Classifications

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Definitions

• This invention relates to the field of video image processing and data communications and in particular to the field of video image encoding.
• Video image encoding techniques are well known in the art.
• Encoding standards such as CCITT H.261, CCJ T H.263, and MPEG provide methods and techniques for efficiently encoding sequences of video images. These standards exploit the temporal correlation of frames in a video sequence by using a motion-compensated prediction, and exploit the spatial correlation of the frames by using a frequency transformation, such as a Discrete Cosine Transformation (DCT).
• DCT Discrete Cosine Transformation
• the resultant frequency component coefficients the measures of energy at each frequency
• the non-uniformly distributed coefficients are quantized, typically producing some non-zero quantized coefficients among many zero valued quantized coefficients.
• the occurrences of many zero valued coefficients, and similarly valued non-zero quantized coefficients allows for an efficient encoding, using an entropy based encoding, such as a Huffman/run-length encoding.
• the aforementioned quantizing process introduces some loss of quality, or precision, in the encoding.
• the quantization step size determines the degree of loss of quality in the encoding process.
• a small quantization step size introduces less round-off error, or loss of precision, than a large quantization step size.
• the quantization step size determines the resultant size of the entropy based encoding.
• a small quantization step size for example, rounds fewer coefficients to a zero level than a large quantization step size, and therefore there will be fewer long runs of zero values that can be efficiently encoded.
• a small quantization step size provides for a high quality reproduction of the original image, but at the cost of a larger sized encoding.
• a large quantization step size provides for a smaller sized encoding, but with a resultant loss of quality in the reproduction of the original image.
• variable sized encodings of an image are often communicated over a fixed bandwidth communications channel, such as, for example, a telephone line used for video teleconferencing, or a link to a web site containing video information.
• the variable length encoded images are communicated to a buffer at the receiving site, decoded, and presented to the receiving display at a fixed image frame rate. That is, for example, in a video teleconferencing call, the sequence of images may be encoded at a rate of ten video frames per second. Because the encodings of each frame are of variable length, some frames may have an encoded length that require more than a tenth of a second to be communicated over the fixed bandwidth communications channel, while others require less than a tenth of a second.
• the aggregate encoded frame transmission rate should equal the video frame rate.
• the receiving buffer size determines the degree of variability about this aggregate rate that can be tolerated without underflowing or overflowing the buffer. That is, if the receiving buffer underflows, a frame will not be available for display when the next period of the video frame rate occurs; if the receiving buffer overflows, the received encoding is lost, and the frame will not be displayable when the next period of the video frame rate occurs. In both cases, a staggering of the frame display occurs and produces a visually disturbing artifact. Techniques are common in the art for controlling the sizes of the variable sized encodings so that the receiving buffer does not overflow or underflow.
• the quantization step size is selected to provide a preferred level of buffer fullness to support a given video frame rate without overflowing or underflowing the receive buffer. Because the receive buffer is of limited size, the quality of the encoding can become unacceptably poor, particularly when communicating via a low bandwidth communications path.
• Another problem with the known methods and techniques is the determination of the initial quantization step size for each frame.
• Conventional techniques use the last determined quantization step size from the prior frame as the initial quantization step size for the subsequent frame to provide, somewhat, for a consistent level of buffer fullness. Because the quantization step size is determined based upon a measure of buffer fullness, the quantization step size of a prior frame is generally a poor estimator of the appropriate quantization step size for a subsequent frame to provide consistent quality.
• the video encodings exploit the temporal correlation of frames in a video sequence by using a motion-compensated prediction, wherein each frame is encoded as changes from the prior frame.
• the first frame of a sequence must be encoded as an independent frame, as well as frames that are encoded to recommence a sequence after a transmission error. Because an independent encoding of a frame typically has significantly more bits than an encoding of changes relative to a prior frame, the quantization step size for the encoding of changes relative to a prior frame is an inappropriate measure for determining the quantization step size for the encoding of an independent frame. As such, either the encoding process requires additional time to adjust the quantization step size to the appropriate level, commensurate with the quality of the prior frames, or an inappropriate step size is used, resulting in varying quality levels, particularly at each independent frame transmission.
• Consistent image quality is provided by controlling the quantization process based on a set of quantizing parameters that include, for example, an initial value and bounds for a quantizing factor that is applied to each frame of the video data. Additionally, the quantizing parameters are modifiable by a user to achieve a user- determinable balance of performance objectives, based on the user's preference for image quality or image update rate. The user-determinable balance is achieved by a suitable modification of the video frame rate and the quantizing parameters commensurate with the selected frame rate.
