51、C# 图片中非白色动态转换成红色，主要应用于指纹图片，呈现红色印泥效果

    public class ImageTool
    {
        
        /// <summary>
        /// 转化图片颜色，输入目标图片宽高数值，那么目标图片会被比例缩放
        /// </summary>
        /// <param name="filePath">原图片路径</param>
        /// <param name="destHeight">目标图片高度</param>
        /// <param name="destWidth">目标图片宽度</param>
        /// <param name="uc">是否执行转化动作</param>
        /// <returns></returns>
        public static string ZoomImage(string filePath, int destHeight, int destWidth, bool uc)
        {
            Bitmap bitmap = new Bitmap(Image.FromFile(filePath));
            if (uc)
            {
                bitmap = UpdateColor(bitmap);
            }
            Bitmap finalmap = ZoomImage(bitmap, destHeight, destWidth);
            
            string zoomFilePath = GetZoomFilePath(filePath);
            finalmap.Save(zoomFilePath);
            return zoomFilePath;
        }
        private static Bitmap UpdateColor(Bitmap bitmap)
        {
            int width = bitmap.Width;
            int height = bitmap.Height;
            Bitmap newmap = new Bitmap(width, height);
            Color pixel;
            for(int i=0; i< width; i++)
            {
                for (int j = 0; j < height; j++)
                {
                    pixel = bitmap.GetPixel(i, j); // 获取原图的像素点颜色值
                    int r, g, b, a;
                    r = pixel.R;
                    g = pixel.G;
                    b = pixel.B;
                    a = pixel.A;

                    // 判断是否为白色背景
                    if (r == 255 && g == 255 && b == 255)
                    {
                        newmap.SetPixel(i, j, Color.FromArgb(255, 255, 255));
                    }
                    else
                    {
                        newmap.SetPixel(i, j, Color.Red);
                    }
                }
            }
            return newmap;
        }
        /// <summary>
        /// 获取目标图片文件名称
        /// </summary>
        /// <param name="filePath">原图片路径</param>
        /// <returns></returns>
        private static string GetZoomFilePath(string filePath)
        {
            string ext = filePath.Substring(filePath.LastIndexOf('.'));
            string zoomFilePath = filePath.Substring(0, filePath.LastIndexOf('.')) + "_zoom" + ext;
            return zoomFilePath;
        }

        /// <summary>
        /// 等比例缩放图片
        /// </summary>
        /// <param name="bitmap">原图片位图对象</param>
        /// <param name="destHeight">目标图片高度</param>
        /// <param name="destWidth">目标图片宽度</param>
        /// <returns></returns>
        private static Bitmap ZoomImage(Bitmap bitmap, int destHeight, int destWidth)
        {
            try
            {
                System.Drawing.Image sourImage = bitmap;
                int width = 0, height = 0;
                //按比例缩放           
                int sourWidth = sourImage.Width;
                int sourHeight = sourImage.Height;
                // 判断原图与目标图片的宽高，如果目标图片的宽或高比原图要小的话，那么执行缩放，计算缩放后的宽高数值
                if (sourHeight > destHeight || sourWidth > destWidth)
                {
                    // 原图宽/目标图片宽 与 原图高/目标图片高，对两者的缩放比例进行比较
                    // 如果宽的缩放比例大于高的缩放比例，则保留目标图片设定的宽度，动态计算缩放后新的高度
                    // 此处计算的width 和 height 均为目标图片中新的图像绘制区域的宽和高，而不是整张图片的宽和高
                    // 目标图片的宽和高始终保持在预定的 destWidth 和 destHeight
                    if ((sourWidth * destHeight) > (sourHeight * destWidth))
                    {
                        width = destWidth;
                        height = (destWidth * sourHeight) / sourWidth;
                    }
                    else
                    {
                        height = destHeight;
                        width = (sourWidth * destHeight) / sourHeight;
                    }
                }
                else
                {
                    width = sourWidth;
                    height = sourHeight;
                }
                // 目标图片位图对象
                Bitmap destBitmap = new Bitmap(destWidth, destHeight);
                // 产生画笔
                Graphics g = Graphics.FromImage(destBitmap);
                // 背景色透明
                g.Clear(Color.Transparent);
                //设置画布的描绘质量         
                g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                // 此时的width 和 height 均为缩放后图片中图像区域的宽高，与原先设定的宽高数值相减之后再除以2，可动态计算出原图绘制到目标图片后的左边距和上边距，目的是使图片尽可能保持居中效果
                g.DrawImage(sourImage, new System.Drawing.Rectangle((destWidth - width) / 2, (destHeight - height) / 2, width, height), 0, 0, sourImage.Width, sourImage.Height, GraphicsUnit.Pixel);
                g.Dispose();
                //设置压缩质量     
                System.Drawing.Imaging.EncoderParameters encoderParams = new System.Drawing.Imaging.EncoderParameters();
                long[] quality = new long[1];
                quality[0] = 100;
                System.Drawing.Imaging.EncoderParameter encoderParam = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
                encoderParams.Param[0] = encoderParam;
                sourImage.Dispose();
                return destBitmap;
            }
            catch
            {
                return bitmap;
            }
        }
    }

下面将图解说明，计算缩放比例的逻辑，为什么要这么写

最终的结果，会是下图的样子