Image Converter — Java source

// Image converter — Java port of the image-converter tool.

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import javax.imageio.IIOImage;
import javax.imageio.ImageIO;
import javax.imageio.ImageWriteParam;
import javax.imageio.stream.ImageOutputStream;

final class ImageConverterTool {

    /** Decode -> (JPEG only) flatten onto white -> encode at quality 0..1. */
    static byte[] convert(byte[] input, String target, float quality) throws IOException {
        BufferedImage src = ImageIO.read(new ByteArrayInputStream(input));

        // JPEG has no alpha channel: draw onto an opaque white canvas first,
        // or transparent pixels render black in the encoded file.
        BufferedImage canvas = "jpeg".equals(target)
                ? new BufferedImage(src.getWidth(), src.getHeight(), BufferedImage.TYPE_INT_RGB)
                : src;
        if (canvas != src) {
            Graphics2D g = canvas.createGraphics();
            try {
                g.setColor(Color.WHITE);
                g.fillRect(0, 0, canvas.getWidth(), canvas.getHeight());
                g.drawImage(src, 0, 0, null);
            } finally {
                g.dispose();
            }
        }

        var writer = ImageIO.getImageWritersByFormatName(target).next();
        try (var out = new ByteArrayOutputStream();
             var ios = ImageIO.createImageOutputStream(out)) {
            var param = writer.getDefaultWriteParam();
            param.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
            param.setCompressionQuality(quality); // 0.85 in the web tool
            writer.setOutput(ios);
            writer.write(null, new IIOImage(canvas, null, null), param);
            return out.toByteArray();
        } finally {
            writer.dispose();
        }
    }

    public static void main(String[] args) throws IOException {
        byte[] png = java.nio.file.Files.readAllBytes(java.nio.file.Path.of(args[0]));
        System.out.write(convert(png, "jpeg", 0.85f));
    }
}