[Guidolib-devel] Graphic devices mechanism.

[Jerome B. <be...@gr...>]

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   Quick note about Guido graphic devices (GDevices)
=============================================================================

This note describes the mechanism currently used by the Guido Engine Library
to display graphics. i.e: how to generate visible score pages, starting from
an internal Guido Graphic Representation structure (Guido GR).

In the first version of the GuidoEngine, a Windows HDC type (graphic device
context handle) was provided to the GR objects, that were directly calling
microsoft APIs to display vector graphics and text.

For the GuidoEngine to be platform-independent, and to avoid being restricted
to only one graphical technology, (even a cross-platform one such as pdf, eps
or OpenGL ...), the choice has been made to provide a c++ object (called
VGDevice) to the GR objects, in place of the previous windows HDC handle.

VGDevice is a c++ pure virtual class that declares all methods required by the
GR objects to communicate their graphical operations. Implementations of
VGDevices are provided by clients applications via derived classes (i.e:
GDevicePostScript, GDeviceWin32...), so that neither GR objects nor any part
of the GuidoEngine depend on a particular graphical implementation.

The main advantage is that VGDevice derived classes can implement any kind of
graphical output : on-screen (platform specific, OpenGL), off-screen (raw
bitmaps), files (pdf, svg, postscript), network streams...

VGDevices derived classes must provides standard graphical functions (Lines,
Arcs, Boxes,Polygons, Text), coordinate transformations (zoom / scaling), and
symbolic music symbols handlers (DrawSymbol method). VGDevice design makes a
clear distinction between text characters and music symbols (although music
symbols are generally glyphs in a music font). Music symbols are identified by
font-independent constants (see MusicalSymbols.h). This mechanism provides
VGDevice objects with a higher abstraction level than a pure graphic layer.


Existing implementations of VGDevices:

GDeviceOSX:             MacOS X Quartz implementation.
GDeviceWin32:           Windows 95/98/Me (gdi)
GDeviceWin2000:         Windows 2000 / XP(gdi+)
GDevicePostScript:      EPS files (encapsulated postscript)
GDeviceWx:              wxWindows DC implementation.

Planned:

- OpenGL gdevice.
- Bitmap and Gif gdevices.

(This document has been posted to CVS, in the doc directory)