Viet.Ho@samba.acs.unc.edu (Viet Ho) (12/31/90)
I did some quick rendering tests on Imagine today and noticed the edges of the CSG spheres are *FACETED* Now I've double checked to make sure these are CSG spheres in the Stage editor. There's only one sphere in the scene. Is Imagine doing polygon mesh approximation on their CSG spheres for the render process? Why would they do such a foolish thing? I use 640X400 in all of my renderings and the edges are most noticable along the darker side of the sphere. This artifact came up in the early TurboSilver renderings, but I didn't paid much attention to them then. -Viet Viet.Ho@samba.acs.unc.edu
amigan@cup.portal.com (R Michael Medwid) (01/01/91)
As far as I can tell Imagine's CSG sphere are just that, one sided "perfect" spheres. The only place I see facets is when I am in the detail editor or in the stage editor and then the wire frame representation is faceted.. but this is to be expected. My only disappointment with CSG spheres in imagine is that they are incompatible with the cycle editor..if you try to load a csg sphere there it gives you a message to the effect :this type of object not supported. But this to is kind of understandable given that the cycle editor likely does its thing based on manipulations of base polygons. -Mike (amigan)
koren@hpfcdc.HP.COM (Steve Koren) (01/04/91)
/ hpfcdc:comp.sys.amiga / Viet.Ho@samba.acs.unc.edu (Viet Ho) / 12:57 am Dec 31, 1990 / I did some quick rendering tests on Imagine today and noticed the edges of the CSG spheres are *FACETED* Now I've double checked to make sure these are CSG spheres in the Stage editor. There's only one sphere in the scene. Is Imagine doing polygon mesh approximation on their CSG spheres for the render process? Why would they do such a foolish thing? I use 640X400 in all of my renderings and the edges are most noticable along the darker side of the sphere. This artifact came up in the early TurboSilver renderings, but I didn't paid much attention to them then. -Viet Viet.Ho@samba.acs.unc.edu ----------
koren@hpfcdc.HP.COM (Steve Koren) (01/04/91)
[I appologize if this response appears more than once; the notes poster program burped repeatedly ] Viet Ho writes: > I did some quick rendering tests on Imagine today and noticed > the edges of the CSG spheres are *FACETED* Now I've double checked > to make sure these are CSG spheres in the Stage editor. There's > only one sphere in the scene. Is Imagine doing polygon mesh > approximation on their CSG spheres for the render process? > Why would they do such a foolish thing? There is a common technique called "Gereaud shading" (yeah, I know I badly mangled the spelling :-) ). It involves representing curved surfaces as a set of polygons, and making the curved surface look smooth by interpolating the normal vectors between centers of the polygons. Since the way the light "reflects" is determined largely by the normal vector, the surface will look smooth even though it is composed of polygons. Of course, you will still see artifacts of this near the edges of the objects. If you look closely at some of the "computer animation" video tapes floating around from the state of the art animation houses (ILM, etc), you can often see this technique being used. Now, I haven't seen Imagine, so I don't know that is necessary what they are doing. But it could be. It has several advantages: firstly, it is easier to render polygons than curved surfaces, and secondly, arbitrary curved surfaces can be modelled easily that way, where trying to model them using some other method would be more difficult. Hope this answers your question. - steve
mark@calvin..westford.ccur.com (Mark Thompson) (01/08/91)
In article <5500044@hpfcdc.HP.COM> koren@hpfcdc.HP.COM (Steve Koren) writes: >There is a common technique called "Gereaud shading" (yeah, I know I >badly mangled the spelling :-) ). It involves representing curved >surfaces as a set of polygons, and making the curved surface look >smooth by interpolating the normal vectors between centers of the >polygons. turn nit-picking mode ON....... What you are trying to describe is Phong shading, not Gouraud. In Phong shading, the averaged normal vectors at each polygon vertex are bilinearly interpolated across the polygons surface in scanline order at each pixel. Lighting calculations are then performed on that pixel based on the interpolated normal vector, the view vector, and the lighting vector(s). In Gouraud shading, the lighting calculations are only performed at the polygon vertices and then these values are interpolated rather than the normal vectors. It is far less computationally intensive but does not handle specular highlights as well as Phong. Most software based renderers today use a Phong shader where as hardware implementations usually do Gouraud shading. %~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~% % ` ' Mark Thompson % % --==* RADIANT *==-- mark@westford.ccur.com % % ' Image ` ...!{decvax,uunet}!masscomp!mark % % Productions (508)392-2480 (603)424-1829 % % % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~