders 800a-n and a video objects compositor 900. As
 with the encoder 100, the decoder 200 may be a microprocessor or digital
 signal processor that is logically divided into these components 700-900
 by program instructions. Alternatively, the components 700-900 may be
 populated by hardware components adapted to perform these functions.
 The DEMUX 700 retrieves the unitary coded signal from the data stream
 channel 300. It distinguishes the coded data of the various video objects
 from each other. Data for each video object is routed to a respective
 video object decoder 800a-n. Other coded data, such as graphics data or
 coded audio data, may be routed to other decoders (not shown).
 The video object decoders 800a-n decode base layer data and any enhancement
 layer data using techniques complementary to those applied at the video
 object encoders 500a-n. The video object decoders 800a-n output decoded
 video objects.
 The video objects compositor 900 assembles a composite image from the
 decoded VOP data of each video object. The video objects compositor 900
 outputs the composite image to a display, memory or other device as
 determined by a user.
 FIG. 4 is a block diagram of a video object encoder 500a of the present
 invention. The video object encoder includes a scalability pre-processor
 510, a base layer encoder 520, a midprocessor 530, an enhancement layer
 encoder 540 and an encoder multiplexer 550. Again, the components of the
 video object encoder 500a may be provided in hardware or may be logical
 devices provided in a microprocessor or a digital signal processor.
 VOP data of a video object is input to the scalability pre-processor 510.
 The scalability pre-processor 510 determines which data is to be coded in
 the base layer and which data is to be coded in the enhancement layer. It
 outputs a first set of VOPs to the base layer encoder 520 and a second set
 of VOPs to the enhancement layer encoder 540.
 The base layer encoder 520 codes base layer VOPs according to conventional
 techniques. Such coding may include the nonscalable coding techniques of
 the MPEG-4 standard. Base layer VOPs are coded by intra coding, predictive
 coding or bi-directionally predictive coding and output on line 522 to the
 encoder multiplexer MUX 550. The base layer encoder also outputs locally
 decoded VOPs on line 524. The base layer encoder obtains locally decoded
 VOPs by decoding the coded base layer data. Effectively, the locally
 decoded VOPs mimic decoded base layer data that is obtained at the decoder
 200.
 The midprocessor 530 receives the locally decoded VOPs and depending on its
 mode of operation, outputs up sampled, down sampled or unchanged VOP data
 to the enhancement layer encoder 540.
 The enhancement layer encoder 540 receives VOP data from the scalability
 preprocessor 510 and locally decoded VOP data possibly having been
 modified by the midprocessor 530. The enhancement layer encoder 540 codes
 the VOP data received from the scalability preprocessor using the locally
 decoded VOP data as a basis for prediction. It outputs coded enhancement
 layer data to the encoder multiplexer 550.
 The encoder multiplexer MUX 550 outputs coded base and enhancement layer
 video object data from the video object encoder.
 FIG. 5 illustrates an example of object based temporal scalability that may
 be achieved by the present invention. There, a first sequence of VOPs
 1010, 1030, 1050, are coded by the base layer encoder 520 and a second
 sequence of VOPs 1020, 1040, are coded by the enhancement layer encoder
 540. In time order, the VOPs appear in the order: 1010, 1020, 1030, 1040,
 1050, . . . .
 The base layer encoder 520 codes VOP 1010 first as an I-VOP. Second, it
 codes VOP 1050 as a P-VOP using VOP 1010 as a basis for prediction. Third,
 it codes VOP 1030 as a B-VOP using VOPs 1010 and 1050 as bases for
 prediction.
 The enhancement layer encoder 540 codes VOP 1020 using base layer locally
 decoded VOPs 1010 and 1030 as bases for prediction. It also codes VOP 1040
 using base layer locally decoded VOPs 1030 and 1050 as bases for
 prediction. Although not shown in FIG. 5, an enhancement layer VOP (such
 as 1040) can look to another enhancement layer VOP as a basis for
 prediction. For example, VOP 1040 could be coded using VOPs 1020 as a
 basis for prediction.
 On decoding, a simple decoder decodes only the coded base layer data. It
 decodes and displays VOPs 1010, 1030, 1050, . . . providing a video
 sequence for display having a first frame rate. A power decoder, however,
 that decodes both base layer and enhancement layer data obtains the entire
 VOP sequence 1010, 1020, 1030, 1040, 1050, . . . . It decodes a video
 sequence having a higher frame rate. With a higher frame rate, an observer
 would perceive more natural motion.
