dart – Robin on Linux

Different default behaviour of JPEG encoding for dart and python

In dart, create JPEG file is like:

import 'dart:io';
import 'package:image/image.dart';

...

List<int> resizedBytes = encodeJpg(resizedImage);
File(outputPath).writeAsBytesSync(resizedBytes)

In python, we usually use:

import cv2

...

cv2.imwrite("output.jpg", output_image)

But they are different! As the source code show, the “encodeJpg()” use 100% quality and YUV444 chroma as default, but cv2 use 95% quality and YUV420 chroma as default.

If you want to write a JPEG file just as “encodeJpg()” do by using python, the code snippet should be:

import cv2

...

params = [cv2.IMWRITE_JPEG_QUALITY, 100, cv2.IMWRITE_JPEG_SAMPLING_FACTOR, cv2.IMWRITE_JPEG_SAMPLING_FACTOR_444]
success, encoded_image = cv2.imencode(".jpg", output_image, params)
if success:
  with open(output_path, "wb") as fp:
    fp.write(encoded_image)

Resizing a image is not as easy as you think

I found a very interesting picture:

The size of this image is about 8MB although it’s blurring. Then I use below python code to try to resize it using different interpolation strategy:

import cv2

img = cv2.imread("/Users/robin/Downloads/ou1.png")

for inter, name in [(cv2.INTER_AREA, "area"), (cv2.INTER_CUBIC, "cubic"), (cv2.INTER_LINEAR, "linear")]:
    after = cv2.resize(img, (310, 310), inter)
    cv2.imwrite(f"{name}.bmp", after)

Only cv2.INTER_AREA works well.

Then let me try dart language:

import 'dart:io';
import 'package:image/image.dart' as img;

void main() async {
  // Load the image
  final file = File('/Users/robin/Downloads/ou1.png');
  final bytes = await file.readAsBytes();
  final image = img.decodeImage(bytes);

  if (image == null) {
    print("Failed to load image");
    return;
  }

  // Resize and save images with different methods
  for (var method in ['area', 'cubic', 'linear', 'average']) {
    img.Image resized;
    if (method == 'area' || method == 'linear') {
      // Use `copyResize` for these, `area` will behave similarly to default
      resized = img.copyResize(image, width: 310, height: 310);
    } else if (method == 'cubic') {
      // Simulate cubic interpolation (approximation)
      resized = img.copyResize(image, width: 310, height: 310, interpolation: img.Interpolation.cubic);
    } else if (method == 'average') {
      resized = img.copyResize(image, width: 310, height: 310, interpolation: img.Interpolation.average);
    } else {
      print("Unknown interpolation method: $method");
      continue;
    }

    // Save the resized image
    final output = File('$method.bmp');
    await output.writeAsBytes(img.encodePng(resized));
    print("Saved resized image using $method interpolation as $method.bmp");
  }
}

The result looks also terrible except just one strategy “average”:

So, better use “Interpolation.average” in dart language.

“area”	“cubic”	“linear”	“average”