[QGIS-Developer] Geometry data model logic?

[Raymond Nijssen]

Hopefully it is just a misconception on my side, but I really don't get 
the logic in the qgis geometry data model.

I tried the following script (sorry, some lines are wrapped by my email 
client):

---

#points

p1 = QgsGeometry().fromWkt('point(0 0)')
print(p1) # <QgsGeometry: Point (0 0)>
print(p1.asPoint()) # <QgsPointXY: POINT(0 0)>
print(p1.centroid()) # <QgsGeometry: Point (0 0)>
print(p1.buffer(1, 1)) # <QgsGeometry: Polygon ((1 0, 0 -1, -1 0, 0 1, 1 
0))>

p2 = QgsPoint(1,2)
print(p2) # <QgsPoint: Point (1 2)>
p2g = QgsGeometry(p2)
print(p2g) # <QgsGeometry: Point (1 2)>
print(p2) # <QgsPoint: Point (1 2)>
print(p2.centroid()) # <QgsPoint: Point (1 2)>
print(QgsPointXY(p2.x(), p2.y())) # <QgsPointXY: POINT(1 2)>
# The next line makes qgis crash after running it 2x !!
#print(QgsGeometry(p2)) # <QgsGeometry: Point (1 2)>

p3 = QgsPointXY(2, 0)
print(p3) # <QgsPointXY: POINT(2 0)>
print(QgsGeometry.fromPointXY(p3)) # <QgsGeometry: Point (2 0)>
print(QgsPoint(p3.x(), p3.y())) # <QgsPoint: Point (2 0)>

# lines

l1 = QgsGeometry().fromWkt('linestring((0 0, 1 1, 1 2))')
print(l1) # <QgsGeometry: LineString (0 0, 1 1, 1 2)>
print(l1.buffer(1, 1).asPolygon())
print(l1.asPolyline()) # [<QgsPointXY: POINT(0 0)>, <QgsPointXY: POINT(1 
1)>, <QgsPointXY: POINT(1 0)>]
print(QgsLineString(l1.asPolyline())) # TypeError: index 0 has type 
'QgsPointXY' but 'QgsPoint' is expected


---

My questions are:

- How is the relation between QgsGeometry, QgsPoint, QgsPoitXY, etc 
meant to be? Is there documentation? Maybe even a schema?

- Having QgsPoint and QgsPointXY etc is not handy, but I understand they 
are necessary for efficiency reasons. However, shouldn't they all have 
easy typecasting functions? For example:
QgsGeometry.asPoint()
QgsGeometry.asPointXY()
QgsGeometry.asLineString()
QgsGeometry.asLineStringXY()
QgsGeometry.asPolygon()
QgsGeometry.asPolygonXY()
QgsGeometry.asMultiLineString()
QgsGeometry.asMultiLineStringXY()
etc..

QgsPoint.asGeometry()
QgsPoint.asPointXY()

QgsLineString.asGeometry()
QgsLineString.asPointXY()

etc..

- Why don't all geometry types inherit from QgsGeometry, making all the 
geometry operators work? For example:
QgsPoint(1,2).buffer(3,5)




Some, IMHO, really odd things are:

- QgsGeometry.asPoint() returns a QgsPointXY

- QgsGeometry.asLineString() does not exist

- QgsGeometry.asPolyLine() returns an array of QgsPointXY's (besides, 
polyline is a strange geometry type in this model)

- QgsLineString([qgsPointXY, qgsPointXY, qgsPointXY, ...]) results in an 
error, while the documentation states:
    QgsLineString(points: Iterable[QgsPointXY]) Construct a linestring
    from list of points. This constructor is more efficient then calling
    setPoints() or repeatedly calling addVertex()

- QgsGeometry(QgsPoint) does work, but destroys my QgsPoint (and makes 
qgis instable)



Hopefully somebody can explain, making this more sense to me. And I'd 
like to contribute to improving this, though I'm not a cpp programmer.

Regards,
Raymond