4
When viewed from outside, the orientation of the polygons of a
surface should point in the same direction (clockwise or coun-
terclockwise). This way, OpenGL will provide methods for face
culling manipulation and display, which can be useful when
dealing with convex surfaces and drawing the front faces and
back faces of a polygonal surface.
The OpenGL documentation available at opengl.org indicates tech-
niques about triangle-strip layouts that can speed up the OpenGL
mode and save data storage space. Since most 3D applications use
or allow the use of OpenGL resources with a 3D card, this informa-
tion can be invaluable if you are going to model and fix triangulated
geometry. If the polygonal mesh is directly generated from regular
geometry such as NURBS surfaces, it is recommended that you
connect the triangles into longer strips. One of the major rules
about triangulation of the mesh refers to the clockwise or counter-
clockwise orientation of the triangle strips. The first triangle can
start with either clockwise or counterclockwise winding, but the
subsequent triangles must be alternated according to the orienta-
tion of the anterior triangle.
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Chapter 13 – Modeling for Games
Figure 13-2: A T-intersection at point A.
Figure 13-3: Alternated triangle strip.