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"
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