asp.net动态输出透明gif图片

最新推荐文章于 2022-09-05 21:20:16 发布

转载最新推荐文章于 2022-09-05 21:20:16 发布 · 74 阅读

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原文链接：http://www.cnblogs.com/vinstudio/archive/2009/12/07/1618681.html

文章标签：

#xhtml #c#

本文介绍如何使用ASP.NET动态生成并输出带有透明背景的GIF图片。通过C#代码实现图片绘制与透明处理，并将处理后的图片作为HTTP响应返回。

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要使用asp.net动态输出透明gif图片，也就是用Response.ContentType = "image/GIF"。

查了国内几个中文资料都没解决，最后是在一个英文博客上找到一个可以用的办法。

http://www.codedblog.com/2007/08/28/generating-a-transparent-gif-image-using-c/

他的解决代码是：

代码

// 存成gif.ashx
<% @ WebHandler Language = " C# " Class = " Gif " %>
using System.IO;
using System.Web;
using System.Drawing;

public class Gif : IHttpHandler {

     /// <summary>
     /// Returns a transparent background GIF image from the specified Bitmap.
     /// </summary>
     /// <param name="bitmap"> The Bitmap to make transparent. </param>
     /// <param name="color"> The Color to make transparent. </param>
     /// <returns> New Bitmap containing a transparent background gif. </returns>
     public Bitmap MakeTransparentGif(Bitmap bitmap, Color color) {
         byte R = color.R;
         byte G = color.G;
         byte B = color.B;
        MemoryStream fin = new MemoryStream();
        bitmap.Save(fin, System.Drawing.Imaging.ImageFormat.Gif);
        MemoryStream fout = new MemoryStream(( int )fin.Length);
         int count = 0 ;
         byte [] buf = new byte [ 256 ];
         byte transparentIdx = 0 ;
        fin.Seek( 0 , SeekOrigin.Begin);
         // header
        count = fin.Read(buf, 0 , 13 );
         if ((buf[ 0 ] != 71 ) || (buf[ 1 ] != 73 ) || (buf[ 2 ] != 70 )) return null ; // GIF
        fout.Write(buf, 0 , 13 );
         int i = 0 ;
         if ((buf[ 10 ] & 0x80 ) > 0 ) {
            i = 1 << ((buf[ 10 ] & 7 ) + 1 ) == 256 ? 256 : 0 ;
        }
         for (; i != 0 ; i -- ) {
            fin.Read(buf, 0 , 3 );
             if ((buf[ 0 ] == R) && (buf[ 1 ] == G) && (buf[ 2 ] == B)) {
                transparentIdx = ( byte )( 256 - i);
            }
            fout.Write(buf, 0 , 3 );
        }
         bool gcePresent = false ;
         while ( true ) {
            fin.Read(buf, 0 , 1 );
            fout.Write(buf, 0 , 1 );
             if (buf[ 0 ] != 0x21 ) break ;
            fin.Read(buf, 0 , 1 );
            fout.Write(buf, 0 , 1 );
            gcePresent = (buf[ 0 ] == 0xf9 );
             while ( true ) {
                fin.Read(buf, 0 , 1 );
                fout.Write(buf, 0 , 1 );
                 if (buf[ 0 ] == 0 ) break ;
                count = buf[ 0 ];
                 if (fin.Read(buf, 0 , count) != count) return null ;
                 if (gcePresent) {
                     if (count == 4 ) {
                        buf[ 0 ] |= 0x01 ;
                        buf[ 3 ] = transparentIdx;
                    }
                }
                fout.Write(buf, 0 , count);
            }
        }
         while (count > 0 ) {
            count = fin.Read(buf, 0 , 1 );
            fout.Write(buf, 0 , 1 );
        }
        fin.Close();
        fout.Flush();
         return new Bitmap(fout);
    }

     public void ProcessRequest(HttpContext context) {
        Bitmap transGif = null ;
         using (Bitmap bmp = new Bitmap( 300 , 50 )) {
             using (Graphics g = Graphics.FromImage(bmp)) {
                g.Clear(Color.Gray);
                g.DrawString( " transparent gif image " ,
                     new Font( " verdana bold " , 14f), Brushes.LemonChiffon, 0f, 0f);
                bmp.MakeTransparent(Color.Gray);
                transGif = MakeTransparentGif(bmp, Color.Black);
            }
        }
         if (transGif != null ) {
            context.Response.Clear();
            context.Response.ContentType = " image/GIF " ;
            transGif.Save(context.Response.OutputStream,
                System.Drawing.Imaging.ImageFormat.Gif);
        }
    }

     public bool IsReusable { get { return false ;}}

}

测试html文件如下

<! DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" >
< html xmlns ="http://www.w3.org/1999/xhtml" >
< head >
< title ></ title >
</ head >
< body style ="background:#999" >
< img src ="gif.ashx" style ="position:absolute" /> 下方的文字
</ body >
</ html >

转载于:https://www.cnblogs.com/vinstudio/archive/2009/12/07/1618681.html