c#工具類之Bitmap縮放幫忙類

using System.Drawing;
using System.Drawing.Drawing2D;

    /// <summary>
    /// BitmapHelper
    /// </summary>
    public static class BitmapScaleHelper
    {
        /// <summary>
        /// 縮放圖片
        /// </summary>
        /// <param name="bitmap">原圖片</param>
        /// <param name="width">新圖片寬度</param>
        /// <param name="height">新圖片高度</param>
        /// <returns>新圖片</returns>
        public static Bitmap ScaleToSize(this Bitmap bitmap, int width, int height)
        {
            if (bitmap.Width == width && bitmap.Height == height)
            {
                return bitmap;
            }

            var scaledBitmap = new Bitmap(width, height);
            using (var g = Graphics.FromImage(scaledBitmap))
            {
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                g.DrawImage(bitmap, 0, 0, width, height);
            }

            return scaledBitmap;
        }

        /// <summary>
        /// 縮放圖片
        /// </summary>
        /// <param name="bitmap">原圖片</param>
        /// <param name="size">新圖片大小</param>
        /// <returns>新圖片</returns>
        public static Bitmap ScaleToSize(this Bitmap bitmap, Size size)
        {
            return bitmap.ScaleToSize(size.Width, size.Height);
        }

        /// <summary>
        /// 按比例來縮放
        /// </summary>
        /// <param name="bitmap">原圖</param>
        /// <param name="ratio">ratio大於1,則放大;否則縮小</param>
        /// <returns>新圖片</returns>
        public static Bitmap ScaleToSize(this Bitmap bitmap, float ratio)
        {
            return bitmap.ScaleToSize((int)(bitmap.Width * ratio), (int)(bitmap.Height * ratio));
        }

        /// <summary>
        /// 按給定長度/寬度等比例縮放
        /// </summary>
        /// <param name="bitmap">原圖</param>
        /// <param name="width">新圖片寬度</param>
        /// <param name="height">新圖片高度</param>
        /// <returns>新圖片</returns>
        public static Bitmap ScaleProportional(this Bitmap bitmap, int width, int height)
        {
            float proportionalWidth, proportionalHeight;

            if (width.Equals(0))
            {
                proportionalWidth = ((float)height) / bitmap.Size.Height * bitmap.Width;
                proportionalHeight = height;
            }
            else if (height.Equals(0))
            {
                proportionalWidth = width;
                proportionalHeight = ((float)width) / bitmap.Size.Width * bitmap.Height;
            }
            else if (((float)width) / bitmap.Size.Width * bitmap.Size.Height <= height)
            {
                proportionalWidth = width;
                proportionalHeight = ((float)width) / bitmap.Size.Width * bitmap.Height;
            }
            else
            {
                proportionalWidth = ((float)height) / bitmap.Size.Height * bitmap.Width;
                proportionalHeight = height;
            }

            return bitmap.ScaleToSize((int)proportionalWidth, (int)proportionalHeight);
        }

        /// <summary>
        /// 按給定長度/寬度縮放,同時可以設置背景色
        /// </summary>
        /// <param name="bitmap">原圖</param>
        /// <param name="backgroundColor">背景色</param>
        /// <param name="width">新圖片寬度</param>
        /// <param name="height">新圖片高度</param>
        /// <returns></returns>
        public static Bitmap ScaleToSize(this Bitmap bitmap, Color backgroundColor, int width, int height)
        {
            var scaledBitmap = new Bitmap(width, height);
            using (var g = Graphics.FromImage(scaledBitmap))
            {
                g.Clear(backgroundColor);

                var proportionalBitmap = bitmap.ScaleProportional(width, height);

                var imagePosition = new Point((int)((width - proportionalBitmap.Width) / 2m), (int)((height - proportionalBitmap.Height) / 2m));
                g.DrawImage(proportionalBitmap, imagePosition);
            }

            return scaledBitmap;
        }
    }