An intermediate data format which is readily convertible to or from JPEG compliant data streams and to or from image data provides, in most circumstances, accelerated encoding and decoding in a degree sufficient to allow additional processing without necessitating increase of processor power even in relatively time-critical applications such as high-speed printers and image browsers. The format features flags which indicate if S8 (indicating that extra bits are required to uniquely encode an orthogonal transform coefficient value) or runs of zero-valued orthogonal transform coefficients longer than sixteen are present in a block of data. The block of data can be tested for these conditions and flags set once per block or once per image. Extensive processing can thus be omitted if either or both of these conditions are not present, as will generally be the case.