 FIG. 6 illustrates an example of object based spatial scalability that may
 be achieved by the present invention. There, VOPs 1110-1140 are coded by
 the base layer encoder 520. Spatially, larger VOPs 1210-1240 are coded by
 the enhancement layer encoder 540. Enhancement layer VOPs 1210-1240
 coincide, frame for frame, with the base layer VOPs 1110-1140.
 The base layer encoder 520 codes the base layer VOPs in the order 1110,
 1130, 1120, . . . . VOP 1110 is coded as an I-VOP. VOP 1130 is coded as a
 P-VOP using VOP 1110 as a basis for prediction. VOP 1120 is coded third as
 a B-VOP using VOPs 1110 and 1130 as a basis for prediction. VOP 1140 is
 coded sometime thereafter using VOP 1130 and another VOP (not shown) as a
 basis for prediction.
 The enhancement layer encoder 540 codes the enhancement layer VOPs in the
 order 1210, 1220,1230, 1240, . . . . As shown in FIG. 6, VOP 1210 is a
 P-VOP coded using VOP 1110 as a basis for prediction. VOP 1220 is coded as
 a B-VOP using base layer VOP 1120 and enhancement layer VOP 1210 as a
 basis for prediction. VOPs 1230 and 1240 are coded in a manner similar to
 VOP 1220; they are coded as B-VOPs using the temporally coincident VOP
 from the base layer and the immediately previous enhancement layer VOP as
 a basis for prediction.
 On decoding, a simple decoder that decodes only the coded base layer data
 obtains the smaller VOPs 1110-1140. However, a more powerful decoder that
 decodes both the coded base layer data and the coded enhancement layer
 data obtains a larger VOP. On display, the decoded video object may be
 displayed as a larger image or may be displayed at a fixed size but may be
 displayed with higher resolution.
 Scalability also provides a graceful degradation in image quality in the
 presence of channel errors. In one application, the coded base layer data
 may be supplemented with error correction coding. As is known, error
 correction coding adds redundancy to coded information. Error coded
 signals experience less vulnerability to transmission errors than signals
 without error coding. However, error coding also increases the bit-rate of
 the signal. By providing error correction coding to the coded base layer
 data without providing such coding to the coded enhancement layer data, an
 intermediate level of error protection is achieved without a large
 increase in the bit rate. Enhancement layer VOPs are not error coded,
 which would otherwise reduce the transmitted bit rate of the unified
 signal. When channel errors occur, the coded base layer data is protected
 against the errors. Thus, at least a basic representation of the video
 object is maintained. Graceful signal degradation is achieved in the
 presence of channel errors.
 FIG. 7 illustrates a block diagram of a video object decoder 800a
 constructed in accordance with an embodiment of the present invention. The
 video object decoder 800a includes a decoder demultiplexer (DEMUX) 810, a
 base layer decoder 820, a midprocessor 830, an enhancement layer decoder
 840 and a scalability post-processor 850. The components of the video
 object decoder 800a may be provided in hardware or may be logical devices
 provided in a microprocessor or a digital signal processor.
 The DEMUX 810 receives the coded video object data from the system
 demultiplexer 700 (FIG. 3). It distinguishes coded base layer data from
 coded enhancement layer data and routes each type of data to the base
 layer decoder 820 and enhancement layer decoder 840 respectively.
 The base layer decoder 820 decodes the coded base layer data to obtain base
 layer VOPs. It outputs decoded base layer VOPs on output 822. In the
 absence of channel errors, the decoded base layer VOPs should represent
 identically the locally decoded VOPs output on line 524 from the base
 layer encoder 520 to the midprocessor 530 (FIG. 4). The decoded base layer
 VOPs are input to the scalability post processor 850 and to the
 midprocessor 830 (line 524).
 The decoder midprocessor 830 operates identically to the encoder
 midprocessor 530 of FIG. 4. If midprocessor 530 had up sampled locally
 decoded VOPs, midprocessor 830 up samples the decoded base layer VOPs. If
 midprocessor 530 had down sampled or left unchanged the locally decoded
 VOPs, midprocessor 830 also down samples or leaves unchanged the decoded
 base layer VOPs. An output of the midprocessor 830 is input to the
 enhancement layer decoder 840.
 The enhancement layer decoder 840 receives coded enhancement layer data
 from the DEMUX 810 and decoded base layer data (possibly modified) from
 the midprocessor 830. The enhancement layer decoder 840 decodes the coded
 enhancement layer data with reference to the decoded base layer data as
 necessary. It outputs decoded enhancement layer VOPs to the scalability
 post-processor 850.
