[Qgis-developer] master slow reading rasters?

[Martin Dobias]

Hi Olivier

On Sun, Mar 30, 2014 at 7:30 PM, Olivier Dalang
<olivier.dalang at gmail.com> wrote:
>
> 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.

The updates currently work as follows: until the first map update (250
ms by default) the canvas displays transformed image for previous
rendering process. After 250 ms this is replaced by real map preview
and being updated every 250 ms until rendering is complete. So, if
your "background" layer is not rendered fast enough, you will get the
undesired effect you mention.

If you turn on parallel rendering (menu Options > Rendering > Render
layers in parallel using all available CPU cores), the effect may be
possibly reduced.

> 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)

I do not know about a ticket, I know Paolo has mentioned that problem
previously (either on the dev list or at the hackfest in Vienna).

I would like address that in the future - probably not for 2.4
release, but maybe for 2.6.

Regards
Martin