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I'm using QGIS 2.14.3.

Is it possible to render bottom layers after higher layers while still keeping them underneath? (or can rendering only be bottom-up?) I have seen the "control rendering order" panel but changing the order here changes the bottom/top order in the canvas.

I'm thinking mostly about rendering heavy bottom rasters or web-based satellite images (openlayers or quickmapservices) after lighter top vector layers, in order not to have to wait for it every time one moves the canvas or zooms.

  • I don't have a direct answer to your question, but a major performance boost can often be gained from doing a lot of your work in the same coordinate system as the web layers so that the tiles won't be reprojected in the background. You naturally don't want to do your major analysis this way, but for viewing and inspecting data, it's helpful. – nicksan Jun 7 '16 at 17:37
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As an alternative, if you want to speed up rendering then you could try the following instead of playing with the rendering order:


  • For Rasters

    Create a virtual raster(s) from multiple raster layers by going to the menubar:

    Raster > Miscellaneous > Build Virtual Raster (Catalog)
    

  • For Vectors

    Create a memory layer(s) from multiple vector layers by downloading the Speedy Layer plugin:

    Plugins > Manage and Install Plugins...
    
  • 1
    Thanks. I actually already have built a virtual raster for the rasters, so do not really have a problem with those, mostly with web-based imagery. – Victor Jun 3 '16 at 13:39
  • @Victor - Interesting problem, hopefully there's a solution :) – Joseph Jun 3 '16 at 13:44
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To add to Joseph's list of suggested speedups, you can alter the refresh interval of the canvas (by default it's a four times a second, 250ms, but you can make this value larger, say 1000 ms).

This might speed up total elapsed time by reducing the number of canvas repaints (the rendering will still be happening in background threads). I'd need to write some pyqgis code to test this idea scientifically, though :)

You can also turn off antialiasing at the expense of image quality, and simplify geometries of vector layers on-the-fly (but that can sometimes cause issues in some projections). Your mileage may vary!

The screenshot below shows the settings you need to tweak.

rendering speedup settings dialog

And there's pyramids too, although those won't help for web map layers.

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