IssueTracker

Can you please describe exactly what method call you're making on ExifInterface? I would expect this to work for exifInterface.setAttribute(TAG_EXPOSURE_TIME, "1/1600") but not exifInterface.setAttribute(TAG_EXPOSURE_TIME, "0.000625").

Thanks, I think part of the cause of the problem you're seeing is that exifInterface.getAttribute(TAG_EXPOSURE_TIME) is returning "6.25E-4" while setAttribute(TAG_EXPOSURE_TIME, value) is ideally expecting "1/1600".

Turns out there is code to handle the case of passing a decimal string to setAttribute, by converting it to a Rational representation of { int numerator, int denominator }, but it (arbitrarily) only handles up to 5 decimal places:

Rational(double value) {
  this(/* numerator= */ (long) (value * 10000), /* denominator */ 10000);
}

It looks like this code has been present for a long time, seems it was present in the original in framework implementation of ExifInterface added in 2016.

So I think there's an obvious improvement we can make here:

• Fix the double -> Rational conversion to support more precision. Ideally we would first try a denominator of 1 / value (since this would give an exact result in the case of this bug, and probably most other cases since shutter speeds that are less than 1 are usually 1/something), something like:

  
  Rational createRationalFromDouble(double value) {
    double inverse = 1 / value;
    if (isMathematicalInteger(value)) {
      return new Rational(1, (long) inverse);
    }
    // Not sure what we can do here
  }
  

  For the else case, maybe we could do something clever with continued fractions: https://en.wikipedia.org/wiki/Continued_fraction

It feels like we should also align the format that getAttribute returns with the format setAttribute expects, in order to explicitly support the copying use-case you've described. That would mean changing getAttribute to return "1/1600" in this case. Unfortunately that is a breaking change for any existing code that is doing Double.parseDouble(exifInterface.getAttribute(TAG_EXPOSURE_TIME)), so not feasible. If we were to introduce this it would need to be a separate method.