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Particular embodiments include a method, an apparatus, and logic embodied in tangible computer-readable medium that when executed carries out a method of encoding an ordered sequence of quantized transform coefficients of a block of image data. One embodiment is a context adaptive variable length co

Particular embodiments include a method, an apparatus, and logic embodied in tangible computer-readable medium that when executed carries out a method of encoding an ordered sequence of quantized transform coefficients of a block of image data. One embodiment is a context adaptive variable length coding method that includes position coding the positions of zero-valued and non-zero valued coefficients by either a mixed method that encodes either the run length of zeroes preceding a non-zero coefficient or the run length of nonzero-valued coefficients preceding a zero-valued coefficients. Another includes position coding that uses a variable length code for two parameters respectively indicating the number of zero-valued coefficient positions and nonzero-valued coefficient positions still to be coded.

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1. A method of operating a decoding apparatus comprising: accepting in the decoding apparatus a bitstream, wherein the bitstream comprises a concatenation of codewords determined by a coding method that encodes an ordered sequence of quantized transform coefficients of a block of image data of an im

1. A method of operating a decoding apparatus comprising: accepting in the decoding apparatus a bitstream, wherein the bitstream comprises a concatenation of codewords determined by a coding method that encodes an ordered sequence of quantized transform coefficients of a block of image data of an image into codewords;recognizing codewords by the decoding apparatus; anddecoding recognized codewords, using the decoding apparatus, wherein the coding method comprises: accepting the ordered sequence,wherein the quantized coefficients in the sequence have a most likely-to-occur amplitude, and at least one other amplitude including a next-to-most likely-to-occur amplitude; encoding the ordered sequence using a context-based adaptive variable length coding method, the context-based adaptive variable length coding method including encoding the positions of the coefficients having a value other than the most likely-to-occur amplitude using at least one position coding method selected from the set of position coding methods consisting of: a first position coding method including repeatedly alternating encoding of (a) a run-length of a run of most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude, and (b) a run length of a run of coefficients having amplitudes other than the most likely-to-occur amplitude and determining if alternating produces shorter codewords than no alternating, the alternating occurring according to whether alternating or not alternating is determined to produce shorter codewords; anda second position coding method including determining a first parameter and a second parameter that are indicative of the number of most likely-to-occur amplitude coefficients and the number of coefficients having other than the most likely-to-occur amplitude whose positions remain to be coded; and jointly encoding the positions of the coefficients according to the first and second parameters using a variable length coding mapping that provides for any pair of parameter values all possible position patterns that have those parameter values,such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords. 2. A method as recited in claim 1, wherein the context-based adaptive variable length coding method further includes: forming a joint event defined by a first parameter indicative of the number of coefficients having other than the most likely-to-occur amplitude and a second parameter indicative of the number of trailing coefficients having the next-to-most likely-to-occur amplitude;encoding the joint event using a first coding method;encoding the signs of the trailing coefficients having the next-to-most likely-to-occur amplitude;encoding the values of the coefficients having other than the most likely-to-occur amplitude using at least one level-coding coding method, the choice of level-coding method depending on at least the amplitude of the coefficient;encoding a value indicative of the total number of coefficients having the most likely-to-occur amplitude using a second coding method; andconcatenating the results of the encodings to form a set of codewords. 3. A method as recited in claim 2, wherein at least some of the first, second, level-coding methods, and position coding method or methods are variable length coding methods, each variable length coding method configured such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords,wherein the position coding method includes repeatedly encoding either a run-length of a run of most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude, or a run length of a run of coefficients having amplitudes other than the most likely-to-occur amplitude interchangeably, either one or the other, andwherein whether a run-length of most likely-to-occur amplitude coefficients or a run-length of other than the most likely-to-occur amplitude coefficients is encoded depends on a function of the number of most likely-to-occur amplitude coefficients remaining to be coded and the number of other than the most likely-to-occur amplitude coefficients remaining to be coded. 4. A method as recited in claim 3, wherein any encoding of one or more run-lengths of runs of consecutive most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude proceeds starting with a run ending with the highest frequency coefficient having other than the most likely-to-occur amplitude and proceeding to runs ending with lower frequency coefficients having other than the most likely-to-occur amplitude, andwherein any encoding of one or more run-lengths of runs of coefficients having amplitudes other than the most likely-to-occur amplitude proceeds in the forward direction starting with a run starting with the lowest frequency coefficient having amplitudes other than the most likely-to-occur amplitude. 