Validation of CG Models - An Introduction to Computer Graphics for Artists - page 121

Fig. 9.8 For rendering, a 1.0 aspect ratio is ideal. Excessively thin triangles can cause render
errors in real time renderers

make use of shortcuts to speed the rendering process. With some of these renderers,
they fi nd the center of a polygon and then compare the Z-depth of that coordinate
with the center of nearby polygons. The Z-depth is the distance from the camera,
measured as a ray perpendicular to the camera lens. Whichever center point has the
smallest z-depth value will be rendered in front of the rest. This algorithm works in
most situations, but the farther a triangle is from a 1.0 aspect ratio, the more likely
it is that some part of the triangle is tilted towards the camera in such a way that its
center is behind another polygon but its frontmost point is in front of that same
polygon. When this is true, the renderer will fl icker by switching between the two
polygons, not sure which should be in front.
An ideal aspect ratio for rendering purposes is 1.0 (Fig. 9.8 ). This is not always
practical due to the shape of your model, but it should be kept in mind as an ideal to
stick as close to as possible without compromising the likeness of your model.

9.3.3 Bow Tie Faces

A bow-tie face is an n-sided face (usually a quad) that has been twisted so that its
normals, if triangulated, would be facing away from each other.
Bow-tie faces can be diffi cult to fi nd if they are quite small or are stretched into
an obscure cavity of your model. Normally, these are made accidentally, due to
improper vertex selection followed by a translate operation. Bow-tie faces are
always non-planar , so if your application has a non-planar highlight function, they
may be located by its use.