[Qgis-developer] Resize QGIS line or polygon about its centroid

[Régis Haubourg]

Oh, I didn't get that you want to actually change the geometry itself not
its style.
I usually don't play with geometry for representation purpose, or sometimes
I store alternate geometries but I keep the original one.
Are you aware of the geometry generator renderer ? You could apply a
translation based on data defined expression to make on the fly
modifications without creating any data. What you need is still unclear to
me, maybe some screenshots or drawings could help.

Cheers
Régis



2017-02-22 15:32 GMT+01:00 C Hamilton <adenaculture at gmail.com>:

> Hi Regis,
>
> I would assume that in this use case the user would not be modifying the
> source polygons, but would make a copy of them which could be a memory
> layer and then resize them. I mentioned resizing small features so that
> they visibly would be better shown on the map, but the other use case would
> be when the user draws a polygon and would then like to shrink or enlarge
> it, then perhaps move it into position. QGIS already has the move tool, but
> I don't see one to shrink or enlarge polygon tool.
>
> I understand how a point label's size can be modified based on some
> attribute, but can an expression be written that would allow a polygon size
> be scaled and centered about the source centroid, based on some attribute
> in the data. If that is possible I don't know how it would be done.
>
> Thanks,
>
> Calvin
>
> On Wed, Feb 22, 2017 at 2:53 AM, Régis Haubourg <regis.haubourg at gmail.com>
> wrote:
>
>> Hi Calvin,
>> what you describe requires to store the specific object size somewhere,
>> which is the exact same requirement with data defined labeling (or any data
>> defined stuff in QGIS, those widgets are everywhere).
>> Modifying the source layer is quite wrong, and sometimes impossible
>> (readonly datasets). You can have a look to EasyCustomLabeling where I
>> coded a duplication of the objects I needed to label into a memory layer.
>> Persistence is made thanks to MemoryLayerSaver plugin that saves them in a
>> separated .mldata file. In you case, a symbol is made of so much markers,
>> sub markers that you can't just define one width or size parameter shortcut
>> button (or mouse scroll + key modifier) so easily.
>>
>> This is anyway quite hacky and justify a planned and funded work for
>> QGIS3 : Auxiliary storage. That work will provide a way to store data or
>> other ressources inside the qgs project. That will provide a nicer and more
>> portable method for custom labeling or styling.
>>
>> Just to end up with your cartographic need, up to now, I've always
>> managed to handle what you describe with conditional rules in expressions
>> without having to manually storing values. I eventually had to store
>> exceptions to rules in the following way:
>>
>> Ex: Styling city point with size proportional to the number of
>> inhabitants , except for one
>>
>> CASE
>>   WHEN city = 'Grenoble' then  22
>>   ELSE scale_exp("altitude", 0, 0, 1, 10, 0.5)
>> END
>>
>> I hope that helps
>> Cheers Régis
>>
>> 2017-02-21 21:14 GMT+01:00 C Hamilton <adenaculture at gmail.com>:
>>
>>> I have an project where I want a feature to be larger than life just so
>>> that it shows up on the map. Ideally I would like is to be able to select
>>> one of more features and with the mouse have it interactively resize them,
>>> but keeping them centered about their centroids. Once I am satisfied with
>>> their size I would either click a button to fix them or hit the Enter key.
>>>
>>> I saw the Affine transformations plugin, but that takes too much
>>> thought. Is this capability already there and am just not finding it or has
>>> it simply not been done? If the latter is the case I may consider writing
>>> the plugin to do it although I think it would be nice for it to be a part
>>> of the standard QGIS editing functions.
>>>
>>> Thanks,
>>>
>>> Calvin
>>>
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>>
>>
>
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