[QGIS-Developer] Incremental drawing possible?

[Nyall Dawson]

On 11 July 2017 at 19:22, Neumann, Andreas <a.neumann at carto.net> wrote:
> Hi Martin,
>
> Sounds interesting. Of course this is not urgent. We now need to concentrate
> on all the stuff that requires refactoring and API changes in the master
> branch.
>
> But it would really be nice, if such rendering improvements could be
> introduced in a future 3.x version. Maybe something for the next QGIS grants
> program?

Andreas - do you compile master builds? I'd love it if you could give
the branch from https://github.com/qgis/QGIS/pull/4777 a test and give
feedback on the open questions in that PR. I suspect you'll be pleased
with the behavior with that change.

Full credit for the changes goes to Sourcepole - I've just forward
ported the feature from their fork and modernised the code for 3.0.

Nyall

>
> Andreas
>
> On 2017-07-11 10:48, Martin Dobias wrote:
>
> Hi Andreas
>
> On Tue, Jul 11, 2017 at 9:38 AM, Neumann, Andreas <a.neumann at carto.net>
> wrote:
>
> Hi Martin,
>
> I noticed that loading and redraw only happens when the mouse button is
> released after the panning operation. While I understand the reasoning for
> this (definitely on the safe side and no unnecessary redraws), I wonder, if
> for WMTS, loading and display could already happen while panning and the
> mouse button isn't yet released, e.g. triggered whenever a certain threshold
> of panning distance (e.g. in percent of the last extent) was surpassed.
>
> This would be even nicer than the current behaviour. Or are there technical
> reasons, not to do that? Google maps works like this. It redraws while
> panning.
>
>
> Good news - there are no technical reasons that would prohibit us from
> such improvement. Background rendering could start earlier, we just
> need to add logic for such behavior. There is already a pending pull
> request from Nyall to render adjacent parts of map canvas once
> rendering of the visible area has finished - that may already be a
> sufficient solution - I have not had time to play with it yet though.
> Maybe these two approaches (render on map dragging, render outside of
> current view) can be combined somehow for the best UX while keeping
> overhead of extra rendering low.
>
> We could even think about moving more towards tile-based approach to
> rendering of map canvas, with the advantage of being able to cache map
> tiles from previous canvas redraws for the same scale. This would
> allow instant appearance of areas previously rendered with lower CPU
> use at the expense of consuming more memory and having more
> complicated logic for labeling.
>
> Regards
> Martin
>
>