Dynamically generating images in ISAPI extension using GDI+ with live demo

• Download demo project - 14.4 Kb

Introduction

Ever wanted to generate "on the heap" images on your web server and send them to the client? It is now possible to do it in your ISAPI extension in just 2 lines of code (well almost).

In fact, this article presents a helper class, to be used in an MFC ISAPI extension that writes GDI+ images to the client browser. All you have to do is to load or generate the GDI+ image!

Requisites

In this article I will suppose that:

• The reader has basic knowledge of GDI+. Here are some nice introductory articles on the CodeProject on this topic: GDI+ section.
• The reader has basic knowledge of MFC ISAPI extension. Here are some nice introductory articles on the CodeProject on this topic:
  • What an ISPAPI extension is? by Mehdi Mousavi
  • Discover ISAPI. Working with GET-POST data by Adrian Bacaianu

Rendering pipe

The rendering pipe is encapsulated in the WriteImage function:

Note that with this method, there is no need to create temporary files on disk since they are directly written to the memory (hGlobal).

Using CGDIpISAPI

Basic use

The use of the class is straight forward: build a CGDIpISAPI object and call WriteImage:

void CHttpServerDerviedClass::Default( CHttpServerContext* pCtxt)
{
    // GDI+ namespace
    using namespace Gdiplus;

    // creating bitmap
    Bitmap bitmap(320,200);
    // drawing on this bitmap
    ...

    // sending bitmap to the browser
    CGDIpISAPI renderer( this, pCtxt, &bitmap );
    // sending image
    renderer.WriteImage();
}

Output options

You can choose the image type (PNG, JPEG, BMP or TIFF). There is an enum for the available codecs. You can also customize the quality for the PNG and JPEG files. It must be between 0 and 100.

All these options can be set at the construction or by setters.

• Setting BMP type

  renderer.SetImageType( CGDIpISAPI::ImageBMP);

• Changing quality

   renderer.SetQuality( 98 );

Initializing GDI+

Do not forget to initialize and de-initialize GDI+. I'm using a helper class for that: CGDIpInitializer. It's use is self-explaining.

 // extension declaration
class CMyExtension : public CHttpServer
{
...
    CGDIpInitializer m_GDIpInitializer;
}

BOOL CMyExtension::GetExtensionVersion(HSE_VERSION_INFO* pVer)
{
    CHttpServer::GetExtensionVersion(pVer);

    // Initialize GDI+
    m_GDIpInitializer.Initialize();
    ...
}

BOOL CMyExtension::TerminateExtension(DWORD dwFlags)
{
    // Shutting down GDI+
    m_GDIpInitializer.Deinitialize();
    ...
}

What not to do...

• When writing an image, do not write any text... So forget about StartContent, EndContent ,WriteClient, etc...

Demo project

The demo projects is a MFC ISAPI extension (for VC7) that illustrates different options of CGDIpISAPI. Note that you also use this class in VC6.

You can test all the features by viewing the test.html file. Of course, before that you must compile the project, put the DLL in your script directory, etc... Below is a screenshot of the output of test.htm:

Reference

1. MSDN Sample: PINBALL
2. What an ISPAPI extension is? by Mehdi Mousavi

Update history

• 4 October 2002
  • Fixed a small bug in the CGDIpISAPI class (was always exporting as JPEG)