It is very useful to make raster global transparency so you can see through, for example if you have one scanned map georeferenced and an aerial photo, you can make scanned map transparent and check how it fits with the aerial photo or other scanned maps.
i use QGis for this and it is very useful and handy.
Find attached an example of the overlay of an aerial photo with a scanned georeferenced map, it is so useful the global transparency.

I don't know why this simple option is missing while there are a lot of feature in raster design and none of them does it quicker and simplier than QGis, please ADD it.
one simple button
