I need to digitize some old maps. But when working, I need to make the backgrounds of old maps as transparent as possible. But the backgrounds of old maps are usually gray and shades of gray. I can make them transparent from the properties section. But I need to specify for each pixel individually. Is it possible to make a certain range of colors transparent in QGIS?
The transparency of the whole layer can be set in layer rendering https://docs.qgis.org/3.22/en/docs/user_manual/working_with_raster/raster_properties.html#layer-rendering
For digitising purposes set the layer rendering to multiply, this makes the layer transparent for the light colours. https://docs.qgis.org/3.22/en/_images/layer_rendering_options.png
Furthermore, you can also increase the layer contrast to make the lighter parts lighter and the darker parts darker.
The global opacity slider can also be used to tweak the image if the dark parts are obstructing the view of certain features. https://docs.qgis.org/3.22/en/docs/user_manual/working_with_raster/raster_properties.html#transparency-properties
