Overview of Graphics Programming

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In VCL applications, the graphics components defined in the Graphics unit encapsulate the Windows Graphics Device Interface (GDI), making it easy to add graphics to your Windows applications.

To draw graphics in an application, you draw on an object's canvas, rather than directly on the object. The canvas is a property of the object, and is itself an object. A main advantage of the canvas object is that it handles resources effectively and it manages the device context for you, so your programs can use the same methods regardless of whether you are drawing on the screen, to a printer, or on bitmaps or metafiles. Canvases are available only at run time, so you do all your work with canvases by writing code.

Note: Since TCanvas is a wrapper resource manager around the Windows device context, you can also use all Windows GDI functions on the canvas. The Handle property of the canvas is the device context Handle.

How graphic images appear in your application depends on the type of object whose canvas you draw on. If you are drawing directly onto the canvas of a control, the picture is displayed immediately. However, if you draw on an offscreen image such as a TBitmap canvas, the image is not displayed until a control copies from the bitmap onto the control's canvas. That is, when drawing bitmaps and assigning them to an image control, the image appears only when the control has an opportunity to process its OnPaint message (VCL applications).

When working with graphics, you often encounter the terms drawing and painting:

  • Drawing is the creation of a single, specific graphic element, such as a line or a shape, with code. In your code, you tell an object to draw a specific graphic in a specific place on its canvas by calling a drawing method of the canvas.
  • Painting is the creation of the entire appearance of an object. Painting usually involves drawing. That is, in response to OnPaint events, an object generally draws some graphics. An edit box, for example, paints itself by drawing a rectangle and then drawing some text inside. A shape control, on the other hand, paints itself by drawing a single graphic.


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