5. A method as recited in claim 2, wherein at least some of the first, second, level-coding methods, and position coding method or methods are variable length coding methods, each variable length coding method configured such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords,wherein the position coding method includes: determining a first parameter and a second parameter that are indicative of the number of most likely-to-occur amplitude coefficients and the number of coefficients having other than the most likely-to-occur amplitude whose positions remains to be coded; andjointly encoding the positions of the coefficients according to the first and second parameters using a variable length coding mapping that provides for any pair of parameter values all possible position patterns that have those parameter values. 6. A method as recited in claim 5, wherein the determining of the first and second parameters is up to a predefined maximum value for each parameter, and wherein variable length coding mappings are provided for pairs of parameters up to the maximum parameter value or values, and the wherein the jointly encoding is repeated until either parameter indicated no more coefficient positions need to be encoded. 7. A method as recited in claim 6, wherein each variable length coding mapping includes a variable length coding table. 8. A method as recited in claim 1, wherein the sequence is such that zero is the most likely-to-occur amplitude and 1 is the next-to-most likely-to-occur amplitude. 9. A non-transitory computer-readable storage medium storing instructions that when executed by one or more processors cause carrying out a method, the method comprising: for an ordered sequence of quantized transform coefficients of a block of image data, the sequence having a most likely-to-occur amplitude, and at least one other amplitude including a next-to-most likely-to-occur amplitude, encoding the ordered sequence using a context-based adaptive variable length coding method, the context-based adaptive variable length coding method including:encoding the positions of the coefficients having a value other than the most likely-to-occur amplitude using at least one position coding method selected from the set of position coding methods consisting of: a first position coding method including repeatedly alternating encoding of (a) a run-length of a run of most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude, and (b) a run length of a run of coefficients having amplitudes other than the most likely-to-occur amplitude and determining if alternating produces shorter codewords than no alternating, the alternating occurring according to whether alternating or not alternating is determined to produce shorter codewords; anda second position coding method including determining a first parameter and a second parameter that are indicative of the number of most likely-to-occur amplitude coefficients and the number of coefficients having other than the most likely-to-occur amplitude whose positions remains to be coded; and jointly encoding the positions of the coefficients according to the first and second parameters using a variable length coding mapping that provides for any pair of parameter values all possible position patterns that have those parameter values,such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords,wherein the encoding the positions uses one or more code mappings determined using assumed or measured statistics for likelihood of occurrence of events. 10. A non-transitory computer-readable storage medium as recited in claim 9, wherein the first and second position coding methods each include using at least one variable length coding method. 11. A non-transitory computer-readable storage medium as recited in claim 9, wherein the first position coding method includes using at least one variable length coding method,wherein the position encoding includes repeatedly encoding either a run-length of a run of most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude, or a run length of a run of coefficients having amplitudes other than the most likely-to-occur amplitude interchangeably, either one or the other, andwherein whether a run-length of most likely-to-occur amplitude coefficients or a run-length of other than the most likely-to-occur amplitude coefficients is encoded depends on a function of the number of most likely-to-occur amplitude coefficients remaining to be coded and the number of other than the most likely-to-occur amplitude coefficients remaining to be coded. 12. A non-transitory computer-readable storage medium as recited in claim 11, wherein any encoding of one or more run-lengths of runs of consecutive most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude proceeds starting with a run ending with the highest frequency coefficient having other than the most likely-to-occur amplitude and proceeding to runs ending with lower frequency coefficients having other than the most likely-to-occur amplitude, andwherein any encoding of one or more run-lengths of runs of coefficients having amplitudes other than the most likely-to-occur amplitude proceeds in the forward direction starting with a run starting with the lowest frequency coefficient having amplitudes other than the most likely-to-occur amplitude. 13. A non-transitory computer-readable storage medium as recited in claim 9, wherein at least some of the first, second, level-coding methods, and position coding method or methods are variable length coding methods, each variable length coding method configured such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords,wherein the encoding of the positions includes: determining a first parameter and a second parameter that are indicative of the number of most likely-to-occur amplitude coefficients and the number of coefficients having other than the most likely-to-occur amplitude whose positions remains to be coded; andjointly encoding the positions of the coefficients according to the first and second parameters using a variable length coding mapping that provides for any pair of parameter values all possible position patterns that have those parameter values. 14. A non-transitory computer-readable storage medium as recited in claim 13, wherein the determining of the first and second parameters is up to a predefined maximum value for each parameter, and wherein variable length coding mappings are provided for pairs of parameters up to the maximum parameter value or values, and the wherein the jointly encoding is repeated until either parameter indicated no more coefficient positions need to be encoded. 