Onward and upward.
Tried to get PSP 9 working, but given up. Another prog killed off by Windows 10.
Any way, here is a screen-grab of DXTBmp after opening tile 0 -149 44 3.
http://i.imgur.com/5L8Rywv.jpg?1
The tiles main image is in the centre of frame, in the top right is a smaller image of the alpha channel. This is the one we are interested in.
DXTBmp has a nice feature that double-clicking on the alpha channel opens it in your paint program.
To set it, click on the icon half-way down the right hand column( a tool-tip shows "Select Editor"). I'm using msPaint.exe in Windows 10, its path is \Windows\System32\mspaint.exe..
If you are using something else, this is where the path to the .exe file is entered.
Here is the alpha channel opened in Paint:
http://i.imgur.com/mQ62qZa.jpg?1
As you can see, most of the image is White, the right-hand edge, and bottom are graded from White to Black with various levels of Grey between.
I used the Darken/Lighten tool in PSP 9. Its all done free-hand, that's why its wobbly...
If you look at the screenshot in #8 above, you can see what effect it has had on the composite image.
Where the alpha channel is White, the tile is opaque. Where it is
not White, some degree of transparency is applied, varying to completely transparent(no main image) where the alpha channel is Black.
(going to take a break here, and do some notes on the various DDS types).
N.