The background color is taken from color or col
if color is missing.
The first entry
is used for background clearing and as the fog color.
The second (if present) is used for background sphere geometry.
If color and col are both missing, the default is found in
the r3dDefaults$bg list, or "white" is used
if nothing is specified there.
If sphere is set to TRUE, an environmental
sphere enclosing the whole scene is drawn.
If not, but the material properties include a bitmap as a texture, the bitmap is drawn in the
background of the scene. (The bitmap colors modify the general color setting.)
If neither a sphere nor a bitmap background is drawn, the background is filled with a
solid color.
The fogScale parameter should be a positive value
to change the density of the fog in the plot. For
fogtype = "linear" it multiplies the density of the
fog; for the exponential fog types it multiplies the density
parameter used in the display.
See
the OpenGL 2.1 reference
for the formulas used in the fog calculations within R (though the
"exp2" formula appears to be wrong, at least on my
system). In WebGL displays,
the following rules are used. They appear to match the
rules used in R on my system.
For "linear" fog, the near clipping plane is
taken as \(c=0\), and the
far clipping plane is taken as \(c=1\). The
amount of fog is \(s * c\) clamped to a 0 to 1
range, where \(s = fogScale\).
For "exp" and "exp2" fog, the observer location
is negative at a distance depending on the field of view.
The formula for the distance is
$$c = [1-sin(theta)]/[1 + sin(theta)]$$
where \(theta = FOV/2\).
We calculate $$c' = d(1-c) + c$$
so \(c'\) runs from 0 at the observer to
1 at the far clipping plane.
For "exp" fog, the amount of fog is
\(1 - exp(-s * c')\).
For "exp2" fog, the amount of fog is
\(1 - exp[-(s * c')^2]\).