Note :
In the Bryce file linked on page one you can see how this terrain was altered.
I used a black/white png (portable network graphic) for clean window edges.
Bryce 5 does not do straight vertical lines well at all and I suggest you use
the png.
You can download the one I made here:
Click Load on the far left window shown.
Browse to where you downloaded the file, VisLightwindow.png in this case, then
click Open.
Click Copy under the left window and Paste
into the Middle window.
Hit Apply shown below.
The resolution (detail level if you will), solidity and shape must be altered
here a bit...
lets carry on my wayward Bryctafarians. Time for a smoke break ; )
Click the leetle down arrow below Grid and select Solid.
It can't cut the window out if it's hollow.
Click Grid and choose at least 512-ultra-fine resolution.
Because Black is low and White is high in the TE you have to reverse the pattern.
Inverting the terrain will make the black areas into Solid White features which
will cut the window pane shape into the walls.
And here's the result - don't leave yet there's more to do...hahahahahaahhaaaaa
Torture is it!
Apply all changes and Exit by clicking the check mark at the bottom right of ther Terrain Editor.