[Qgis-developer] Re: Improving rendering speed

[Gavin Macaulay]

Hi,

I've had a look at using QPainter's clip region instead of QgsClipper. It
seems to have a bug in it reminiscent of the original problem we had with
not clipping- using a clip region allows one to zoom in further than
without, but it still gets confused eventually. I suspect that the problem
is when the coordinates are approaching the limit of the floats in which
they are stored (when I did the QgsClipper code I think I took some care
to cope with this gracefully).

For the moment we'll have to leave QgsClipper in. Note that the QgsClipper
clipping overhead only happens when zoomed in enough to cause the feature
screen coordinate points to go beyond +/-32000, although it does check
every feature to see if it goes beyond +/-32000 (but that's a fairly quick
operation).

Gavin

> Hi,
>
> I think that the problem with QImage will be a very similar problem as
> with X11, but is most likely to now extend to other platforms :(
>
> QPainter has the ability to set a clip region, and this may be a way to
> get Qt to do the clipping, although I seem to remember trying this in
> Qt3. However in the end someone has to do the clipping, and whether it's
> Qt or Qgis should really only depend on which code can do it fastest.
> I'll try a clip region on QPainter later tonight.
>
> Gavin
>
> Martin Dobias wrote:
>> I've updated that wiki page - with regard to Gavin's testing, there's
>> still need of clipping features. Currently not due bug in X11 but
>> because of bug in Qt - in rasterizer for QImage. This will probably
>> affect also other platforms, not only X11. Please could someone test
>> it on the other platforms?
>>
>> Martin
>>
>>
>> On 3/18/06, Martin Dobias <wonder.sk at gmail.com> wrote:
>>> Hi,
>>>
>>> I've started to do some research on how to make rendering faster. My
>>> benchmarks and thoughts on optimizing are on wiki:
>>> http://wiki.qgis.org/qgiswiki/Rendering_Performance
>>>
>>> I'd like hear some feedback from you!
>>>
>>> Bye,
>>> Martin
>>>
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