1. An apparatus for conditional variable length coding of image data, comprising: (a) a computer processor; and (b) programming in a non-transitory computer readable medium and executable on the computer processor for performing conditional variable length coding of image data following steps comprising: (b)(i) categorizing an input distribution having multiple contexts, into multiple groups, for said conditional variable length coding; (b)(ii) wherein said multiple contexts comprise at least a quantization value and either previous residual input or neighboring residual input; (b)(iii) selecting one group of contexts for the current residual input, from multiple groups based on context of input distribution of image data; (b)(iv) selecting a progressive coding parameter set for the selected one group; (b)(v) performing progressive coding during variable length coding (VLC) to compress a residual in adaptive entropy coding of the image data; and (b)(vi) repeating steps (b)(iii) through (b)(v) as the current residual input changes wherein a different group of contexts and associated progressive coding parameter set are selected for adaptive entropy coding of the image data.