C#中保存Gif文件设置透明无效果

大家好，又见面了，我是你们的朋友全栈君。

在C#中使用Graphics.Clear(Color.Transparent)生成gif格式透明文件时，设置透明不起作用。使用Bitmap.MakeTransparent 方法同样也不能使gif文件透明。这两个问题也是微软的设计使然，和gif文件格式有关系。

如果要在c#中编程实现透明色，必须另写代码，下面是一位老外的代码

///

<summary>

///
Make the image transparent.

///
The input is the color which you want to make transparent.

///

</summary>

///

<param name=”color”>
The color to make transparent.
</param>

///

<param name=”bitmap”>
The bitmap to make transparent.
</param>

///

<returns>
New memory stream containing 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();