The embarrassment of using GDI + to draw Png Images

Problem scenario:

After reading the WebCast (III) of Yi Zhiming, I used the GDI + plot part and followed the code, and reported an error ":

"System. Runtime. InteropServices. ExternalException: A generic error occurred in GDI + ."

Code behavior:

Bmp. Save (Response. OutputStream, ImageFormat. Png );

Development Environment:

1. WindowsXP, vs2005, vs2008, vs2010beta2;

2. Windows2003, vs2005, vs2008, vs2010beta2

Code profiling:

Requirement: create a web application and run the start page to output an ellipse.

Code placed in Page_Load

Error code:

Code

Bitmap bmp = new Bitmap (200, 60 );
Graphics g = Graphics. FromImage (bmp );
G. Clear (Color. White );
G. DrawEllipse (new Pen (new SolidBrush (Color. Blue), 10, 10,180, 40 );
Response. ClearContent ();
Response. ContentType = "image/png ";
Bmp. Save (Response. OutputStream, ImageFormat. Png );

Change png to gif or jpeg, and the program runs normally.

Correct code 1: (updated code lines: 1, 8, and 9)

Output images with MemStream

Code

1 MemoryStream MemStream = new MemoryStream ();
2 Bitmap bmp = new Bitmap (200, 60 );
3 Graphics g = Graphics. FromImage (bmp );
4g. Clear (Color. White );
5g. DrawEllipse (new Pen (new SolidBrush (Color. Blue), 10, 10,180, 40 );
6 Response. ClearContent ();
7 Response. ContentType = "image/png ";
8 bmp. Save (MemStream, ImageFormat. Png );
9 MemStream. WriteTo (Response. OutputStream );

The Code in line 1 can be changed to Response. BinaryWrite (MemStream. ToArray ());

Code 2:

Specify the image storage location for saving

Code

1 string sIconFileName = Server. MapPath ("test.png ");
2 Bitmap bmp = new Bitmap (200, 60 );
3 Graphics g = Graphics. FromImage (bmp );
4G. Clear (Color. Transparent );
5G. DrawEllipse (new Pen (new SolidBrush (Color. Blue), 10, 10,180, 40 );
6Response. ClearContent ();
7Response. ContentType = "image/png ";
8bmp. Save (sIconFileName, ImageFormat. Png );

 

Cause analysis:

Png is a special case. Its decoder requires two-way stream. Therefore, we first need to read the stream to the memory stream, and then use the output stream to write the memory stream.

The gif decoder has a built-in parallel mechanism for processing.

Additional reading:

Http://www.west-wind.com/Weblog/posts/6008.aspx

Http://www.evolt.org/article/To_PNG_or_not_to_PNG/22/60134/index.html