Hi all, As said in my post (a few minutes ago), it sounds like we do not all agree on what is a primitive! OpenGL says a primitive can be a strip/fan ( http://www.opengl.org/wiki/Primitives ).
However, I agree it sounds very strange to have same behaviour for BIND_PER_PRIMITIVE and BIND_PER_PRIMITIVE_SET. Should we introduce "BIND_PER_ELEMENT" or such? Should we rename the BIND_PER_PRIMITIVE and BIND_PER_PRIMITIVE_SET? Thoughts? Sukender PVLE - Lightweight cross-platform game engine - http://pvle.sourceforge.net/ ----- "Wojciech Lewandowski" <lewandow...@ai.com.pl> a écrit : > Hi Robert, Jason, > > I am lazy guy, and I agree with both of you, its only neccessary to > make it > backward compatibile ;-) > > However, for purely academic reasons I will respond to Jason. But > don't > treat my post as call for revolution. Its just my opinion how > PER_VERTEX, > PER_PRIMITIVE, PER_PRIMITIVE_SET should be resolved for Triangle > strips. > But, I will state it again, if it was differently done before, then > lets > keep it as it used to be. > > Now lets get to our academic discussion: > > In Jason's post BIND_PER_PRIMITIVE does the same as > BIND_PER_PRIMITIVE_SET > for Triangle strips. Note that if this was correct behaviour then > TriangleStrips with BIND_PER_PRIMITIVE binding could use fast paths. > > Instead, IMHO thats how these bindings should be translated to > glBegin/glEnd > OpenGL code: > > // Color BIND_PER_VERTEX : N vertices, N colors > glBegin(GL_TRIANGLE_STRIP); > glColor4f( ... ); // set color for each vertex > glVertex3f( ... ); > glColor4f( ... ); > glVertex3f( ... ); > ... > glColor4f( ... ); > glVertex3f( ... ); > glEnd(); > > // Color BIND_PER_PRIMITIVE : N vertices, N-2 colors > glBegin(GL_TRIANGLE_STRIP); > glVertex3f( ... ); // two verts without colors > glVertex3f( ... ); > glColor4f( ... ); // starting from third vertex, we pass color > for each > provoking vertex of triangle > glVertex3f( ... ); > glColor4f( ... ); > glVertex3f( ... ); > ... > glColor4f( ... ); > glVertex3f( ... ); > glEnd(); > > // Color BIND_PER_PRIMITIVE_SET : N vertices, 1 color > glColor4f( ... ); // set color once > glBegin(GL_TRIANGLE_STRIP); > glVertex3f( ... ); // no need for colors > glVertex3f( ... ); > ... > glVertex3f( ... ); > glEnd(); > > > With above interpretation, only PER_PRIMITIVE binding cannot be done > with > fast paths DrawArrays. So general recommendation to avoid > BIND_PER_PRIMITIVE as slow path trigger, still makes sense. > > Second: I would not treat Performer as ultimate reference for > resolving all > doubts. Performer did not offer PER_PRIMITIVE_SET binding and also had > > additional PFGS_FLAT_LINESTRIPS, PFGS_FLAT_TRISTRIPS in addition to > classic > PFGS_LINESTRIPS, PFGS_TRISTRIPS we have in OSG. > > > I actually wonder what the colors would look like here. Did you > actually > > run this code? My guess would be that the final vertex is green, > but the > > final triangle would blend from red to green across its surface, > because > > its two other vertices were red (as specified in the code). I > could be > > wrong (I haven't run the code myself), but that's what I would > expect. > > Even if you consider each triangle in the strip a different > "primitive", > > you still couldn't get a set of completely red triangles, followed > by a > > completely green triangle, which is what the OP is looking for. > > I ran it and yes you can. I guess, thats what PFGS_FLAT_TRISTRIPS was > doing > in Performer. With flat shading (activated in example), one triangle > is red > and second is green. With smooth shading second triangle is colored > wtih > red -> green gradient. > > >> Last argument is actually a postulate for OSG clarity. We have > >> BIND_PER_PRIMITIVE_SET flag. Shouldn't this flag be rather used for > the > >> situation where we want to one normal / one color etc for all > triangles > >> in > >> tristrip ? > > > If I understand you correctly, then yes. BIND_PER_PRIMITIVE in the > case > > of triangle strips should mean the same normal/color for each entire > > > triangle strip (that's how Performer used to treat it as well). If > I > > remember correctly, the OP was looking to get different normals for > each > > triangle in the strip (to produce a faceted appearance, I think). I > don't > > believe this is possible even in pure OpenGL. The only way to do it > is to > > use distinct triangles for primitives. > > Well... what you write sounds like you don't agree with me, because I > > postulate that that BIND_PER_PRIMITIVE should differ from > BIND_PER_PRIMITIVE_SET and BIND_PER_VERTEX. And yes I will say that > again, > its possible to make faceted apperance with triangle strips in OpenGL. > Thats > was my point in my first post and thats what this short example I > posted was > doing. Btw good point about GL_TRIANGLES instead of GL_TRIANGLE ;-) > > Highest Regards ;-) > > Wojtek > > _______________________________________________ > osg-users mailing list > osg-users@lists.openscenegraph.org > http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
