[QGIS-Developer] QGIS Georeferencer improvements

Olivier Dalang olivier.dalang at gmail.com
Tue Mar 31 16:37:43 PDT 2020

I'm +1 for B as well !

Two window makes it much easier to mix things up. And with live preview, B
will get really intuitive (unless working with very heavy non-linear
deformations, but then it's a mess anyways).

About initial placement, if we have live preview, maybe the image could
just "float" (filling the canvas, not affected by zoom or pan) until
there's a point pair to pin it. That would make it clear that the raster
isn't georeferenced yet, work well with the "add more GCP = get more
precision" feeling, and most importantly avoid annoying high factor zoom
in/out if the initial position/scale didn't match the desired position.

Looking forward for this improvement very much :-)



On Tue, 31 Mar 2020 at 23:48, Nyall Dawson <nyall.dawson at gmail.com> wrote:

> On Wed, 1 Apr 2020 at 07:27, Martin Dobias <wonder.sk at gmail.com> wrote:
> > 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")
> Well - kind of. We do some stuff in image coordinates, yet confusingly
> require a CRS selection when first loading a raster (which I believe
> is used for nothing at all), and then the map coordinates of GCPs have
> an innate CRS too (which can be changed when you actually save the
> image, causing all sorts of weirdness!)
> > 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...
> I think option B makes much more sense. It may act different to what
> users are used to, but my experience is that users find the current
> approach confusing. I certainly do myself, and that's a bad sign. I've
> also wasted a lot of time over the years fixing mine/others
> georefencing errors caused by the confusing mix of CRS options in the
> current georeferencer.
> Reasons I like option B:
> - the live updates you've mentioned above
> - we know exactly what CRS already recorded GCPs are in, so we can
> easily transform them to a different CRS if at time of saving the user
> does decide to pick a different CRS vs what they've done the
> registration in
> My one question: where do you initially place the image? Do you just
> throw it in the middle of the current canvas extent as a starting
> point?
> Nyall
> >
> > Thanks for your thoughts :-)
> >
> > Cheers
> > Martin
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