Toolkit2/Toolkit.Avalonia/ImageResizer.cs

using Avalonia.Media.Imaging;
using SkiaSharp;

namespace Toolkit.Avalonia;

public class ImageResizer : 
    IImageResizer
{
    public Bitmap Resize(Stream stream, 
        int targetWidth, 
        int targetHeight, 
        bool maintainAspectRatio)
    {
        using SKBitmap original = SKBitmap.Decode(stream);

        float widthRatio = (float)targetWidth / original.Width;
        float heightRatio = (float)targetHeight / original.Height;
        float scale = maintainAspectRatio ? Math.Max(widthRatio, heightRatio) : Math.Min(widthRatio, heightRatio);

        int newWidth = (int)(original.Width * scale);
        int newHeight = (int)(original.Height * scale);

        using SKBitmap resized = new(newWidth, newHeight);
        using SKCanvas canvas = new(resized);

        canvas.Clear(SKColors.Transparent);
        canvas.DrawBitmap(original, new SKRect(0, 0, newWidth, newHeight));

        SKBitmap cropped;
        if (maintainAspectRatio)
        {
            int cropX = (newWidth - targetWidth) / 2;
            int cropY = (newHeight - targetHeight) / 2;

            cropped = new SKBitmap(targetWidth, targetHeight);
            using SKCanvas croppedCanvas = new(cropped);
            SKRect cropRect = new(cropX, cropY, cropX + targetWidth, cropY + targetHeight);
            croppedCanvas.Clear(SKColors.Transparent);
            croppedCanvas.DrawBitmap(resized, cropRect, new SKRect(0, 0, targetWidth, targetHeight));
        }
        else
        {
            cropped = resized;
        }

        using SKImage image = SKImage.FromBitmap(cropped);
        using MemoryStream outputStream = new();
        image.Encode(SKEncodedImageFormat.Png, 100).SaveTo(outputStream);
        outputStream.Seek(0, SeekOrigin.Begin);

        return new Bitmap(outputStream);
    }
}