[QGIS-Developer] Export PDFs from the processing modeler

[Nyall Dawson]

On Thu, 6 Sep 2018 at 14:16, Olivier Dalang <olivier.dalang at gmail.com> wrote:
>
> Dear List,
>
> (I took the liberty to CC in some of you that I thought may be interested by this)

Count me in too -- I'm very interested here, given my history with
both Print Layout and Processing work!

> I'm dreaming of using the QGIS graphical modeler to output PDFs using print composers. This would allow to build very interesting report generators using our favorite tool. As far as I understand, currently, it is not possible.
>
> I'd be interested in working to implement it, to start with as a plugin (prototype), and once this works see if we can integrate into core.
>
> What I thought of so far is to be able to specify as inputs :
> - a QGIS project [file] (using the current project if empty - esp. useful in conjunction with the great new embedded models feature)
> - a composer name [string] (use the first one if empty)

I've got (Python) code which allows Print Layout parameter choices
(exposed as a combo box showing layouts within the current project),
and a layout "map item" parameter (links to the print layout
parameter, and shows a nice combo box of map items in the selected
layout). I can clean this up and share, but honestly my preference
would be to get this in to master as c++ code.

I think ideally the "print layout" parameter would expose both layouts
in the current project + a "..." file selector allowing choice of a
Print Layout template file (.qpt) (this should be relatively
straightforward -- basically a new temporary layout would be created
using the selected qpt). Possibly we could/should have an option in
that drop down to allow selection of a layout from a different project
file too, but this would add considerable complexity, and I'm unsure
if there's a concrete use case for this which couldn't be handled by
qpt template files alone.

> - N layers [layer] (I don't think we can support variable inputs count, so could be 5)
> - N layer ids [string] (id of the layer in the project to be replaced by corresponding input)

There's already multi layer parameters available for algorithms (which
also support layer order), so these could be reused here.

> Do you have some ideas about implementation ?

I'd see a number of ways we could expose print layouts/atlas to
Processing. Some potential algorithms could be:

Exporting algorithms
---------------------------
- Export layout. Just does a direct export of the selected layout to a
specified file. Nice and easy! [1]
- Export atlas. As above, but for a pre-existing atlas and selected
folder/file destination.
- Something like "export layout as atlas", where users select and
existing layout/qpt, map item, and "coverage layer", and the algorithm
iterates over the features in that layer just like an atlas, but using
a map layer generated elsewhere in the model as the coverage layer.

Utility algorithms
----------------------
- Create layer from layout map item extent. Creates a new polygon
layer with a polygon feature of the are currently visible within a
selected layout map item. Handy for quickly creating a polygon layer
with the exact coverage of a map item for styling or atlas creation
purposes. (I've implemented this one already as a PyQGIS alg)
- Create atlas feature template: After selecting a layout map item and
specifying a desired "scale" and "origin point", the algorithm creates
a new polygon layer with a single feature of the required size which
makes the map item match the given scale [2]. The use case here is to
create the feature, then "copy and move" it multiple times over the
area of interest for an atlas.
- Some equivalent of the ESRI "Create strip map index"  tool/QGIS
PolyStrip plugin. Select a map item, desired scale, and line feature
layer and the algorithm creates rotated template features along the
lines to allow an atlas following the line. Options for overlap
margins, max map rotation, etc.

Layout creation algorithms
-----------------------------------

We could also have algorithms for assisting with editing/creating
layouts themselves, e.g. algorithms which created grids of guides,
grided layouts, etc. I'd LOVE LOVE LOVE a layout "spell check" tool!

Anyway, count me in here. I'm interested in helping in whatever way
desired - I can either mentor someone else to code this, or would be
interested in doing it directly if there's a sponsor available.
Alternatively we could write up all the desired specs and run a crowd
funding campaign to raise funds!

Nyall

[1] Would work super-well with a potential "QGIS variable" parameter,
which displays as the QGIS variable editor and which would allow
tweaking of expression variables to inflence the map render
[2] I'd also like to see a proper parameter widget for "scale"
parameters, which uses the nice QGIS scale widget instead of just
direct number entry.