[Qgis-developer] Symbol levels

[Martin Dobias]

On Wed, Apr 16, 2014 at 1:49 PM, Hugo Mercier <hugo.mercier at oslandia.com> wrote:
>> On Tue, Apr 15, 2014 at 3:07 PM, Hugo Mercier <hugo.mercier at oslandia.com> wrote:
>>>
>>> The UI asks for numeric levels, but, if I am correct, only relative
>>> orders between two symbol layers are important.
>>> Was there something else originally planned ? Something like a global
>>> level to be able to order symbol layers of different vector layers ?
>>
>> No, it was meant just for levels within one layer.
>
> Ok, so ... why the UI asks the user for integers ? And what is the
> meaning of two layers with the same number ?

Integers are there for convenience... with more complex scenarios
(e.g. roads with highway crossings) it may be handy to have numbers
instead of just a pure list, and to possibly have "holes" between the
levels.

> What about a UI with the list of symbol layers and some up/down buttons ?

That would be possible, but maybe harder to use. In the table view,
you can see symbols and their levels at once. In a simple list with
up/down buttons you need to split the symbols into symbol layers and
it is more difficult to identify what are the levels for one symbol
(IMHO).


>>> Another point : apart from performance issue, I cannot see any reason
>>> why *not* to enable symbol levels. Are there situations were users do
>>> want different symbol layers to overlap ?
>>
>> I think there are such situations, from the top of my head:
>> - with symbol levels you cannot see the rendering progress (everything
>> is rendered at the end) - this makes quite a difference with big
>> layers
>
> If I am correct, you can still see the progress. You will see the first
> symbol layer, then the second one, etc.

Yes, but only once all features have been fetched from the provider -
and that interval usually dominates the rendering time. So, with a
layer that takes 5 seconds to render, there will be maybe 4 seconds
when you see nothing from the layer.

Regards
Martin