public Rect GetTransparentBounds(BitmapSource source) {
    var width = source.PixelWidth;
    var height = source.PixelHeight;
    var pixelBytes = new byte[height * width * 4];
    source.CopyPixels(pixelBytes, width * 4, 0);

    double? ax1 = null, ax2 = null, ay1 = null, ay2 = null;
    Rect result = new Rect();
    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {
            byte a = pixelBytes[(y * width + x) * 4 + 3];
            if (a != 0xff) {
                if (ax1.HasValue) {
                    ax1 = Math.Min(ax1.Value, x);
                    ax2 = Math.Max(ax2.Value, x);
                } else {
                    ax1 = x;
                    ax2 = x;
                }

                if (ay1.HasValue) {
                    ay1 = Math.Min(ay1.Value, y);
                    ay2 = Math.Max(ay2.Value, y);
                } else {
                    ay1 = y;
                    ay2 = y;
                }
            }
        };
    }

    if(ax1.HasValue && ay1.HasValue){
        result.X = ax1.Value;
        result.Y = ay1.Value;
        result.Width = ax2.Value - ax1.Value;
        result.Height = ay2.Value - ay1.Value;
    }

    return result;
}