• the processing system in accordance with this invention allows for alternative encodings in dependence upon the desired performance objectives. If quality images are preferred, fewer, but more detailed, images are transmitted per second; if accurate motion depiction is preferred, more, but less detailed, images are transmitted. If neither image quality nor accurate motion have priority, images of moderate detail are transmitted at a moderate image update rate.
• the sets of parameters for effecting the desired performance objective are predefined, and include, for example, an initial quantizing factor for encoding independent frames of images after the occurrence of a communications error.
• FIG. 1 illustrates an example block diagram of a video processing system in accordance with this invention.
• FIG. 2 illustrates an example block diagram of a video encoding system in accordance with this invention.
• FIG. 1 illustrates an example block diagram of a video processing system in accordance with this invention, as would be used, for example, for videoconferencing.
• a camera 180 provides video input 101 corresponding to an image scene 181 to a video encoding system 100.
• the encoding system 100 converts the video input 101 into encoded frames 131 suitable for communication to a receiver 200 via a communications channel 141.
• the communications channel 141 is represented as a communications network, such as a telephone network, although it could also be a wireless connection, a point to point connection, or combinations of varied connections between the encoding system 100 and the receiver 200.
• the source of the video input 101 may be prerecorded data, computer generated data, and the like.
• the encoded frames 131 may contain less information than the available information at the video input 101.
• the performance of the video processing system is based on the degree of correspondence between the encoded frames 131 and the available video input 101.
• image quality is used herein to be a measure of the accurate reproduction of an image
• motion quality is used herein to be a measure of the accurate depiction of motion in a sequence of images.
• the system is configured to provide a proper balance between image quality and motion quality, the proper balance being defined by the designers of the system.
• the proper balance is typically established by defining an acceptable video frame rate of the encoded frames 131 given the available bandwidth of the communications channel 141 and the available buffering at the receiver 200, and then providing as much image quality as possible at that chosen frame rate.
• the desired performance of a video processing system is often dependent upon the context within which the video processing system is used. For example, when using a videophone to call home, it may be desirable to accurately convey facial detail, whereas when using the same videophone for a business meeting, it may be more important to provide a continual update of fast moving events. It may also be important to accurately convey facial expressions at other times during the business meeting, or to accurately convey motion during a call home. Also, providing as much image quality as possible for frames is not necessarily desirable, because it is often more visually disturbing to view frames of varying quality than to view frames of consistent quality, even if that consistent quality is less than a sporadically achievable higher quality.
• the video encoding system 100 is configured to provide consistent image quality at a chosen frame rate; and, in accordance with another aspect of this invention, the video encoding system 100 is configured to allow a user of the video processing system to control the choice of the proper balance between image quality and motion quality, based on user preferences 205.
• FIG. 2 illustrates an example block diagram of a video encoding system 100 in accordance with this invention that provides consistent image quality and allows for a tradeoff of image quality and motion quality based upon a user's preference.
• the video encoding system 100 encodes the video input 101 for communication to the receiver 200, and includes a transform device 110, a quantizer 120, an encoder 130, and a buffer regulator 140, as would be similar to a conventional video encoding system.
• the video encoding system 100 also provides a source 150 of quantizing parameters 151 that affect the operation of the quantizer 120.
• Video input 101 is transformed by the transform device 110 to produce a set of coefficients 111 that describe the image content of each frame.
• the transform device 110 employs a variety of techniques for efficiently coding each frame as a set of coefficients 111.
• an initial frame of the sequence of images is transformed using a Discrete Cosine Transform (DCT) to provide a set of DCT coefficients that correspond to the image of the initial frame.
• DCT Discrete Cosine Transform
• the transform device 110 compares the next frame of the sequence to the first frame, and transforms the differences between the frames as a set of movements of individual blocks in the first frame (motion vectors) 112, and a set of differences between the image details of the blocks in the first and next frames (error terms).
• the transform device 110 then provides a set of DCT coefficients 111 corresponding to the error terms.
• Subsequent frames of the sequence are similarly transformed to motion vectors 112 and error term DCT coefficients 111.
• images are reconstructed as a sequence of modifications to the first frame, by applying the inverse of these functions via the decoder 220.
• the transform device 110 transforms the next frame in a sequence as an independent frame whenever such an error is detected.
• This independent frame is.independent of prior frames and contains a set of coefficients 111 that correspond directly to the image of this frame, thereby forming a first frame to a new sequence. Because this frame is independent of all prior frames, it is independent of the effects of the prior communications error, as are all subsequent frames.
