The Merge SOP applies all incoming attributes to all input geometry. Each input geometry may have its own set of attributes.
Three spheres are wired into a Merge SOP. The first has no attributes applied. The second has a color attribute (
Cd) applied by a Point SOP. The third has a normal attribute (
N) applied by another Point SOP.
The Merge SOP does NOT know how to build attributes, but can apply them. As a result, all applied attribute values are set to zero.
This is why the first two spheres display and render black. They have normal attributes applied, but their values are set to zero.
In addition, the first and last spheres have a color attribute applied, but their values are set to zero.
It is better to set attributes explicitly, instead of relying on the Merge SOP to do so.