Problems at Shared Vertices - Modeling and Rendering

Shading discontinuities can arise while two adjacent polygons fail to share a vertex which lies along their common edge. Identify the three polygons of figure 17, wherein vertex C is shared through the two polygons on the right, although not by the large polygon on the left. The shading information found directly at C for the polygons at the right will classically not be the same as the information interpolated at such point from the values at A and B for the polygon at the left.

Figure: Vertex C is shared by the two polygons on the right, but not by the larger Rectangular polygon on the left.

As an outcome, there will be a discontinuity in the shading. The discontinuity can be eliminated via inserting in the polygon on the left an extra vertex that shares C's shading information. We can pre-process a static polygonal database in sequence to eliminate this matter; conversely, if polygons will be split on the fly for example: through using the BSP-tree visible-surface algorithm, so extra bookkeeping can be finish to initiate a new vertex in an edge that shares an edge which is split.

Figure: Problems with computing vertex normal. Vertex normal are all parallel