[QGIS-Developer] Rendering (simple) labels before background raster layer finishes?
Johannes Kröger (WhereGroup)
johannes.kroeger at wheregroup.com
Thu Sep 9 01:06:46 PDT 2021
Hey list,
I have a fast vector layer in the foreground and a slow raster tile
layer in the background. The vector layer has some simple labels.
Neither layer uses any blending modes to my knowledge.
For rendering the labels QGIS waits until the layers have finished
rendering which takes a while.
Our users can use the labels to orient themselves and to decide where to
pan and zoom.
They could work faster if the labels would appear early on, before the
slow raster layer finished rendering.
So I would like to have the labels render earlier, before the raster
layer finishes loading, and then "just" composite(?) the scene for the
final canvas image when done. Is there a reasonable way in PyQGIS to
achieve that?
When I manually disable the raster layer, move the map, let it finish
rendering (so the vector data and labels are visible) and then re-enable
the raster layer, it plops up in the background without the labels being
visibly re-rendered themselves so it seems technically possible.
As a hacky approach I tried disabling the raster layer right when the
https://qgis.org/pyqgis/master/gui/QgsMapCanvas.html#qgis.gui.QgsMapCanvas.renderStarting
signal, then re-enabling it when the render is done but of course that
introduces "white flashes" from the background vanishing, then
reappearing. Maybe it would be possible to tell QGIS to not request any
new data/render for the raster layer for a while but use the cached
render it currently has until the labels are done, then fetch and render
for the updated extents?
Cheers, Hannes
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