layersopacityqgisqgis-3transparency
I just switched from QGIS 2.18 to version 3.0.3. In the earlier version, setting layer transparency was easily done with the "transparency" slider from the Layer properties->style tab. See image below:
The newer QGIS version, however, does not appear to display such a transparency bar, as seen in next image. Is there a simple way to change a layer transparency in this version of QGIS?
A little python script like this would do it
for layer in iface.legendInterface().layers():
layer.renderer().setOpacity(0.2)
Just copy and paste that into the python console (which is shown via Plugins->Python console) and hit enter twice. The script will loop through all layers and set their opacity to 20%.
For QGIS 3.x you need a slightly different Python script because the API has changed significantly:
layers = qgis.core.QgsProject.instance().layerTreeRoot().layerOrder()
for layer in layers:
layer.setOpacity(0.2)
# Redraw the layers so you can see the effect
iface.mapCanvas().refreshAllLayers()
You might want to look at VRTs (Virtual Rasters).
You can build these in QGIS, using Raster > Miscellaneous > Build virtual raster (Catalog), or directly on the command line using gdalbuildvrt
VRTs are small XML files which stitch together lots of rasters into one, "big" virtual raster which can be loaded and processed as if it was a single raster.
If you set the sourcenodata to "0 0 0" (assuming 3 bands, r,g,b) it should set pixels to transparent by setting the nodata value to 0. You probably want settings to look something like this:-
I don't have any batches of aerial imagery to hand to test this, but it should allow you to merge all your images and set a single NODATA value for all of them across multiple bands.
I tried this by creating a VRT from a single aerial raster (without a nodata value) and it had the desired effect of setting nodata to 0, and the empty areas to transparent, so it's worth a try.
Also tried this from the Processing/GDAL menu in QGIS 2.16.3, and this option was missing. So use the old-style menu option or the command line to try this out.
Best Answer
You have to click on the layer rendering arrow (see the bottom of your second image) to access the slider and other option.
in QGIS 2 it was on by default, in QGIS 3 it's off by default...