asp.net 動態輸出透明gif圖片

最後更新：2018-12-08 來源：互聯網

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查了國內幾個中文資料都沒解決，最後是在一個英文部落格上找到一個可以用的辦法。
他的解決代碼是：

代碼複製代碼代碼如下://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;}}
}

測試html檔案如下

複製代碼代碼如下:<!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>

本文章原先以中文撰寫並發佈於 aliyun.com，亦設英文版本，僅作資訊用途。本網站不對文章的準確性，完整性或可靠性或其任何翻譯作出任何明示或暗示的陳述或保證。如對該文章有任何疑慮或投訴，請傳送電郵至 info-contact@alibabacloud.com 並提供相關疑慮或投訴的詳細說明。職員會於 5 個工作天內與您聯絡，一經驗證之後，即會刪除該侵權內容。

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