Smarter Map Redraws [grant 2020]

[PeterPetrik]

qgis/QGIS-Enhancement-Proposals#181

• only for RasterLayers so far, should be easily extensible for other types
• there is developer define in QgsMapRendererParallelJob SIMULATE_SLOW_RENDERER, line 338, you can uncomment it to force very slow renderers (for testing)

general overview of the implementation: The QgsMapLayerRenderer has flag mReadyToCompose. To use the "smart map redraws" for the particular renderer, the flag must be set to false in the constructor and set to true once there is something to show to user (so the renderer actually drawn some data to resulting/destination image already). The rule now is that whenever the flag is set to false, the rendering job tries to fetch the cached image from the QgsMapRenderingCache for particular layer and transform it to the current drawn extent instead of showing QImage with all transparent pixels

Work funded by QGIS grant 2020.

[roya0045]

This makes me want to improve expression evaluation...

[nyalldawson]

@PeterPetrik
This doesn't actually seem to have any effect for me -- see the screencast below. It doesn't show so well in the screencast, but even the pan operations results in a ~0.25 second white canvas flash for me

[PeterPetrik]

@PeterPetrik
This doesn't actually seem to have any effect for me -- see the screencast below. It doesn't show so well in the screencast, but even the pan operations results in a ~0.25 second white canvas flash for me

actually removed the mReadyToCompose=false from raster renderer contructor when doing "final" changes for official PR... will fix ASAP

[nyalldawson]

Ok, that's certainly MUCH nicer now! Great stuff!

A few suggestions:

1. I think we should implement this for vector layers too, by setting mReadyToCompose only after the first feature is rendered. I've tested and this helps with some semi-common situations like a inverted polygon using the shapeburst renderer, which can be rather slow to draw.
2. It's actually possible for us to have multiple rendered versions of a single layer covering different extents, because the "preview job" functionality in the canvas automatically draws neighbouring tiles after the main job finishes. If we allowed the render cache to store more than one image for a layer, then potentially we could draw ALL of these during the intermediate compositing. This would mean that zooming out would nicely show the data from the surrounding areas while the redraw occurs. What do you think?

[saberraz]

Before:

After:

[vincentschut]

A HUGE thank you for this! This makes qgis feel so much better for me!

[PeterPetrik]

Best to thanks to QGIS grant scheme and all the QGIS sponsors :)