More of this Feature • Page 1: GDI Jargon • Page 2: Drawing: Lines • Page 3: Drawing: Shapes • Page 4: Draw vs. Paint • Page 6: Pictures: TBitmap • Page 7: Ways To Kill Flicker • Page 8: GDI, The Hard Way • Page 9: API Drawings

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Related Resources • Graphics programming in Delphi • Screen zooming • Double buffering • Win API in Delphi

   Using Handles and Stuff
When using Delphi, you can quite happily use the standard VCL functions (e.g. Canvas.TextOut) without worrying. However, what if you don't want the VCL graphics functions, which just wrap up the Windows API versions? Well, you can use the Windows API functions directly - but wait, they all want HDCs! What's a HDC?

Just about everything in Windows uses a "Handle". The handle is just a way of identifying your object. Windows have handles, buttons have handles, etc. Therefore, you should not be surprised to learn that all your objects have these handles as a property - for example, "MyForm.Canvas.Handle". Here's a quote from Win32.hlp, explaining handles: "An application must obtain an object handle and use this handle to examine or modify the system resource (or both). In the Microsoft® Win32® application programming interface (API), handles are usually implemented as indirect pointers, but this is not always the case."

The HDC type is a Handle to a Device Context. If you remember from the first part of this tutorial, I said that TCanvas encapsulates most of the DC functions. Well, you just use your object's Canvas Handle whenever Windows wants a HDC. Nice and easy.

Having said all that, I might have confused you. If so, here are some examples of using Windows API functions versus VCL ones. You'll see they are almost the same.

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