JPEG images have different pixel values across multiple devices

Yes, the pixel color values are different across devices. This is very annoying especially if you want to compare colors. The solution is to compare visually equal colors (by human perception).

One of the best methods to compare two colors by human perception is CIE76. The difference is called Delta-E. When it is less than 1, the human eye can not recognize the difference.

You can find wonderful color utilities class (ColorUtils), which includes CIE76 comparison methods. It is written by Daniel Strebel,University of Zurich.

From ColorUtils.class I use the method:

static double colorDifference(int r1, int g1, int b1, int r2, int g2, int b2)

r1,g1,b1 - RGB values of the first color

r2,g2,b2 - RGB values ot the second color that you would like to compare

If you work with Android, you can get these values like this:

r1 = Color.red(pixel);

g1 = Color.green(pixel);

b1 = Color.blue(pixel);

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:

1. The phones may run different versions of Android where the implementation of BitmapFactory changed (e.g. perhaps inPreferQualityOverSpeed was broken and then fixed, or vice versa), or
2. The phones may provide different hardware features (e.g. vector instruction set, DSP co-processor, etc.) that BitmapFactory leverages. 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.

Much of a JPEG decoder's work involves real number calculations. Usually this is done using fixed point integer arithmetic for performance. This introduces rounding errors. Slight variations are a natural part of working with JPEG.

I suppose you should also check if compressed PNGs appear the same way across devices.

http://pngquant.org/

If the answer is yes, then the only thing remaining would be to figure out how to convert programmatically on the phone those images to that same kind of compressed png.