It should work to some extent. Not sure if it will look right when a circular region is on the horizon. But it’s worth a try. [/QUOTE]
btw, does yours work correctly when the circular region is on the horizon? I’m guessing we are talking about the same issue…the issue that you can see in picture #2 that I attached. In that case, you are seeing the disk from the side, and since the disk is not curved the part of the earth that sticks above disk is visible. Had you been looking straight down at it you would think that that piece of earth was covered by the disk but in the side view you see that it is not.
Clearly, if this is what you are experiencing also, then this solution would not work for the extreme case of say coloring half the earth one color. i.e. say you wanted to highlight the upper hemisphere and so you drew a disk through the center of the earth and when you turned of the depth test and viewed the earth from above you would see what you expect. But when you now viewed the earth from the side (ie towards the equator) you would only see the edge of the disk and both the top and bottom hemispheres would not be colored.
Sigh, that kind of sucks. I say that is an “extreme” case but not really…not for space visualization it is not. The reason is that some satellites have very high orbits and as such their ground coverage may in fact be the entire hemisphere of the earth and in that case this solution would not be optimal.
Well, at least I have some more options. This is kind of fun, if not frustrating.