 The scalability post-processor 850 generates composite video object data
 from the decoded base layer data and the decoded enhancement layer data.
 In the case of temporal scalability, the scalability post-processor 850
 reassembles the VOPs in the correct time ordered sequence. In the case of
 spatial scalability, the scalability post-processor outputs the decoded
 enhancement layer data. The decoded base layer data is integrated into the
 decoded enhancement layer VOPs as part of the decoding process.
 FIG. 8 illustrates a block diagram of the scalability pre-processor 510
 (FIG. 4). The scalability pre-processor 510 includes a temporal decimator
 511, a horizontal and vertical decimator 512 and a temporal demultiplexer
 513. It can perform spatial resolution reduction (horizontal and/or
 vertical) and temporal resolution reduction by dropping intermediate
 pictures or VOPs as necessary. VOPs input to the scalability pre-processor
 are input on line 514. The scalability pre-processor outputs VOPs to the
 base layer decoder on line 515 and other VOPs to the enhancement layer
 decoder on line 516.
 The temporal decimator 511 reduces the VOP rate of both the base layer and
 the enhancement layer by dropping predetermined VOPs.
 The temporal demultiplexer is used for temporal scalability. For a given
 VOP input to it, the temporal demultiplexer 513 routes it to either the
 base layer decoder (over output 515) or to the enhancement layer decoder
 (over output 516).
 The horizontal and vertical decimator 512 may be used for spatial
 scalability. Each VOP input to the scalability pre-processor (or, at
 least, those output from the temporal decimator) is output directly to the
 enhancement layer decoder over line 516. The VOPs are also input to the
 horizontal and vertical decimator where image data of each VOP is removed
 to shrink them. The shrunken VOPs output from the horizontal and vertical
 decimator are output to the base layer encoder over line 515.
 FIG. 9 is a block diagram of an enhancement layer encoder 540 for video
 objects constructed in accordance with the present invention. The
 enhancement layer encoder 540 includes a VOP Motion Compensated DCT
 Encoder 541, a VOP Interlayer Motion Estimator 542 ("VIME") and a VOP
 Interlayer Motion Compensated Predictor 543. It receives the enhancement
 layer VOPs from the scalability pre-processor 510 at input 544 and the
 locally decoded base layer VOPs (possibly modified) at input 545. The
 enhancement layer encoder outputs the coded enhancement layer data on
 output 546.
 The enhancement layer encoder 540 receives the enhancement layer VOPs from
 the scalability pre-processor 510 on input 544. They are input to the VOP
 Motion Compensated DCT Encoder 541 and to the VOP Interlayer Motion
 Estimator 542. The VOP Motion Compensated DCT Encoder 541 is a motion
 compensated transform encoder that is adapted to accept a predicted VOP
 and motion vectors as inputs. The motion vectors are generated by VIME
 542, a normal motion estimator that has been adapted to accept enhancement
 layer VOPs from input 544.
 VIME 542 performs motion estimation on an enhancement layer VOP with
 reference to a locally decoded base layer VOP. It outputs motion vectors
 to the VOP Interlayer Motion Compensated Predictor 543 and, selectively,
 to the VOP Motion Compensated DCT Encoder 541.
 The VOP Interlayer Motion Compensated Predictor 543 is a normal motion
 compensated predictor that operates on the locally decoded base layer VOPs
 received from the midprocessor 530. It obtains a prediction from one or
 two possible sources of prediction. In a first prediction, prediction is
 made with reference to a first VOP. In a second prediction, prediction is
 made with reference to a second VOP. A third prediction obtains an average
 of the first and second predictions. The source of predictions, the first
 and second VOPs, may be located in either the base layer or enhancement
 layer. Arrows in FIGS. 5 & 6 illustrate exemplary prediction directions.
 In an MPEG-4 system image data of video objects is organized into blocks of
 image data. Prediction according to the three predictions described above
 may be performed on a block by block basis. Thus a first block of a VOP
 may be predicted using prediction 1 (First VOP), a second block may be
 predicted using prediction 2 (second VOP), and a third block may be
 predicted using prediction 3 (both VOPs). In the embodiment, the first and
 second VOPs are properly viewed as possible sources for prediction because
 they may be used as sources for prediction but are not necessary used.
 The VOP Interlayer Motion Compensated Predictor 543 outputs predicted VOPs.
 The output of the VOP Interlayer Motion Compensated Predictor 543 or the
 locally decoded base layer VOPs are input to the VOP Motion Compensated
 DCT Encoder 541.
