Am 17.06.2018 um 19:59 schrieb jr:

>> As for the `no_image` keyword, marking an object in this manner will (or
>> at least is intended to) cause it to /not/ "be there" - i.e. have /no
>> effect whatsoever/ - in the context of primary rays, while still "being
>> there" in the context of secondary rays (e.g shadow computations and
>> reflections).
> 
> what does "primary" and "secondary" rays mean, in the context?  that is, in a
> scene with only emissive media objects and zero light_source(s).

Primary rays are rays traced (backward) from the camera into the scene.

Seconary rays are rays traced (backward) from any "point of interest"
other than the camera. Typically, the point of interest is an
intersection of the primary ray with an object's surface, but it could
also be a point in a media container where scattering effects are computed.

Primary rays are traced to determine which object surfaces are "seen"
directly in the scene (including surfaces defined to have an invisible
texture), and what interiors (if any) lie between the camera and the
first opaque surface.


Secondary rays may be traced for different purposes:

- A "shadow ray" would be traced from an object's surface to a light
source, to compute whether that light source directly illuminates the
object or whether a shadow is cast on the surface by other object
surfaces or interiors; or from a (sample) point in scattering media to a
light source, to compute whether that light source directly illuminates
that region of space or whether the volumetric equivalent of a shadow is
cast on that region of space.

- A "reflection ray" would be traced from a (reflective) object's
surface in the direction given by the law of reflection, to determine
what objects and interiors are reflected by the surface.

- A bunch of "radiosity rays" would be traced from a (sample) point on
an object's surface in somewhat random directions to determine how much
indirect illumination the point receives from other objects (including
emissive media).

- A bunch of "subsurface rays" would be traced from a point slightly
below an object's surface in somewhat random directions to determine how
much light is transported through the bulk of the object to the
corresponding point on the surface.


In POV-Ray, "refraction rays" are also considered "primary rays" for
historical and technical reasons. These would be traced from a
(transparent or semi-transparent) object's surface in the direction
given by snell's law, to determine what other objects and interiors can
be seen "behind" the object.

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