The JPEG standard does not require that decoder implementations produce bit-for-bit identical output images. Unfortunately the standards document specifying decoder requirements, ISO 10918-2, is apparently not freely available online but Wikipedia says:
…the JPEG standard (and the similar MPEG standards) includes some precision requirements for the decoding, including all parts of the decoding process (variable length decoding, inverse DCT, dequantization, renormalization of outputs); the output from the reference algorithm must not exceed:
- a maximum 1 bit of difference for each pixel component
- low mean square error over each 8×8-pixel block
- [etc.]
Differences between different decoder outputs using the same input are generally due to differing levels of internal precision, particularly in performing the IDCT. Another possible source of differences is smoothing, which attempts to reduce “blockiness” artifacts.
Like you, I would expect that setting inPreferQualityOverSpeed would produce the same output but nothing actually guarantees that. I can think of at least a couple ways that you could get small variations on two different phones:
- The phones may run different versions of Android where the implementation of
BitmapFactorychanged (e.g. perhapsinPreferQualityOverSpeedwas broken and then fixed, or vice versa), or - The phones may provide different hardware features (e.g. vector instruction set, DSP co-processor, etc.) that
BitmapFactoryleverages. Even differences in scalar floating-point units can cause discrepancies, especially with JIT compilation producing the actual machine instructions.
Given the wiggle room in the standard plus your experimental observations, it appears the only way to guarantee bit-for-bit agreement is to perform decoding within your own application. Perhaps you can find some alternative Android-compatible library.