>   Could you profundize on this? What exactly is the difference between
> scanline rendering and raytracing which makes the former better for
> rendering a huge amount of objects?

This was answered well in Catmull et al's original
paper on REYES in SIGGRAPH 87, but I'll explain.

In a raytracer, all of the scene's geometry must
be retained in memory because secondary rays due
to reflection, refraction, shadows, etc. could
be aimed anywhere, thus random access to the geometry
database must be possible. Since memory is
getting more plentiful and cheaper, this is
not as big a deal as it was before, but for
scenes that approach reality, the hardware
is not quite there yet. The memory requirement
is also not a simple aggregate of the scene
objects; an octree or other bounding hierarchy
is needed to make searches take practical time,
and this adds considerable storage overhead.

In a scanline renderer, each object is considered
only once, so an object only exists in memory
while being drawn. POV-Ray is actually well-named;
it is indeed a "persistence" of vision. :)

There is also the "many textures" problem -- if you
want to use lots of different textures, they
must, like scene geometry, be preloaded in
a raytracer, because textures will be arbitrarily
accessed. Thus you get a lot of trashing as
textures are paged in and out. In a scanline
renderer, an object's texture is loaded and
used for all of the drawing of that object.
The artist can also arrange objects to be
provided in "texture order" so that each
texture is loaded only once.

There's also the accelerated video possiblity,
especially now that 3D cards are getting
so fast and cheap. In raytracing, one usually
adds more computers in a cluster or farm
(although this drives up one's electricity bill).

Ray

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