Hi Martin,
On 03/10/2014 21:10, Martin Dobias wrote:
Hi Vincent
On Fri, Oct 3, 2014 at 1:10 PM, Vincent Mora<vincent.m...@oslandia.com> wrote:
We can use expressions to make the single symbol layer behave:
- graduated : properties are continuous functions of attributes,
- categorized or rule based : properties are discrete function of attributes
The user interfaces for categorized, graduated, rule-based could remain the
same, it's just that it would generate the expressions for the single symbol
instead of configuring as specific class. Several categories could be
defined to affect various parameters (color and size are the first use
case).
Can you tell me if the idea is sound ?
If I understand correctly, you propose to use single symbol renderer
with expressions (for color and for size) in places where we currently
use graduated/categorized renderers. Is that correct?
Yes, it is correct. Although the marker, which can be composed of
several simple symbols, seems to be a better base.
If it is, how
would it handle the case where symbols of individual classes are
completely different? (not just varying in size/color)
A discrete function can handle the shape (it is already a parameter that
can be defined by an expression). A little complexity comes with
composed symbols (markers): in a given layer, different markers (from
different classes) may be composed of a different number of symbols, but
if we handle the 'null' case for the shape, then the number of symbols
for a marker is only a max, and markers with less symbols will be
handled just fine.
It would be also good to check the impact on the rendering performance
- maybe if more than just few classes are used, the evaluation with
expressions could slow things down significantly. (the expression
engine could enjoy some performance improvements).
The performance issue is definitely a concern.
I can think of some strategies to avoid a systematic evaluation of
expressions for all features, but they imply some sort of
classification, so I'm not sure it's a net gain on the code complexity
front. So I would prefer keeping the approach simple (i.e. evaluate
expressions for all features) and profile the code to see if expression
evaluation can be improved.
I've made a preliminary test with graduated color (5 classes) one the
one hand and continuous color defined by expression on the other hand.
For 250k points, this gives roughly a factor 2 loss for rendering time
(5sec for graduated, 10sec for expressions). I think this is rather
encouraging.
Do you think this 2x loss is a killer for this approach (considering the
improvements that can be made to expression evaluation) ?
Regards
Martin
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