[Qgis-user] Let's discuss on a Mask feature

[Vincent Picavet]

Hi,

> users often ask me how they can mask parts of their map that are not inside
> their area of interest.
> As a quick an fast answer, we developped Mask plugin available on
> repository.  It take selected objects of a polygon layer, and creates a new
> memory layer, populated with a great rectangle with holes in place of
> selected objects. We have that way a layer that can  be drawn on top of
> others, masking features.
> 
> *We miss two main features:
>    - draw labels only for objects parts inside mask*
>    - *have the ability to generate a mask on the fly when using Atlas tool
> in Master.* That means, each time Map in composer in Atlas is zoomed on, we
> would like to generate a map of coverage object. We would also like to be
> able to generate a mask with buffer options to allow to show beiond strict
> boundary of mask.

As to solve this, I though of a solution being redrawing a geometrical object 
in the composer on top of the map, based on the objects to mask. This needs 
implementation of a polygon object in the composer, which is not yet present. 
But it would probably be simpler than doing a full mask feature 
implementation.

We could also have a button in the composer to «create a mask for this map 
based on the currently selected features in the map window».

The only drawback to this approach is that the labels will be covered by the 
mask, and will appear truncated if part of it lies under the mask. I do not 
know if this would be a blocker.

Therefore the question is : do we need this in the main map window, or this 
solution for composer would be enough ?

We'd be glad to implement that in composer and Atlas if this is a good 
solution for you.

Vincent

> *Do others need this?*
> 
> *Dear dev's, does that sound possible, and not too a CPU killer task? *I
> guess intersecting all objects on screen with some complex polygons could
> be hard. What about invalid polygons? Should we suggest simplifying mask
> polygons on the fly?
> 
> Please tell me about this, I would be happy to fund this If everybody
> agrees with that need and implementation.
> 
> 
> 
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