Hi all,
Interesting discusssion. I didn't guess that it wouyld bring so much comments!
Well this cannot be a regression as I'm implementing it! I just need to make it
work "the old style" way.
However, I would expect that a primitive is one of {point, line, triangle,
quad}, even id OpenGL says a strip is also a primitive (is there a naming issue
there? Should we have something like "elements", "primitive" and "primitive
set"???). So, for me, PER_PRIMITIVE is a binding for them, and
PER_PRIMITIVE_SET is a binding for a "bunch" of them :
TRIANGLES, TRIANGLE_STRIP, TRIANGLE_FAN : primitive = one triangle / primitive
set = all triangles
QUADS, QUAD_STRIP : primitive = one quad / primitive set = all quads
Once again, that's only my point of view.
Actually, OpenGL can do this binding (even if it's slow), so why not supporting
it if it's not that difficult? Should we name it differently?
Thank you all for your comments and ideas.
Cheers,
Sukender
PVLE - Lightweight cross-platform game engine - http://pvle.sourceforge.net/
----- "Jason Daly" <[email protected]> a écrit :
> On 1/8/2011 6:19 AM, Robert Osfield wrote:
> > Hi All,
> >
> > Perhaps we should be asking the question what was the behavior
> prior
> > to the refactor to I did for GL3/OpenGLES support. Sukender did
> your
> > Geometry work previously? Is this a regression or just a behaviour
> > that you weren't expecting?
>
> Good question!
>
> ---------------
>
> Somehow I missed Wojtek's post, so I'll reply to that here:
>
> >> glBegin(GL_TRIANGLE)/glEnd() code with 2 triangles and one normal.
> It will
> >> be correct OpenGL code. Would you say that two triangles correspond
> to one
> >> OSG primitive or two OSG primitves in this case ? And if you do not
> pass
> >> normal before second triangle, OpenGL will use last normal passed
> (ie the
> >> one from first triangle):
> >>
> >> glBegin(GL_TRIANGLE);
> >> glNormal3f(...);
> >> glVertex3f(...); //1
> >> glVertex3f(...); //2
> >> glVertex3f(...); //3
> >> // no normal and its no error !
> >> glVertex3f(...); //4
> >> glVertex3f(...); //5
> >> glVertex3f(...); //6
> >> glEnd();
>
> It's two primitives. Yes, you can use the same normal for two
> separate
> triangles, but that doesn't mean it's not two primitives. Actually
> your
> code is slightly incorrect, the glBegin() line should read:
>
> glBegin(GL_TRIANGLES);
>
> I'm not pointing this out just to be pedantic. It's evidence to
> support
> my position that it's actually two primitives (i.e.: two triangles)
> in
> that case :-)
>
>
> >> In the same way OpenGL assumes that last passed normal is used for
> the
> >> triangle in triangle strip. Triangle Srip is just another method of
> passing
> >> vertices to OpenGL and each triangle may have its own unique
> normals/colors.
> >> If you don't agree, just do a reverse test: see if below would
> render both
> >> triangles with the same color or different colors. They will
> differ, and
> >> this is also correct OpenGL code:
> >>
> >> glShadeModel( GL_FLAT );
> >> glBegin(GL_TRIANGLE_STRIP);
> >> glColor4f( 1, 0, 0, 1 ); // RED
> >> glVertex3f(0, 0, 0);
> >> glVertex3f(0, 1, 0);
> >> glVertex3f(1, 0, 0);
> >> glColor4f( 0, 1, 0, 1 ); // GREEN
> >> glVertex3f(1, 1, 0);
> >> glEnd();
>
> Yes, I mentioned that in my previous post. It doesn't take away from
>
> the fact that the triangle strip is considered a single primitive.
>
> 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.
>
> >> 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.
>
> --"J"
> _______________________________________________
> osg-users mailing list
> [email protected]
> http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
_______________________________________________
osg-users mailing list
[email protected]
http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