 FIG. 10 is a block diagram of a midprocessor 530, 830 constructed in
 accordance with an embodiment of the present invention. The midprocessor
 530, 830 includes a horizontal interpolator 531 and a vertical
 interpolator 532 on a first processing path, a horizontal decimator 533
 and a vertical decimator 534 on a second processing path and a third,
 shunt path 535. It receives VOPs on input 536 and outputs VOPs on an
 output 537.
 The horizontal interpolator 531 and vertical interpolator 532 are enabled
 when the midprocessor 530, 830 operates in an up sampling mode. For each
 VOP, the horizontal interpolator 531 and vertical interpolator 532 enlarge
 the VOP and calculate image data for data point(s) between original data
 points.
 The horizontal decimator 533 and vertical decimator 534 are enabled when
 the midprocessor 530, 830 operates in down sampling mode. The horizontal
 decimator 533 and vertical decimator 534 reduce the VOP and remove image
 data for certain of the original data points.
 The shunt path 535 outputs untouched the VOPs input to the midprocessor
 530, 830.
 FIG. 11 is a block diagram of the enhancement layer decoder of video
 objects 840 of FIG. 7. The enhancement layer decoder 840 includes a VOP
 Motion Compensated DCT Decoder 841 and a VOP Interlayer Motion Compensated
 Predictor 842. The coded enhancement layer data is input to the
 enhancement layer decoder on input 843. Decoded base layer VOPs received
 from the midprocessor 830 are input to the enhancement layer decoder on
 input 844. The enhancement layer decoder 840 outputs decoded enhancement
 layer VOPs on output 845.
 The VOP Motion Compensated DCT Decoder 841 decodes motion vectors as well
 as the prediction mode from the coded enhancement layer data and outputs
 them to the VOP Interlayer Motion Compensated Predictor 842 along with
 decoded enhancement layer previous VOP. The VOP Interlayer Motion
 Compensated Predictor 842 also receives the decoded base layer VOPs from
 line 844. The VOP Interlayer Motion Compensated Predictor 842 outputs
 predicted VOPs back to the VOP Motion Compensated DCT Decoder 841. Based
 upon either the enhanced layer previous decoded VOPs or the decoded base
 layer VOPs, or their combination, the VOP Motion Compensated DCT Decoder
 841 generates the decoded enhancement layer VOPs. Among the combinations
 allowed at the encoder are one-half of previous decoded enhancement layer
 VOP and one-half of the base layer VOP, as well as one-half of a previous
 and a next decoded VOP of base layer.
 FIG. 12 is a block diagram of the scalability post-processor 850. It
 includes a temporal multiplexer 851 and a temporal interpolator 852. The
 scalability post-processor 850 receives decoded base layer data on input
 853 and decoded enhancement layer VOPs on input 854. It outputs composite
 video object data on output 855.
 The temporal multiplexer 851 reassembles the VOPs from the base layer and
 the enhancement layer into a single stream of VOPs. The temporal
 interpolator 852 is used for temporal scalability to rearrange VOPs into
 the correct time ordered sequence. For spatial scalability, the decoded
 base layer VOPs may be ignored; the decoded enhancement layer data
 bypasses the temporal multiplexer 851.
 The temporal interpolator 852 increases the frame rate of the VOPs in a
 manner that complements the temporal decimator 511 of the video object
 encoder 500a (FIG. 8). If the temporal decimator 511 was bypassed for
 encoding, the temporal interpolator 852 may be bypassed during decoding.
 As has been shown, the present invention provides a system providing
 scalability, either temporal scalability, spatial scalability or both.
 VOPs are separated into base layer VOPs and enhancement layer VOPs and
 coded as such. On decoding, a specific decoder may decode the coded base
 layer data with or without the coded enhancement layer data, depending on
 it processing power and channel conditions.
 The present invention also provides a general scalability syntax while
 coding.
 Generalized scalability allows predictions to be correctly formed at the
 decoder by embedding the necessary codes indicating the specific type of
 temporal scalability or spatial scalability to be derived. The reference
 VOPs for prediction are selected by reference_select_code as described in
 Tables 1 and 2. In coding P-VOPs belonging to an enhancement layer, the
 forward reference can be one of the following three: the most recent
 decoded VOP of enhancement layer, the most recent VOP of the lower layer
 in display order, or the next VOP of the lower layer in display order.
