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I have large raster tile sets working well in QGIS (Ordnance Survey UK data). Sometimes I want to grey/fade these to make my data clearer to see. Attempting to access properties on QGIS for the VRT effectively crashes QGIS for the larger tile sets (it works for the smaller tile set, but takes a long time... but for the layers with more tiles I've not waited long enough to see if it works because more than a minute or ten becomes silly).

Can I access the properties in another way - for instance by taking a text editor to something (the project file presumably)? Should I do this then display the layers twice (once colour and once grey) so they can be turned on and off at will?

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I think I answered my own question - answer here for others to use...

There's a line in the project file:

grayscaleMode="0"

(or ='1' for grey)

After waiting for a significant time it was possible to access the properties dialogue for all the VRT layers - it just took an inordinate length of time. What I've now done is to set up the project so that it displays two copies of each map layer, one set to be grey one set as colour.

There are alternate modes for the greyscale - I think I chose 'lightness' rather than 'luminosity' or another option. I'm not sure how the project file would change with these options.

What I'd do next time around is to insert two copies of the colour VRT into my project, then to (save and) exit QGIS and change this grayscaleMode="0" flag to "=1" in a text editor for one of the layers. This would save the lengthy wait to access the properties within QGIS.

I have NOT tested this!

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  • AND I would also use a new feature which I just discovered - visibility presets (QGIS 2.6). At the top of the layers panel is a small eye button/icon. Set up your layers as you want them (i.e. which are visible and which hidden, in my case showing grey basemaps), then use this button to save this setting ("Add preset..."). Then set up an alternate preset (in my case showing colour basemaps). Easy to switch one to the other. Not sure how this works if new layers are added.
    – Rostranimin
    Commented Jan 16, 2015 at 11:22

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