[QGIS-Developer] QGIS Georeferencer improvements

[Martin Dobias]

Hi Nyall

On Fri, Mar 13, 2020 at 8:45 PM Nyall Dawson <nyall.dawson at gmail.com> wrote:
>
> On Sat, 14 Mar 2020 at 01:13, Martin Dobias <wonder.sk at gmail.com> wrote:
>
> >
> > 1. Georeferencer opens in a new window. In order to see the main map
> > canvas and the georeferenced layer side by side, it is necessary to do
> > some resizing of windows which is inconvenient. It would be maybe more
> > practical to have the georeferenced image in a dock widget in the main
> > window - and all georeferencer related map tools included in the main
> > QGIS window as well. (Or maybe the other way around - to keep
> > georeferencer as a separate window, but allow the main map canvas to
> > be embedded in it?)
>
> I would personally prefer the second option. I think a significant
> amount of users doing regular georeferencing would have dual screen
> setups (or large screens) and would appreciate having the a larger
> georeferencer window then a dock would allow.

I have been thinking about this and what are the pros and cons of
having a separate window for georeferencing vs doing georeferencing in
main map canvas. Actually rather than discussing whether to have one
or two windows, the key bit to decide on is something else - what
would be the coordinate system of the georeferencer canvas:

A. image coordinates - that is what we have in QGIS georeferencer now:
the image gets loaded without any distortion and stays that way all
the time. Picking of map coordinates for GCPs has to be done in a
separate map canvas. (i.e. one click in "georeferencer canvas", and
one click in "reference map canvas")

B. map coordinates - that is what ArcGIS does. The image is placed at
some initial position in the map, picking of image coordinates and map
coordinates for GCPs is done in the same map canvas. (i.e. two clicks
in single map canvas)

The option B has the advantage that it is quite easy/intuitive to have
live preview of the image transformation as GCPs are being added. With
option A the live preview of the transform is not really possible (we
are still in image coordinates) - and the preview would need to be in
a separate map canvas.

So where I am hesitating is whether to stick to option A which is
probably familiar to QGIS users - or with option B that may provide
better UX thanks to the live update of the warped raster (or vector).
I guess we will need to choose one or the other - supporting both
options would be probably too confusing. I am trying to figure out
which way to take, because this will also have great impact on the
workflow on how GCPs would be picked on the map(s) using map tools...

Thanks for your thoughts :-)

Cheers
Martin