 In B-VOPs, the forward reference can be one of the two: the most recent
 decoded enhancement VOP or the most recent lower layer VOP in display
 order. The backward reference can be one of the three: the temporally
 coincident VOP in the lower layer, the most recent lower layer VOP in
 display order, or the next lower layer VOP in display order.
 TABLE 1
 Prediction Reference Choices For P-VOPs
 in The Object-Based Temporal Scalability
 ref_select.sub.--
 code Forward Prediction Reference
 00 Most recent decoded enhancement VOP belonging
 to the same layer.
 01 Most recent VOP in display order belonging to
 the reference layer.
 10 Next VOP in display order belonging to the
 reference layer.
 11 Temporally coincident VOP in the reference
 layer (no motion vectors)
 TABLE 2
 Prediction Reference Choices For B-VOPs
 In The Case of Scalability
 ref_select Forward Temporal Backward Temporal
 code Reference Reference
 00 Most recent decoded Temporally coincident VOP in
 enhancement VOP of the reference layer
 the same layer (no motion vectors)
 01 Most recent decoded Most recent VOP in display
 enhancement VOP of order belonging to the
 the same layer. reference layer.
 10 Most recent decoded Next VOP in display order
 enhancement VOP of belonging to the
 the same layer. reference layer.
 11 Most recent VOP in Next VOP in display
 display order belonging order belonging to the
 to the reference layer. reference layer.
 The enhancement layer can contain P or B-VOPs, however, in scalability
 configurations of FIG. 4 and FIG. 5, the B-VOPs in the enhancement layer
 behave more like P-VOPs at least in the sense that a decoded B-VOP can be
 used to predict the following P or B-VOPs.
 When the most recent VOP in the lower layer is used as reference, this
 includes the VOP that is temporally coincident with the VOP in the
 enhancement layer. However, this necessitates use of lower layer for
 motion compensation which requires motion vectors.
 If the coincident VOP in the lower layer is used explicitly as reference,
 no motion vectors are sent and this mode can be used to provide spatial
 scalability. Spatial scalability in MPEG-2 uses spatio-temporal
 prediction, which is accomplished as per FIG. 5 more efficiently by simply
 using the three prediction modes: forward prediction (prediction direction
 1), backward prediction (prediction direction 2), interpolated prediction
 (prediction directions 1 and 2) available for B-VOPs.
 Since the VOPs can have a rectangular shape (picture) or an irregular
 shape, both the traditional as well as object based temporal and spatial
 scalabilities become possible. We now provide some details by which
 scalability can be accomplished for arbitrary shaped VOPs by extending the
 technique of chroma-keying known in the art. Normally, scalable coding of
 arbitrary shaped objects requires explicit transmission of shape
 information of each VOP, however, by use of a simpler technique of
 chroma-keying in which only rectangular VOPs containing arbitrary shaped
 VOP are coded such that in the region outside of arbitrary shape of
 interest a key color (not present anywhere in the VOP) is inserted by the
 encoder and specified in the bitstream allowing deletion by the decoder,
 the only caveat is that the key color insertion/deletion is performed not
 only on arbitrary shape VOPs of lower (here, a base) layer but also in
 enhancement layer. Thus it becomes possible at the decoder to recover VOPs
 of scalable arbitrary shape since coding is really performed on
 rectangular VOP windows in the same manner as coding of pictures.
 The class hierarchy introduced in FIG. 2 can be used to implement a
 practical bitstream representation that may allow ease of access for
 object manipulation and editing functionalities. For illustrative
 purposes, they are described with reference to syntax elements from
 "MPEG-4 Video Verification Model Version 2.1," ISO/IEC JTC1/SC29/WG11,
 MPEG 96/776 (March 1996) (herein, "VM 2.1"). Tables 3-6 illustrate by
 example some bitstream details of video syntax class and meaning of
 various syntax elements in each class, particularly for reorganized or new
 syntax elements.
