[Qgis-developer] Suppress rendering of small features

Nyall Dawson nyall.dawson at gmail.com
Thu Sep 12 03:11:44 PDT 2013

> This is possible using the Rule-Rendering
> also is possible to use the rule-rendering to define a rule that in a
> scale interval and with a dimension interval the feature is render using
> a point rendering and change in a polygon rendering when the scale grow.

While it is possible to set up rules to achieve a similar result, it's
quite a lot of work. It's also important to note that the feature I
propose would mean it's still possible to use graduated or categorized
renderers on a layer - again, the end result (scale/sized based
filtering together with categorized rendering) would be achievable by
creating a bunch of rules, but it would be a lot of work and the end
result would be much more rigid....

> As I understand, the feature to suppress small/large features would be
> optional.

Yes - definitely optional and off by default!

> How would you define small? I guess something can already be done with
> scale based visibility, but that needs preprocessing of the data and
> assigning some kind of "size class".

I'm thinking of using the same method as the "Suppress labelling of
features smaller than" feature from the labelling engine. This gives
you a choice of feature size in mm to use as a threshold, and in my
experience works quite well.

I think it's important to note that hiding small features isn't just
about a performance win - it's also useful from a cartographic
perspective. My main use case involves a large water body dataset I
frequently use, which contains everything from large lakes and inlets
to very small dams and reservoirs. I need to have the large features
show up at all scales, but I don't want all these smaller features
cluttering up my map at small scales. This feature would mean I could
easily hide the little lakes from my map at small scales, but they'd
show up as I zoom in.
Sure, this could all be done by assigning a hierarchy to the features
and setting up a bunch of rules in a rule based renderer - but it
would be so much quicker and easier to just tick a box and tweak the
threshold size to suit.

> Another interesting thing would be to simplify features when you zoom
> out. Maybe simplifying would be quicker than rendering thousands of
> unnecessary vertices.

Wasn't there a summer of code project with this goal a few years back?
Does anyone know what the outcome of that project was?


More information about the Qgis-developer mailing list