[Qgis-developer] master slow reading rasters?

[Olivier Dalang]

Hi !

>> One unpleasant effect is: when zooming in, it first loads a preview,
then the image
>> disappears, then it shows at full resolution.

I have the same problem here on master, but it seems to affect vectors as
well as rasters.
It's more easy to spot (and more annoying) with layers taking a long time
to load/render, such as big rasters, complex vectors, or layers from a slow
distant server.

As far as I see, here what's happening upon user interaction (zoom or pan)

1) the previously displayed image is displayed offsetted/resampled
(instantaneous)
2) the image disappears for a short amount of time (probably during the
recalculation of the image)
3) the recomputed image is show.

The problem seems to be that the previous image is removed while rendering,
instead of being kept until the layer has been refreshed.

This can be also very annoying when working with a lot of layers.
Since layers are made invisible while rendering, they all reappear
progressively, one after the other, leading to a map canvas which seems
almost animated after finishing to interact with it.
Here's a video of the effect, displaying a lot of shapefile layers :
https://www.dropbox.com/s/ofa3pe5mqss3gm2/LayerRendering_01.avi
(sorry, it's quite abstract, but because of rights, I can't give a better
view of this data)

Do you confirm ? Did someone open a ticket related to this already ?
(didn't find one in the list)

Thanks !

Olivier
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