清除图片周围的空白区域

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    class ImageUtil
    {
        public static void ClearBlank(string sourceFileName, string targetFileName)
        {
            var srcBmp = new Bitmap(sourceFileName);
            var rect = new Rectangle(0, 0, srcBmp.Width, srcBmp.Height);
            var srcBmpData = srcBmp.LockBits(rect, ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
            var srcPtr = srcBmpData.Scan0;
            var scanWidth = srcBmpData.Width * 3;
            var srcBytes = scanWidth * srcBmp.Height;
            var srcRgbValues = new byte[srcBytes];
            Marshal.Copy(srcPtr, srcRgbValues, 0, srcBytes);
            /*************************************************************************************/
            //从下向上  
            int k = 0;
            int lastRow = 0;
            var found = false;

            for (var i = srcBmp.Height - 1; i >= 0; i--)
            {
                for (var j = 0; j < srcBmp.Width; j++)
                {
                    k = 3 * j;

                    var r = srcRgbValues[i * scanWidth + k + 0];
                    var g = srcRgbValues[i * scanWidth + k + 1];
                    var b = srcRgbValues[i * scanWidth + k + 2];

                    if (r + g + b < 255 * 3)
                    {
                        lastRow = i;
                        found = true;
                        break;
                    }
                }
                if (found)
                {
                    break;
                }

            }

            //从上向下
            k = 0;
            found = false;
            var firstRow = 0;
            for (var i = 0; i < srcBmp.Height; i++)
            {
                for (var j = 0; j < srcBmp.Width; j++)
                {
                    k = 3 * j;

                    var r = srcRgbValues[i * scanWidth + k + 0];
                    var g = srcRgbValues[i * scanWidth + k + 1];
                    var b = srcRgbValues[i * scanWidth + k + 2];

                    if (r + g + b < 255 * 3)
                    {
                        firstRow = i;
                        found = true;
                        break;
                    }
                }
                if (found)
                {
                    break;
                }

            }

            //从左向右
            k = 0;
            found = false;
            var firstCol = 0;
            for (var i = 0; i < srcBmp.Width; i++)
            {
                for (var j = 0; j < srcBmp.Height; j++)
                {
                    k = 3 * i;

                    var r = srcRgbValues[j * scanWidth + k + 0];
                    var g = srcRgbValues[j * scanWidth + k + 1];
                    var b = srcRgbValues[j * scanWidth + k + 2];

                    if (r + g + b < 255 * 3)
                    {
                        firstCol = i;
                        found = true;
                        break;
                    }
                }
                if (found)
                {
                    break;
                }
            }
            //从右向左
            k = 0;
            found = false;
            var lastCol = 0;
            for (var i = srcBmp.Width - 1; i >= 0; i--)
            {
                for (var j = srcBmp.Height - 1; j >= 0; j--)
                {
                    k = 3 * i;

                    var r = srcRgbValues[j * scanWidth + k + 0];
                    var g = srcRgbValues[j * scanWidth + k + 1];
                    var b = srcRgbValues[j * scanWidth + k + 2];

                    if (r + g + b < 255 * 3)
                    {
                        lastCol = i;
                        found = true;
                        break;
                    }
                }
                if (found)
                {
                    break;
                }
            }
            //解锁位图  
            srcBmp.UnlockBits(srcBmpData);

            const int yuLiang = 3;
            var t = CutImage(srcBmp, new Rectangle(firstCol - yuLiang, firstRow -yuLiang, lastCol - firstCol + 2* yuLiang, Math.Abs(lastRow - firstRow) + 2* yuLiang));
            t.Save(targetFileName, ImageFormat.Png);
            t.Dispose();
        }
        private static Bitmap CutImage(Image img, Rectangle rect)
        {
            var b = new Bitmap(rect.Width, rect.Height, PixelFormat.Format32bppArgb);
            var g = Graphics.FromImage(b);
            g.DrawImage(img, 0, 0, rect, GraphicsUnit.Pixel);
            g.Dispose();
            return b;
        }
    }
}