julian@osu-dbs.UUCP (04/11/84)
The recent submission of hidden surface software bears explanation. The collection is known as scene assembler and friends, or `scn_assmblr'. Most of the system was written by my adviser, Dr. Franklin C. Crow, while he was at the Computer Graphics Research Group, Ohio State University (he is now at Xerox PARC). The rest was written by me. Since he left, I have been primarily responsible for maintaining our version of the system. Let me warn you about a few things. 1. The coordinate space is HDV (horizontal-depth-vertical). It corresponds exactly to longitude-latitude-altitude for a ground based observer, which is why it was selected. We prefer XYZ, and switched to XYZ a long time ago. You can convert from HDV to XYZ with a right handed rotation of 90 degrees about X. 2. "delete" command in scn_assmblr does not work. 3. If you change the color of something in scn_assmblr, it will attempt to update the *.obj file on disk, which of course requires write access. This is so `poly_zsort' can get the object's color, but unfortunately precludes object instancing. We changed `scn_assmblr' to pipe the color to `poly_zsort'. 4. `poly_zsort' is a heuristic; it is not a full blown hidden surface algorithm implementation. You must follow the rules about polygons with approximately equal area and dimensions for it to work anywhere near correctly. Forget intersecting objects. 5. `poly_zsort' does not do bounds checking as it fills up arrays. Furthermore, commands must be typed in a particular order for it to work correctly. There are other minor problems you will encounter as you get familiar with the system. The senior members of our group are constantly playing with the software to make the interface more robust and consistent. Believe me, two dozen animators can find lots of bugs. We have recently written (plug time) a paper describing our ideas on the graphics pipeline and how we've implemented it here, with `scn_assmblr' forming the central scene generator. We will get around to sending it in somewhere someday, probably IEEE Computer Graphics & Applications. In addition, Dave Zeltzer and myself have papers describing the two animation systems in use at CGRG, both of which pipe to scn_assmblr. You can make nice pictures with `scn_assmblr' and friends, but be prepared to put in lots of work. Julian "a tribble took it" Gomez Computer Graphics Research Group, Ohio State Univ. [ucbvax!]decvax!cbosg!osu-cgrg!julian