15. A non-transitory computer-readable storage medium as recited in claim 9, wherein the sequence is such that zero is the most likely-to-occur amplitude and 1 is the next-to-most likely-to-occur amplitude. 16. An apparatus comprising: an input buffer operative to accept a bitstream of codewords by a context-based adaptive variable length coding method that encodes an ordered sequence of quantized transform coefficients of a block of an image, the quantized coefficients, each quantized coefficient having an amplitude from a set of amplitudes that includes a most likely-to-occur amplitude and at least one other amplitude, including a next-to-most likely-to-occur amplitude;a codeword recognizer coupled to the input buffer and operative to recognize a codeword of the codewords in the bitstream, the codeword recognizer comprising one or more processors and a non-transitory computer readable medium comprising instructions to instruct at least one of the one or more processors to recognize the codeword;a decoder operative to decode the recognized codeword, wherein the context-based adaptive variable length coding method includes encoding the positions of the coefficients having a value other than the most likely-to-occur amplitude using at least one position coding method selected from the set of position coding methods consisting of: a first position coding method including repeatedly alternating encoding of (a) a run-length of a run of most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude, and (b) a run length of a run of coefficients having amplitudes other than the most likely-to-occur amplitude and determining if alternating produces shorter codewords than no alternating, the alternating occurring according to whether alternating or not alternating is determined to produce shorter codewords; anda second position coding method including determining a first parameter and a second parameter that are indicative of the number of most likely-to-occur amplitude coefficients and the number of coefficients having other than the most likely-to-occur amplitude whose positions remain to be coded; and jointly encoding the positions of the coefficients according to the first and second parameters using a variable length coding mapping that provides for any pair of parameter values all possible position patterns that have those parameter values,such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords. 17. An apparatus as recited in claim 16, wherein at least one of the position coding methods is a variable length coding method, each variable length coding method configured such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords. 18. An apparatus as recited in claim 16, wherein the sequence is such that zero is the most likely-to-occur amplitude and 1 is the next-to-most likely-to-occur amplitude. 19. A non-transitory computer-readable storage medium storing instructions that when executed by one or more processors cause carrying out a method comprising: accepting a bitstream, wherein the bitstream comprises a concatenation of codewords determined by a coding method that encodes an ordered sequence of quantized transform coefficients of a block of image data of an image into codewords;recognizing of codewords; anddecoding recognized codewords, wherein the coding method comprises: accepting the ordered sequence, wherein the quantized coefficients in the sequence have a most likely-to-occur amplitude, and at least one other amplitude including a next-to-most likely-to-occur amplitude; encoding the ordered sequence using a context-based adaptive variable length coding method, the context-based adaptive variable length coding method including encoding the positions of the coefficients having a value other than the most likely-to-occur amplitude using at least one position coding method selected from the set of position coding methods consisting of: a first position coding method including repeatedly alternating encoding of (a) a run-length of a run of most likely-to-occur amplitude coefficients preceding a coefficient having other than the most likely-to-occur amplitude, and (b) a run length of a run of coefficients having amplitudes other than the most likely-to-occur amplitude and determining if alternating produces shorter codewords than no alternating, the alternating occurring according to whether alternating or not alternating is determined to produce shorter codewords; anda second position coding method including determining a first parameter and a second parameter that are indicative of the number of most likely-to-occur amplitude coefficients and the number of coefficients having other than the most likely-to-occur amplitude whose positions remain to be coded; and jointly encoding the positions of the coefficients according to the first and second parameters using a variable length coding mapping that provides for any pair of parameter values all possible position patterns that have those parameter values,such that relatively more likely-to-occur sequences of quantized coefficients are represented by relatively short sets of codewords and relatively less likely-to-occur sequences of quantized coefficients are represented by relatively long sets of codewords. 20. A non-transitory computer-readable storage medium as recited in claim 19, wherein the context-based adaptive variable length coding method further includes: forming a joint event defined by a first parameter indicative of the number of coefficients having other than the most likely-to-occur amplitude and a second parameter indicative of the number of trailing coefficients having the next-to-most likely-to-occur amplitude;encoding the joint event using a first coding method;encoding the signs of the trailing coefficients having the next-to-most likely-to-occur amplitude;encoding the values of the coefficients having other than the most likely-to-occur amplitude using at least one level-coding coding method, the choice of level-coding method depending on at least the amplitude of the coefficient;encoding a value indicative of the total number of coefficients having the most likely-to-occur amplitude using a second coding method; andconcatenating the results of the encodings to form a set of codewords.