Development/Architecture/KDE3/Structured Graphics - Revision history

Dhaumann: Development/Architecture/KDE 3 Architecture/Structured Graphics moved to Development/Architecture/KDE3/Structured Graphics

2007-03-11T21:34:57Z

Development/Architecture/KDE 3 Architecture/Structured Graphics moved to Development/Architecture/KDE3/Structured Graphics

Dhaumann: Category:Architecture

2007-01-23T09:34:27Z

Category:Architecture

CuCullin: Category KDE3

2007-01-10T02:37:09Z

Category KDE3

Dhaumann: use qt3 linker

2007-01-07T18:27:19Z

use qt3 linker

Dhaumann at 18:22, 7 January 2007

2007-01-07T18:22:12Z

Dhaumann: port http://developer.kde.org/documentation/library/kdeqt/kde3arch/graphics/qcanvas.html

2007-01-07T18:20:56Z

port http://developer.kde.org/documentation/library/kdeqt/kde3arch/graphics/qcanvas.html

New page

== Introduction: Structured graphics with QCanvas ==

QPainter offers a powerful imaging model for painting on widgets and pixmaps.
However, it can also be cumbersome to use. Each time your widget receives
a paint event, it has to analyze the QPaintEvent::region() or
QPaintEvent::rect() which has to be redrawn. Then it has to setup a
QPainter and paint all objects which overlap with that region. For example,
image a vector graphics program which allows to drag objects like polygons,
circles and groups of them around. Each time those objects move a bit, the
widget's mouse event handler triggers a paint event for the whole area covered
by the objects in their old position and in their new position. Figuring
out the necessary redraws and doing them in an efficient way can be difficult,
and it may also conflict with the object-oriented structure of the program's
source code.

As an alternative, Qt contains the class
[http://doc.trolltech.com/3.3/qcanvas.html QCanvas] in which you put
graphical objects like polygons, text, pixmaps. You may also provide additional
items by subclassing
[http://doc.trolltech.com/3.3/qcanvasitem.html QCanvasItem] or one of
its more specialized subclasses. A canvas can be shown on the screen by one or
more widgets of the class
[http://doc.trolltech.com/3.3/qcanvasview.html QCanvasView] which you have
to subclass in order to handle user interactions. Qt takes care of all repaints
of objects in the view, whether they are caused by the widget being exposed,
new objects being created or modified or other things. By using double buffering,
this can be done in an efficient and flicker-free way.

Canvas items can overlap each other. In this case, the visible one depends on
the z order which can be assigned by QCanvasItem::setZ(). Items can also be
made visible or invisible. You can also provide a background to be drawn
"behind" all items and a foreground. For associating mouse events with objects,
in the canvas, there is the method QCanvas::collisions() which returns a list
of items overlapping with a given point. Here we show a screenshot of a canvas
view in action:

[[Image:q3canvas.png|frame|center|QCanvas screenshot]]

Here, the mesh is drawn in the background. Furthermore, there is a
QCanvasText item and a violet QCanvasPolygon. The butterfly is a
QCanvasPixmap. It has transparent areas, so you can see the underlying
items through it.

A tutorial on using QCanvas for writing sprite-based games can be
found [http://zez.org/article/articleview/2/1/ here].

''Initial Author:'' [mailto:bernd@kdevelop.org Bernd Gehrmann]

← Older revision		Revision as of 09:34, 23 January 2007
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	[[Category:KDE3]]		[[Category:KDE3]]
		+	[[Category:Architecture]]

← Older revision		Revision as of 02:37, 10 January 2007
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	''Initial Author:'' [mailto:bernd@kdevelop.org Bernd Gehrmann]		''Initial Author:'' [mailto:bernd@kdevelop.org Bernd Gehrmann]
		+
		+	[[Category:KDE3]]

@@ Line 15: / Line 15: @@
 As an alternative, Qt contains the class
-[http://doc.trolltech.com/3.3/qcanvas.html QCanvas] in which you put
+{{qt3|QCanvas}} in which you put
 graphical objects like polygons, text, pixmaps. You may also provide additional
 items by subclassing
-[http://doc.trolltech.com/3.3/qcanvasitem.html QCanvasItem] or one of
+{{qt3|QCanvasItem}} or one of
 its more specialized subclasses. A canvas can be shown on the screen by one or
 more widgets of the class
-[http://doc.trolltech.com/3.3/qcanvasview.html QCanvasView] which you have
+{{qt3|QCanvasView}} which you have
 to subclass in order to handle user interactions. Qt takes care of all repaints
 of objects in the view, whether they are caused by the widget being exposed,

@@ Line 35: / Line 35: @@
 view in action:
-[[Image:q3canvas.png|frame|center|QCanvas screenshot]]
+[[Image:q3canvas-demo.png|frame|center|QCanvas screenshot]]
 Here, the mesh is drawn in the background. Furthermore, there is a