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Some Notes on OGL with wxPython

From: "andrew cooke" <andrew@...>

Date: Thu, 8 Jan 2009 17:57:41 -0300 (CLST)

WxPython, the Python binding for WxWindows, whicah calls GTK on Linux, and
which gives fairly nice-looking GUIs, includes a little-documented module
called ogl.

The documentation generated from source is at
http://www.wxpython.org/docs/api/wx.lib.ogl-module.html and there's some
additional notes at http://wiki.wxpython.org/index.cgi/wxOGL

Here's a few more details I just learnt:

- Kind of obvious, but things are drawn in the order provided to
canvas.AddShape(), so supply "top" things last.

- To generate text use a ogl.TextShape.  This is like a RectangleShape, in
that it has a width and height - these form the clipping area.  Add text
to the shape using AddText(...).  You can also add text to other shapes,
but it will be clipped by the boundary (TextShape's boundary is
invisible).

- To add a line, use ogl.LineShape().  The exact sequence of calls is
tricky, but this seems to work:
  line = ogl.LineShape()
  line.MakeControlPoints(2) # has to be at least 2
  line.SetEnds(x1, y1, x2, y2) # sets the first + last control points
  line.InsertControlPoint(x, y) # see below
  line.Initialise()
  line.SetSplit(True) # for a spline
  canvas.AddShape(line)
The InsertControlPoint() adds an additional point in the list of control
points; it is always inserted just before the end (ie (x2, y2) above), so
repeatedly calling this "fills in" the line from start to end.

Andrew

More wxPython and OGL

From: "andrew cooke" <andrew@...>

Date: Mon, 12 Jan 2009 15:00:51 -0300 (CLST)

Still don't completely "get" this, but here's some more details:

By default, it's possible to drag shapes around on the canvas (mouse left
click and drag).  If a line is from shapeA to shapeB then
  shapeA.AddLine(line, shapeB)
attaches them, so that dragging is coherent (note that the shapes must
already be drawn; the line is drawn "underneath").

Dragging can be disabled by using
  shape.SetDraggable(False)

Response to clocking etc is mediated via the ShapeEvtHandler.  However
there's something odd with the architecture - Shape inherits from this
class but also has an instance (Shape._eventhandler).  As far as I can
tell the inherited handler is unused and you override fucntinality (to
intercept events etc) by setting the instance handler
(Shape.SetEventHandler).

However, you cannot just set the handler with a subclass, you must also
define the "previous" event handler to be what was already defined.  In
other words:
  myEventHandler = ....
  myEventHandler.SetPreviousHandler(shape.GetEventHandler())
  shape.SetEventHandler(myEventHandler)

Whether an object responds to a click or not is defined by the sensitivity
filter.  If the object is insensitive then the event is propagated to the
parent.  This means that if you have one shape on top of another (in my
case, overwritten text) then you can make sure that events detected by the
top layer shape are forwarded by:
  lowerShape = ...
  canvas.AddShape(lowerShape)
  upperShape = ...
  canvas.AddShape(upperShape)
  upperShape.SetParent(lowerShape)
  upperShape.SetSensitivityFilter(0)

The event handler associated with the lower shape will then receive events
as expected.

Andrew

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