• another independent first frame transformation is effected.
• inter-frame or predicted frame is used to identify a frame encoding that is based on one or more prior frames
• intra-frame or independent frame is used to identify a frame encoding that contain a complete encoding of the image content of the frame, independent of any other frame.
• the transform device 110 may effect other transformations of the video input 101, in addition to or in lieu of the example transformation presented above, using conventional or novel transformation techniques.
• copending application "Low Bit Encoding Scheme for Video
• the coefficients 111 are quantized, or rounded, by the quantizer 120.
• the coefficients 111 may be very precise real numbers that result from a mathematical transformation of the image data, such as the aforementioned coefficients of a frequency transformation. Communicating each of the bits of each of the very precise real numbers would provide for a very accurate reconstruction of the image at the receiver 200, but would also require a large number of transmitted bits via the channel 141.
• the quantizer 120 converts the coefficients 111 into quantized coefficients 121 having fewer bits.
• the range of the coefficients 111 may be divided into four quartiles, wherein the quantized coefficient 121 of each coefficient 111 is merely an identification of the quartile corresponding to the coefficient 111.
• the quantized coefficient 121 merely requires two bits to identify the quartile, regardless of the number of bits in the coefficient 111.
• the quantizing factor is a measure of the quantization step size and is inversely proportional to the number of divisions, or quantization regions, of the range of the input parameter being quantized. The quantizing factor determines the resultant size of each quantized coefficient.
• the quantizing factor of 1/4 of the range of the input requires two bits to identify the quantized region associated with each coefficient 111; a quantizing factor of 1/8 the range of the input requires three bits, and so on.
• the range of the coefficients may be divided into uniform or non-uniform sized quantization regions, and the association between a coefficient 111 value and a quantized coefficient 121 value may be linear or non-linear.
• the encoder 130 encodes the quantized coefficients 121, using, in a preferred embodiment, an entropy encoding that produces different sized encodings based on the information content of the quantized coefficients 121. For example, run-length encoding techniques common in the art are employed to encode multiple sequential occurrences of the same value as the number of times that the value occurs. Because each frame of the video input 101 may contain different amounts of image information, the encoded frames 131 from the encoder 130 vary in size. The independent frames, for example, will generally produce large encoded frames 131, as compared to the inter-frames that are encoded as changes to prior frames.
• the encoded frames 131 are communicated to the channel 141 via the buffer regulator 140.
• the channel 141 is a fixed bit rate system, and the buffer regulator 140 provides the variable length encoded frames 131 to the channel 141 at the fixed bit rate. Because the encoded frames 131 are of differing lengths, the frames are communicated via the channel 141 at a varying frame rate.
• the receiver 200 includes a buffer 210 that stores the encoded frames 131 that are arriving at a varying frame rate and provides these frames for processing and subsequent display as video output 201 at the same fixed frame rate as the video input 101.
• the buffer regulator 140 is provided a measure of the size of the receiver buffer 210 and controls the amount of data that is communicated to the receiver 200 so as not to overflow or underflow this buffer 210.
• the buffer regulator 140 controls the amount of data that is communicated to the receiver 200 by controlling the amount of data that the quantizer 120 produces, via buffer control commands 142.
• the buffer regulator 140 controls the amount of data that the quantizer 120 produces by providing a buffer control command 142 that effects a modification to the quantizing factor based on a level of fullness of the receiver buffer 210.
• the buffer regulator is configured to allow the quantizing factor to be within an acceptable range of values.
• the buffer regulator 140 may specify a minimum and maximum allocated size for subsequent blocks of the current frame, from which the quantizer 120 adjusts its quantizing factor only to the degree necessary to conform.
• the buffer regulator 140 may merely provide an increment/decrement buffer control command 142 to the quantizer 120 as required.
• the quantizer 120 Upon receipt of an increment/decrement control command 142, the quantizer 120 increments/decrements the quantizing factor, respectively; absent an increment/decrement command 142, the quantizer maintains the prior value of the quantizing factor.
• Other techniques for modifying the quantizer factor in dependence upon a measure of the fullness of the receive buffer 210 would be evident to one of ordinary skill in the art.
• the quantizing factor is also dependent upon the quantizing parameters 151 from the quantizing parameter source 150.