 TABLE 3
 Video Session
 No. of
 Syntax bits
 VideoSession() {
 video_session_start_code 32
 do {
 do {
 VideoObject()
 } while (nextbits() = =
 video_object_start_code)
 if (nextbits() != session_end_code)
 video_session_start_code 32
 } while (nextbits() != video_session_end_code)
 video_session_end_code 32
 }
 TABLE 3
 Video Session
 No. of
 Syntax bits
 VideoSession() {
 video_session_start_code 32
 do {
 do {
 VideoObject()
 } while (nextbits() = =
 video_object_start_code)
 if (nextbits() != session_end_code)
 video_session_start_code 32
 } while (nextbits() != video_session_end_code)
 video_session_end_code 32
 }
 TABLE 5
 Video Object Layer
 No. of
 Syntax bits
 VideoObjectLayer() {
 video_object_layer_start_code 28
 layer_id 4
 layer_width 10
 layer_height 10
 quant_type_sel 1
 if (quant_type_sel) {
 load_intra_quant_mat 1
 if (load_intra_quant_mat)
 intra_quant_mat[64] 8*64
 load_nonintra_quant_mat 1
 if (load_nonintra_quant_mat)
 nonintra_quant_mat[64] 8*64
 }
 intra_dcpred_disable 1
 scalability 1
 if (scalability) {
 ref_layer_id 4
 ref_layer--sampling_direc 1
 hor_sampling_factor_n 5
 hor_sampling_factor_m 5
 vert_sampling_factor_n 5
 vert_sampling_factor_m 5
 enhancement_type 1
 }
 do {
 VideoObjectPlane()
 } while (nextbits() = =
 video_object_plane_start_code}
 next_start_code()
 }
 layer_id: It uniquely identifies a layer. It is a 4-bit quantity with
 values from 0 to 15. A value of 0 identifies the first independently coded
 layer.
 layer_width, layer_height: These values define the spatial resolution of a
 layer in pixels units.
 Scalability: This is a 1-bit flag that indicates if scalability is used for
 coding of the current layer.
 ref_layer_id: It uniquely identifies a decoded layer to be used as a
 reference for predictions in the case of scalability. It is a 4-bit
 quantity with values from 0 to 15.
 ref_layer_sampling_direc: This is a 1-bit flag whose value when "0"
 indicates that the reference layer specified by ref_layer_id has the same
 or lower resolution as the layer being coded. Alternatively, a value of
 "1" indicates that the resolution of reference layer is higher than the
 resolution of layer being coded resolution.
 hor_sampling_factor_n, hor_sampling_factor_m: These are 5-bit quantities in
 range 1 to 31 whose ratio hor_sampling_factor_n/ hor_sampling_factor_m
 indicates the resampling needed in horizontal direction; the direction of
 sampling is indicated by ref_layer_sampling_direc.
 vert_sampling_factor_n, vert_sampling_factor_m: These are 5-bit quantities
 in range of 1 to 31 whose ratio
 vert_sampling_factor_n/vert_sampling_factor_m indicates the resampling
 needed in vertical direction; the direction of sampling is indicated by
 ref_layer_sampling_direc.
 enhancement_type: This is a 1-bit flag that indicates the type of an
 enhancement structure in a scalability. It has a value of i1i when an
 enhancement layer enhances a partial region of the base layer. It has a
 value of i0i when an enhancement layer enhances entire region of the base
 layer. The default value of this flag is i0i.
 TABLE 6
 Video Object Plane
 No. of
 Syntax bits
 VideoObjectPlane() {
 video_object_plane_start_code 32
 vop_temp_ref 16
 vop_visibility 1
 vop_of_arbitrary_shape 1
 if (vop_of_arbitrary_shape) {
 vop_width 10
 vop_height 10
 if (vop_visibility) {
 vop_composition_order 5
 vop_hor_spatial_ref 10
 marker_bit 1
 vop_vert_spatial_ref 10
 vop_scaling 3
 :
 :
 }
 :
 /* syntax to derive shapes by
 deleting key color */
 :
 }
 vop_coding_type 2
 if (vop_coding_type = = 1 .vertline..vertline.
 vop_coding_type = = 2)
 {
 vop_fcode_forward 2
 if (vop_coding_type = = 2) {
 vop_fcode_backward 2
 vop_dbquant 2
 }
 else {
 vop_quant 5
 }
 if (!scalability) {
 separate_motion_texture 1
 if (!separate_motion_texture)
 combined_motion_texture_coding()
 else {
 motion_coding()
 texture_coding()
 }
 }
 else {
 :
 /* syntax to derive forward and backward shapes by 1
 deleting key color */
 :
 }
 ref_select_code 2
 if (vop_coding_type = = 1 .vertline..vertline.
 vop_coding_type = =
 2) {
 forward_temporal_ref 10
 if (plane_coding_type = = 2) {
 marker_bit 1
 backward_temporal_ref 10
 }
 }
 combined_motion_texture_coding()
 }
 }
 The meaning of the syntax elements of video object planes is specified in
 VM2.1.
 Accordingly, the present invention provides a video coding system and
 syntax supporting generalized scalability. The system finds application
 with limited or noisy channels and with decoders of varying processing
 power.