[Extgraph-developer] Improved drawing of zoomed canvas etc.

[Phil S. <p.s...@gn...>]

Hi. I have joined Extgraph list as busy hacking SimpleGraph for my own
application. Added jointed links and also a host of new node types for
my application.

However, I have got to an issue that might be of general interest. 
Simplegraph has relatively involved painting method. It paints to a bitmap
which is then bitlblt to the canvas. This has a few downsides, particularly
in that as you zoom in, instead of seeing more text, text is simply 
enlarged, lines are thickened etc. I want to zoom so that can see more text 
and keep lines at same weight.  I also looked at less involved way of 
painting but see that it is done because zoom method includes arbitary
rounding. This can be avoided but developer perhaps didn't spot it. I tried
with setworldTransform or setviewport setwindowextext calls but hit the
rounding issue as the code is structured at the moment.

Proper scaling on zoom requires transforming the graph coords by
x' = x*scale + scaledoffset. To avoid problem with rounding, think the
actual calculation is
x' = muldiv(x,M,D) + offset, where M is max(clientHeight, clientwidth) and
D is M*100/Zoom percent

The easier way to introduce this scaling would be a derivative of
TCanvas say TScaleableCanvas with overriddern draw methods to scale
the x,y coordinates before calling inherited draw methods.

Sadly this approach is no go. MoveTo, LineTo etc etc are static methods
in TCanvas. You can replace with static MoveTo, LineTo, but then the
drawing calls have to reference TScaleCanvas to the replaced method.
Once you do this, you cant pass TMetafileCanvas or Printer.Canvas to
the draw methods. 

Unless someone has a better idea, I think the only way to achieve what I
want is explicit scaling of coordinates in the drawing calls. This would a
big impact on ExtGraph, perhaps rather more than you would want.


----------------------------------------------------------
Phil Scadden, Institute of Geological and Nuclear Sciences
764 Cumberland St, Private Bag 1930, Dunedin, New Zealand
Ph +64 3 4799663, fax +64 3 477 5232