FILE SIZE BOUNDED JPEG TRANSCODER (FSBJT)

A technique that reduces the size of an existing JPEG file or set of DCT coefficients to satisfy a certain bit budget by setting to zero coefficients whose magnitude is below a certain threshold and which occur after a certain ordinal number in the zig-zag scan. The cutoff ordinal number is chosen using a clever savings calculation strategy. This strategy is implemented by filling appropriate savings values in an array of savings values, Savings[1], . . . , Savings[63]. The value Savings[n] is exactly the number of bits saved by reducing the thresholding cutoff ordinal number from n +1 to n. When a non-zero coefficient is set to zero, bits are saved because two runs of zeros (the one preceding it and the one following it) get combined into a single, longer run of zeros. The exact number of bits saved can be calculated by adding the bits needed to code the previous and next runs, and subtracting the bits needed to code the combined run. Some special conditions (runs longer than 16 and end-of-block conditions) need to be carefully factored into the computation.