Post #8by doctorjoe » 27.06.2005, 04:55
Just to explain why the foreground orbits was a hard problem.
OpenGL has a z-depth buffer that stores information about how close or far something is in order to figure out if someone should be drawn in front or behind something else. The trouble with this depth buffer is that it doesn't have enough accuracy to do this correctly if there is a wide variation in distances, which happens to be the case in celestia where some things could be meters away, and some things could be light years.
So what celestia does is to bypass all of this and use depth buffers to draw an individual object and bypass the system in drawing the relation between objects. It sorts the objects in order of distance and draws them from back to front, with the front objects block the back ones.
The trouble with this is that you then can't render the orbits correctly since the orbits are lines that move back and forth between objects.
What the patch does is to draw the orbits three times. Once for the background. Once to draw then in front of the planet the object is orbiting, and once more to draw the intersection between the orbit and the object. This is less bad than it sounds, before there is some code that removes unnecessary redraws. Also all of the calculations for where the orbits are drawn are only done once.
Also the software that started this thread "cheats." The planets they draw are huge, and so they can put everything into one depth buffer without accuracy problems. Pretty, but wrong.
Also, as far as the "hole in the sky" bug. That is because the code that draws the clouds doesn't know anything about oblateness.
