[Qgis-developer] georeferencer plugin - map follow mode

[Manuel Massing]

Hi Maxim, Hi Tim,

before eveybody gets their hope up - this is really a quite unspectular
feature, unfortuately not a real-time reprojection of the raster data
(although output-sensitive warping shouldn't be hard to implement). 
The patch just adds the ability to display the same viewport in the main
canvas and the georeferencer canvas (by using the transform estimated from
GCPs). Very similar to Aaron Racicot's RefMap plugin.

> Guys do I understand correctly that with Manuels patch you will be able to
> click on a known place on a already georeferenced map in main canvas window
> and then in the corresponding place in the georegister window and it will
> use the main canvas windows coords for that point removing the need for
> manual entry of gcps? If so that will be great!

In fact, the georeferencer already supports getting the map coordinate
from the main canvas (add GCP, then choose "From Map") - my patch just aims 
at improving/accelerating the workflow when referencing against against a
layer in the main canvas.

I'll give a concrete use-case to illustrate this:

Suppose you have a scanned map you wish to georeference against a vector layer
(e.g. containing street networks). Also, you are lucky to have a
dual-head setup, so you open the georeferencer plugin on a separate screen, 
and open the vector layer in the main canvas. The map features a lat/long
grid or some easy recognisable features, allowing you to add a few GCPs
quickly. Tentatively warping the input and placing it as a backdrop to the
vector layer reveals that while is a good approximation, you will have to add
maybe 50 more GCPs to get acceptable agreement with the reference. 

So you start to roam the main canvas for recognizable correspondencies (now
aided by your backdrop) and quickly spot one. But in order to place a GCP,
you have to find the same location in the plugin canvas. After ten minutes
of zooming, panning and swearing, you see this little checkbox called 
"Link Current View to Main Canvas" *click* and the viewport magically
jumps to the location you see in the main canvas.

Hope anyone can relate :-) At least for me this is a real timesaver.

I have attached a work-in-progress patch (against SVN), just as a crude little 
demo with the following
caveats:

	-does not yet report if there are insufficient control points to estimate the 
      transform
	-helmert transform is currently broken (but very probably also the case for
      SVN)
	-UI is terrible
		-Can anyone suggest a better description than "Link Main Canvas To Current 
	      View"?
		-probably ought to place the various dialog elements in a tab widget
		 to preserve canvas real estate
	-No refactoring for the warping code yet, so some duplication in the
	 interest of a "small" patch

I hope you are still interested in getting this into SVN.

cheers,

	Manuel
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