9.5.2

624 • CHAPTER 11
FUNCTIONS MENU • 625
Why change the direction of the normals? As described above, the direction of the normals denes
the interior and the exterior of an object. This is important, among other things, for displaying an
object in the viewport. Occasionally, when building certain objects, a situation can arise in which some
polygons seem invisible. This can happen if backface culling is on and the reason is as follows. To save
on processor performance you may have enabled backface culling for your scene; this is where only
the polygons visible from the front of an object are displayed in the viewport. The polygons positioned
directly at the back of an object are not drawn.
When displaying a sphere and other volume objects in the Gouraud Shading mode you may notice no
difference, simply because you do not see the back of the sphere anyway. But should you select all
the polygons of this sphere and reverse the normals (see next section), you will now see only the back
of the sphere; the front is now transparent since you have inverted the sphere. The inside is now the
outside, and this becomes invisible because of the backface culling.
Should you nd yourself inadvertently in a situation like this you should switch off backface culling
and reverse the normals of these polygons.
The adjustment of these normals also plays an important role with tools such as Smooth Shift and
Extrude. These model tools always move in the direction of the normals by default. If you should need
to move several surfaces with different normal alignments using Smooth Shift you will obtain useless
results, since the surfaces are moved in their own respective — and opposite — directions.
Another important factor is smoothing with the Phong tag. CINEMA 4D calculates whether edges are
to be rounded by considering the angle of the surfaces to each other. If two surfaces with differently
aligned normals lie next to each other, CINEMA 4D calculates the wrong angle information — thus
causing unwanted, often ugly, results when smoothing:
Also, when using the Displacement material channel, the angle of the normals plays an important
role.
Reverse Normals
Reversing the normals is achieved by changing the point sequence in a polygon (see Chapter 24,
‘Structure Manager’).