Because a surface generally appears with a known set of other surfaces, its proportion of the total visible surface area is another constraint on its identity in the complete object context. This proportion can vary because of self-occlusion, but is otherwise a constant. The precise relative area is, in theory, determinable for all viewing positions, but in practice only the range defined by representative positions is considered.

The relative surface area calculation is trivial once
the individual component's absolute areas have been calculated.
The surface cluster (Chapter 5) is the context for the relative area
calculation.

Table 6.11 summarizes the results of the relative surface area
calculation for the same image regions as in Table 6.8.
Again, the same good performance is noted.
A point to note about the relative area is that valid evidence
can still be computed even if only the relative distances (as compared to the
absolute distances) to the object's surfaces are available.
This point also holds for objects with fixed geometry, but variable size:
the relative proportion of the total size remains the same.

**Final Comments**

This chapter showed that surface data allowed a variety of general identity-independent three dimensional properties. These were directly computable for curves, surfaces and surface clusters, using simple computations that estimated the properties acceptably. More work is needed to extend the number of properties, improve the algorithms and determine their stability over a wider range of objects.