Abstract base class for: drawing a picture to a graphics context, storing and restoring the picture.

The class defines the protocol for a number of concrete picture classes. Its main role is to support glass doors used in object embedding.

The class provides a protocol for the provision of scaling and cropping functions by derived classes, together with default implementations.

Its main function is Draw(), which draws the picture onto the graphics context at a particular point. It also has two important pure virtual functions: ExternalizeL() and GetOriginalSizeInTwips().

A picture has both an original representation and an on-screen representation. The original representation has a size in twips, and can somehow be drawn. The on-screen representation is assumed to be drawn under the control of an application which may wish to re-size or scale the original in some way, to crop it at the edges, and/or to ensure it fits within a particular defined area on the screen.

The class provides several functions that allow an application to set scaling and cropping before invoking the Draw() function to draw the picture on-screen. It is up to the internal workings of the function to determine the order of application cropping and scaling.