Section 2.4.3.1 Plane says:

"This is followed by a float value that gives the distance along the normal
that the plane is from the origin (that is only true if the normal vector
has unit length; see below)."

The comment in parentheses is wrong and should be deleted.  It doesn't
matter if the normal vector has non-unit length.  The float value is always
the distance that the plane is from the origin.

Here is a simple test example:

camera {
    location <0, 1, -5>
    look_at <0, 0, 0>
}

light_source {
    <-100, 100, -100>
    color rgb <1, 1, 1>
}

sphere {
    <0, 0, 0>, 0.1
    pigment { color rgb <1, 0, 0> }
}

sphere {
    <0, -1, 0>, 0.1
    pigment { color rgb <1, 0, 0> }
}

sphere {
    <sqrt(0.5), -sqrt(0.5), 0>, 0.1
    pigment { color rgb <1, 0, 0> }
}

plane {
    <0, 5, 0>, -1
    pigment { color rgb <1, 1, 1> }
}

plane {
    <-10, 10, 0>, -1
    pigment { color rgb <1, 1, 1> }
}

Lloyd Lee-Lim

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