• the quantizing parameters 151 include an initial quantizing factor Qi, that is used as an initial value for quantizing an independent frame, and minimum Qmin and maximum Qmax parameters that control the extent of the quantizing factors. Because of the inherent differences between an independent frame and an inter-frame, separate initial quantizing factors are provided for each of the frame types, as discussed below. For clarity, the initial quantizing factor for the inter-frames, or predicted frames, is termed Qp herein.
• the quantizing factor determines the level of precision of the quantized encoding
• initializing each frame to a given value provides for a more consistent image quality at the receiver 200.
• the initial quantizing factor are chosen to provide reasonably sized encodings at a given frame rate, as discussed below, fewer adjustments of the quantizing factor by the buffer regulator 140 will, in general, be required. This improved efficiency is particularly apparent in the severing of an independent frame's quantizing factor from its immediate predecessor, because the immediate predecessor is generally an inter-frame, having fundamentally different characteristics.
• the initial quantizing factor Qi, Qp is determined heuristically, experimentally, or algorithmically, based upon the bandwidth of the channel 141, the frame size and frame rate of the video input 101, and the size of the receive buffer 210.
• the frame size and the size of the receive buffer 210 is specified by an accepted communication standard, thereby allowing the video encoding system 100 of one vendor to communicate with a receiver 200 of another vendor without fear of an underflow or overflow of receive buffer 210.
• the bandwidth of the channel 141 is determined by the provider of the channel 141, and typically depends upon the class of service. For example, an ISDN communications link will typically have a substantially higher bandwidth than a common telephone communications link.
• the initial quantizing factor Qi, Qp is inversely proportional to both the size of the buffer 210 and the bandwidth of the channel 141.
• a larger bandwidth of the channel 141 allows a highly detailed (i.e. low quantizing factor) encoding to be communicated to the receive buffer 210 in a shorter period of time.
• the size of the buffer 210 is typically directly proportional to the bandwidth of the channel 141 and the frame size of the video input 101.
• a large buffer 210 allows for a high degree of variability among the sizes of the encoded frames 131, and thus highly detailed encodings can be communicated more often than when the buffer 210 size requires that all frames be constrained to near a consistent nominal size.
• the initial quantizing factor Qi, Qp is directly proportional to the frame rate of the video input 101. A higher frame rate requires more frames to be communicated per second; thus, given a fixed bandwidth of channel 141, a higher frame rate requires less detail (i.e. higher quantizing factor) in each of the coded frames 131.
• a nominal size of the encoded frame 131 can be defined that is equal to the bandwidth of channel 141 divided by the frame rate.
• the receive buffer 210 allows some encoded frames 131 to be larger than the nominal size, and some encoded frames 131 to be correspondingly smaller, so that the average encoded frame size for full bandwidth utilization substantially equals the nominal encoded frame size. Note that if an encoded frame is substantially larger than the nominal frame size, at least some of the subsequent frames must be smaller than the nominal frame size.
• an encoded independent frame has more information than an encoded inter-frame, and thus should be allocated a frame size that is larger than the nominal frame size.
• the allocated frame size should not be so large that the subsequent inter- frames are constrained so as to substantially reduce their image quality.
• the initial quantizing factor Qi is the factor that, on average, produces an encoded frame 131 that is approximately equal to twice the nominal frame size, based on the bandwidth of the channel 141 and the frame rate of the video input 101.
• the initial quantizing factor Qp for inter-frames in general, is lower than the determined initial quantizing factor Qi for independent frames.
• a lower quantizing factor is selected because the inter-frames typically have less information to transfer than the independent frames, and thus can support the use of a lower quantizing factor while still providing smaller encoded frames 131.
• the initial quantizing factor Qp is a factor that, on average, produces an encoded frame 131 that is somewhat less than the nominal frame size, based on the bandwidth of the channel 141 and the frame rate of the video input 101. Note that for a given bandwidth of the channel 141, size of the receiver buffer
• the bandwidth of the channel 141 is typically fixed, as is the size of the receiver buffer 210.
• the frame rate of the video input 101 is adjusted so as to allow for a preferred level of image quality. That is, if the image quality is unacceptable, the frame rate of the video input 101 is reduced, to allow for the communication of larger encoded frames 131.
• the frame rate of the video input 101 causes unacceptable motion quality
• the frame rate of the video input is increased, thereby requiring a reduction in image quality to allow for the communication of smaller encoded frames 131 at the higher frame rate.
• the frame rate of the video input 101 can be adjusted via a variety of means common in the art.
• the frame rate is adjusted by communicating an appropriate video control command 102 to the source of the video input 101, such as the video camera 180 of FIG. 1, if it has an adjustable frame rate.
