McTaggart, Kevin wrote:
> When drawing a square, a call to glNormal3f doesn't appear to affect how
> the square is drawn. I'm trying to get a coloured front and dark back
> by using the following:
>
> glColorMaterial(GL_FRONT_AND_BACK, GL_EMISSION)
> glPolygonMode(GL_FRONT, GL_FILL)
> glPolygonMode(GL_BACK, GL_LINE)
> glColor3f(0.0, 1.0, 0.0)
> glBegin(GL_POLYGON)
> glNormal3f(0.0, -1.0, 0.0) # Doesn't appear to have any
> influence
> glVertex3f(1.0, -1.0, -1.0)
> glVertex3f(1.0, -1.0, 1.0)
> glVertex3f(-1.0, -1.0, 1.0)
> glVertex3f(-1.0, -1.0, -1.0)
> glEnd()
>
> It appears that calling glNormal3f has no influence on which is the
> front or back, and that I have to rearrange the order in which vertices
> are called (clockwise versus counter-clockwise). Any hints on how to
> get glNormal3f to influence which is front and back?
>
AFAIK this is working correctly. That is, the "front" and "back" faces
of a polygon are determined by winding rules, not normals. You can
reverse the direction by setting the winding rule to CCW or CW, but I
don't *think* you can make it use normals... after all, a polygon can
have a different normal at every vertex, one pointing backward, another
forward, then how do you decide which is "back" and which is "front".
I would imagine you might be able to write a shader program that
implements the rules for you so that it *looks* like the normals are
used in your situation, but I don't think you can readily do it in
general OpenGL, as the engine is going to look at the front/back status
long before it gets around to lighting calculations (I'm guessing, but
that would make sense to me).
HTH,
Mike
--
________________________________________________
Mike C. Fletcher
Designer, VR Plumber, Coder
http://www.vrplumber.com
http://blog.vrplumber.com
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