More of this Feature • Page 1: GDI Jargon • Page 2: Drawing: Lines • Page 3: Drawing: Shapes • Page 5: Handles and Stuff • 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

   Graphics That Stay Put
The disappearance of your graphics from above highlights an important distinction in GDI graphics - the difference between "Drawing" and "Painting".

Drawing is what we did above. This simply means that you draw some graphics when you want. The graphics appear, but only until the window gets refreshed.

Painting is slightly different to drawing. Whenever a window needs redrawn, Windows sends a message. This can be handled by the "OnPaint" event. Any code that you put in the OnPaint handler will be called, and graphics drawn in OnPaint will not disappear when the form gets hidden and shown. This is the fundamental difference between drawing and painting: drawing is only temporary, while painting gets called whenever the window needs refreshed, and generally deals with redrawing the entire screen.

It's time for a simple demonstration to make the distinction obvious. Dump the following procedure into a project:

procedure TForm1.DrawSomeText;
begin
  Canvas.TextOut(10, 10, 'Some text');
end; 

This looks innocent enough. If you dump a button onto the form and call DrawSomeText from the button's OnClick handler you will notice the same problem as before - the text disappears from the form when it moves off-screen. HOWEVER, if you call DrawSomeText from the form's OnPaint event handler the text stays there (finally!). This is all it takes - just an OnPaint handler.

The difference between painting and drawing should now be obvious. Generally, we would want to deal with drawing, dumping things onto the screen in a transient way. At the same time, we could store the changes somehow. One method for this is to create a bitmap and draw the change to both the bitmap and the screen. When it's time to paint we draw the bitmap to screen, which will be up-to-date. This will be discussed in a later tutorial.

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