Is it possible to edit layer transparency in .sld or .qml file for use in QGIS?

Question

Say I have a shapefile with 20 graduated classes that I'm using in QGIS. I want to save the layer style so that I can use it on other shapefiles, but I can't edit the layer transparency without editing each class (20) individually.

Is it possible to edit the layer transparency in a .sld or .qml file? I arbitrarily set the transparency of one class to 53% and another to 78% (just for the sake of testing), saved both a .sld and a .qml, but I couldn't find these values anywhere in the two files. I also searched for things like 'opacity' and 'transparency' but I don't see an indication of how this is saved. However, my changes (of 53% and 78%) are reflected when loading a .qml file with the shapefiles.

It's not the end of the world to have to edit each class individually at first if the changes are reflected after saving a .qml. But I would still prefer something more reproducible since this would be really easy to do with a simple find/replace, and I may have to change the transparency of all classes at various times in the future.

Answer 1

The transparency is given by the so called Alpha Channel (Alpha compositing)

In a .qml file:

<symbols>
  <symbol alpha="0.10" clip_to_extent="1" type="fill" name="0">

This is the alpha value of the symbol (Layer transparency) and the value is 0.10 which corresponds to a transparency value of 0.9

If you try with multiple values of transparency, you can see that

alpha value = 100 - transparency value

There may be another value (Layer rendering/Layer transparency) and in this case, the values are the same (80 and 80)

<blendMode>0</blendMode>
<featureBlendMode>0</featureBlendMode>
<layerTransparency>80</layerTransparency>

Also, there is greater precision when exporting the .qml file than what is seen on the QGIS transparency slide bar. For example, if you set that bar to 10%, and save the .qml, you may see something like:

<symbols>
  <symbol alpha="0.0985" clip_to_extent="1" type="fill" name="0">

instead.