Graphics Overview

Provides an abstract interface for drawing to devices, such as screens and printers.

The Bitmaps API and Printing API provide concrete implementations of the abstract drawing interfaces defined in this API, for screens and printers respectively. The Window Server Client Side API extends the Bitmaps API's classes in turn for drawing in windows (the normal case). For some applications, this polymorphism can be important, as it allows shared code for screen drawing and printing.

Most drawing takes place within a control, as defined in the UI Control Framework. That API provides support functions for using graphics contexts.

The API has five key concepts: basic geometry classes, graphics device (CGraphicsDevice), graphics context (CGraphicsContext), colour (TRgb), and units conversion (TPoint, TSize and MGraphicsDeviceMap) and zooming (TZoomFactor).

Point: a pair of (x,y) co-ordinates. It is provided by TPoint.

Size: a width and height. It is provided by TSize.

Rectangle: described by the co-ordinates of its top-left and bottom-right corners, or by its top-left hand corner and its size. It is provided by TRect.

Region: defines an area made up a collection of rectangles. They are handled by a family of classes derived from TRegion.

A graphics device represents the medium being drawn to. It is the provider of graphics contexts through which drawing is done.

The base class for graphics devices is CGraphicsDevice. Concrete devices implement derived classes.

A graphics context provides a large number of drawing operations, with some state settings defining how the drawing is performed. The settings are:

• a pen, for drawing shape outlines and text, with colour, line size and style settings

• a brush, for filling areas, with colour, style and origin settings

• for text, a current font and a justification mode

• a drawing mode: controls how colours drawn interact with the screen colour

• a co-ordinate origin, typically the top left corner of the screen

• an internal drawing position, for relative drawing operations

Operations include drawing shapes (points, lines, polylines, arcs, rectangles, polygons, ellipses, rounded rectangles and pie slices), text, and bitmaps.

The base class for graphics contexts is CGraphicsContext. Concrete devices provide derived classes that can provide additional operations suitable to the device.

A colour is specified by a 24-bit colour value. The system then maps these values into device-level values. The colour class is TRgb.

Graphics code can be written in a device-independent way by handling lengths/positions using twips (1/1440th inch), and only converting into pixels when actually drawing. Graphic devices implement a twips/pixels mapping interface for this conversion. The interface is defined by MGraphicsDeviceMap.

You might want drawing code to draw at different possible levels of magnification (zoom). In that case, the mapping is performed by a TZoomFactor, which also implements MGraphicsDeviceMap.

Bitmaps Overview

Printing Overview

UI Control Framework Overview

Window Server Client Side Overview