Image Converter — C# source

// Image converter — C# port of the image-converter tool.

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Linq;

static class ImageConverterTool
{
    // Decode -> (JPEG only) flatten onto white -> re-encode at quality.
    // WebP needs a modern codec (ImageSharp / SkiaSharp); System.Drawing
    // covers the PNG + JPEG path that mirrors the web tool's core logic.
    static byte[] Convert(byte[] input, string target, long quality = 85)
    {
        using var src = new Bitmap(new MemoryStream(input));

        // JPEG has no alpha channel: paint onto an opaque white canvas,
        // or transparent pixels render black in the encoded file.
        Bitmap canvas;
        if (target == "jpeg")
        {
            canvas = new Bitmap(src.Width, src.Height, PixelFormat.Format24bppRgb);
            using var g = Graphics.FromImage(canvas);
            g.Clear(Color.White);
            g.DrawImage(src, 0, 0, src.Width, src.Height);
        }
        else
        {
            canvas = src;
        }

        var format = target == "jpeg" ? ImageFormat.Jpeg : ImageFormat.Png;
        var codec = ImageCodecInfo.GetImageEncoders()
            .First(c => c.FormatID == format.Guid);
        var eps = new EncoderParameters(1);
        // Quality is 0..100; the web tool's 0..1 slider multiplies by 100.
        eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);

        using var outMs = new MemoryStream();
        canvas.Save(outMs, codec, eps);
        if (canvas != src) canvas.Dispose();
        return outMs.ToArray();
    }

    static void Main()
    {
        byte[] png = File.ReadAllBytes("input.png");
        File.WriteAllBytes("output.jpg", Convert(png, "jpeg", quality: 85));
    }
}