m3_lightmeter/lib/utils/ev_from_bytes.dart

import 'dart:developer';
import 'dart:math' as math;
import 'dart:typed_data';

import 'package:exif/exif.dart';
import 'package:m3_lightmeter_resources/m3_lightmeter_resources.dart';

Future<double?> evFromImage(Uint8List bytes) async {
  try {
    final tags = await readExifFromBytes(bytes);
    final iso = double.tryParse("${tags["EXIF ISOSpeedRatings"]}");
    final apertureValueRatio = (tags["EXIF FNumber"]?.values as IfdRatios?)?.ratios.first;
    final speedValueRatio = (tags["EXIF ExposureTime"]?.values as IfdRatios?)?.ratios.first;
    if (iso == null || apertureValueRatio == null || speedValueRatio == null) {
      log('Error parsing EXIF: ${tags.keys}');
      return null;
    }

    final aperture = apertureValueRatio.numerator / apertureValueRatio.denominator;
    final speed = speedValueRatio.numerator / speedValueRatio.denominator;

    return log2(math.pow(aperture, 2)) - log2(speed) - log2(iso / 100);
  } catch (e) {
    log(e.toString());
    return null;
  }
}