[Qgis-developer] Utilising QGraphicsScene in a first-class fashion for QgsMapCanvas

[Brendan Morley]

Dear fellow developers,

I've been experimenting (in my personal code base) with the use of
QGraphicsScene / QGraphicsView / QGraphicsItem as I believe TrollTech
intended them.

My proof of concept seems to work (so far) and I now want to see if
there is any interest to collaborating on this further within the QGIS
dev team.


Here's what I have so far:

1. Current State of Trunk

a. QgsMapCanvas is now a QGraphicsView
b. The QGraphicsView is attached to a QGraphicsScene.
c. The QGraphicsScene is populated by a QGraphicsRectItem, 
d. The QGraphicsRectItem is the QgsMapCanvasMap.
e. The QgsMapCanvasMap renders itself by fetching the drawable features
and rendering onto its own QPixmap.
f. The QgsMapCanvasMap paints itself by painting its QPixmap. 
g. The QGraphicsScene coordinates are 1:1 with the QGraphicsView
coordinates.
h. The 1:1 mapping is maintained by a proprietary implementation.
e. Panning is handled in a proprietary implementation.

2. Design

a. All drawable features are represented by a corresponding
QGraphicsItem.
b. Vector features become QGraphicsItems with an attached Path
(using .addPath).
c. Vector features also have attached ToolTips if possible.
d. Raster layers would continue to be QGraphicsRectItems.
e. Incremental drawing to be restored.
f. Ability to halt a drawing operation to be restored.


3. Deprecations

a. The QgsMapToPixel object is obsolete, since Map coordinates are
equivalent to Scene coordinates and Pixel coordinates are equivalent to
View coordinates.
b. The QgsMapCanvasMap is obsolete, since vector features can be added
directly to the QGraphicsScene rather than being abstracted through the
QgsMapCanvasMap.

4. Probable Deprecations

a. The Pan tool should be refactored to exploit the built-in dragging of
the QGraphicsView.
b. The feature selection method should be refactored to exploit the
built-in selection functionality of the QGraphicsScene.


5. Benefits

a. I have already got incremental rendering working - this was broken
every since we ported to Qt4.
b. Ability to exploit ToolTips.
c. Ability to move features as a whole, rather than point by point.
d. Ability to rotate the map with only a little extra code.
e. Qt4.3 looks like having perspective transformations.  We can start
simulating those pseudo-3D road navigation devices!
(http://zrusin.blogspot.com/2006/09/perspective-transformations.html)
f. Let TrollTech worry about enhancing this code rather than us.


Thanks,
Brendan.