snapshot of https://github.com/flutter/samples.git (main, 2026-09-16; blobs > 131072B omitted)

asset_transformation/grayscale_transformer/bin/grayscale_transformer.dart

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+import 'dart:io';
+
+import 'package:args/args.dart';
+import 'package:image/image.dart';
+
+const inputOptionName = 'input';
+const outputOptionName = 'output';
+
+int main(List<String> arguments) {
+  // The flutter tool will invoke this program with two arguments, one for
+  // the `--input` option and one for the `--output` option.
+  // `--input` is the original asset file that this program should transform.
+  // `--output` is where flutter expects the transformation output to be written to.
+  final parser = ArgParser()
+    ..addOption(inputOptionName, mandatory: true, abbr: 'i')
+    ..addOption(outputOptionName, mandatory: true, abbr: 'o');
+
+  ArgResults argResults = parser.parse(arguments);
+  final String inputFilePath = argResults[inputOptionName] as String;
+  final String outputFilePath = argResults[outputOptionName] as String;
+
+  try {
+    final Image image = decodeImage(File(inputFilePath).readAsBytesSync())!;
+    final Image imageInGrayscale = grayscale(image);
+    File(outputFilePath).writeAsBytesSync(encodeJpg(imageInGrayscale));
+
+    return 0;
+  } catch (e) {
+    // The flutter command line tool will see a non-zero exit code (1 in this case)
+    // and fail the build. Anything written to stderr by the asset transformer
+    // will be surfaced by flutter.
+    stderr.writeln(
+      'Unexpected exception when producing grayscale image.\n'
+      'Details: $e',
+    );
+    return 1;
+  }
+}