• the frame rate change is effected via the use of a rate buffer in the transform device 110.
• the rate buffer common to one of ordinary skill in the art, receives frames of image data 101 at the highest rate the video source provides, and the processes within the transform device 110 sample the image data 101 from the rate buffer at the desired frame rate for communication to the receiver 200.
• a conventional rate buffer system also includes filters that smooth the visual anomalies that may be caused by this sampling process.
• a preferred embodiment of this invention allows for the above modifications of frame rate and quantizing factor based on a user preference 205.
• the modification of frame rate can be effected by a continuous user adjustment via a user control 230 in the receiver 200.
• a continuous control allows for a continuous adjustment of image quality.
• the user is provided a limited set of options via the user control 230, for ease of operation, and for ease of design of the aforementioned rate buffer.
• the user options in a preferred embodiment are: higher image quality, higher motion quality, or a best tradeoff.
• the best tradeoff corresponds to the convention choice of an acceptable frame rate that provides an acceptable image quality given the bandwidth of the channel 141.
• the higher image quality is effected by reducing the best tradeoff frame rate by approximately twenty percent.
• the higher motion quality is effected by increasing the best tradeoff frame rate by approximately twenty percent.
• the initial quantizing factors Qi, Qp are dependent upon the frame rate. Therefore, in accordance with this invention, as the frame rate is modified based on the user preference, so also are the initial quantizing factors Qi, Qp.
• the quantizing parameters 151 of FIG. 2 include the initial quantizing factors Qi, Qp, as well as a minimum Qmin and maximum Qmax set of parameters that bound the extent of the quantizing factor as it is adjusted by the buffer regulator 140.
• the Qmin and Qmax parameters are also modified based on the user preference for higher image quality or higher motion quality. As discussed above, the minimum Qmin and maximum Qmax parameters are provided in order to provide a consistency in the image quality.
• the quantizer 120 reduces the quantizing factor to Qmin in response to the buffer control commands 142, it is not reduced further. If additional bits are required to be provided to the channel 141 to prevent a receiver buffer underflow, the buffer regulator 140 inserts null bits, rather than providing more details to the encoded frame 131. Correspondingly, once the quantizer 120 increases the quantizing factor to Qmax, it is not increased further. If fewer bits must be provided to the channel 141 to avoid a receiver buffer overflow, the buffer regulator 140 reduces the frame rate, for example by not transmitting the frame, causing a subsequent momentary freeze of the image at the receiver 200, rather than an introduction of a poor quality image. Alternatively, the buffer regulator 140 may effect an explicit frame rate reduction at the transform 110, using the frame rate modification techniques presented above.
• the Appendix to this specification provides tables of preferred values of frame rates and quantizing parameters (Qi, Qp, Qmin, Qmax) for common frame formats used for videoconferencing and commonly used channel bit rates.
• Table 1 provides the frame rates and quantizing parameters for a QCIF format, which has a frame size of 176 by 144 pixels, using a 20 kbps bandwidth channel.
• the frame rate is set to five frames per second, whereas if the user selects a higher motion quality, the frame rate is set to ten frames per second.
• the quantizing parameters are set higher than those at the lower frame rates.
• Tables 2, 3, and 4 provide the frame rates and quantizing parameters for a CIF format, which has a frame size of 352 by 288 pixels, using a 100 kbps, 200 kbps, and 300 kbps bandwidth channel, respectively.
• Tables 2, 3, and 4 provide the frame rates and quantizing parameters for a CIF format, which has a frame size of 352 by 288 pixels, using a 100 kbps, 200 kbps, and 300 kbps bandwidth channel, respectively.
• quantizing parameters 151 are presented as being predefined in a quantizing parameter source 150, they could be dynamically computed, using for example a machine learning or expert system approach that determines appropriate quantizing parameters based on prior user preferences and feedback.
• the system may also automatically generate the user preferences 205, based for example on experiential data, thereby anticipating the user's desires.

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• Theoretical Computer Science (AREA)
• Compression Or Coding Systems Of Tv Signals (AREA)
• Compression, Expansion, Code Conversion, And Decoders (AREA)

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• 1999
  • 1999-12-15 WO PCT/EP1999/010223 patent/WO2000040032A1/en not_active Application Discontinuation
  • 1999-12-15 JP JP2000591812A patent/JP2002534864A/ja active Pending
  • 1999-12-15 EP EP99967977A patent/EP1057344A1/en not_active Withdrawn

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