Christian Walther <cwa### [at] gmxch> wrote:

> What you're seeing on the right side in the second scene is the back
> surface of the sphere (through the transparent front surface). Its
> normal points inward, and therefore the slope pattern value is between 0
> and 0.5. Try replacing your sphere { 0, 1 } by a difference { sphere {
> 0, 1 } plane { -z, 0 } }.
>
>   -Christian

Thanks.  I have to confess, in my wacky theorizing I never thought of the
normals on the INSIDE surface being used.  But it makes perfect sense.
Right now, I can't for the life of me think of a use for that particular
"feature"...but give me time!!!

And it didn't occur to me to simply "wipe out" the offending part of the
sphere. A perfect solution. HOWEVER... if I change the index values of my
color map to, say,

[0.0 color rgb <1,0,0>]
[0.7 color rgb <1,0,0>]
[0.8 color rgbt 1]
[1.0 color rgbt 1]

then the offending part can't be truncated--because along with the normals
on the OUTER surface applying red to the sphere about 70% of the way from
left to right, the INNER normals are covering the surface 70% of the way
from right to left!  So it looks like a solid sphere, no transparency
anywhere.

I wonder if this was forseen when the slope pattern was conceived or
written. A solution, off the top of my head, would be to eliminate the use
of normals on the inside of an object. Though that may leave others WANTING
that feature; or may have other unintended consequences.

Ken




Ken

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"Trevor G Quayle" <Tin### [at] hotmailcom> wrote:
> >

>
> I didn't mean this was an actual mistake in the docs.  The docs makes sense
> if you remember that they are using <0,-1,0>.  If you assume (as I did the
> first time I went through) that "up" is <0,1,0>, then the docs seem wrong.
> Hope I didn't confuse you any more than necessary...
>

Thanks.  What a relief! ;-)

Yeah, I had to reorient my own thinking about "slopes" when I first read the
slope pattern docs. My school math classes all those years ago drilled into
me that a slope of 0 was, so to speak,  "horizontal", and a slope of +1 was
45-deg. upward. (And a slope approaching vertical had a value approaching
infinity!) Yet it makes sense why POV treats "slope" differently, confining
its value between 0 and 1 to represent object normals varying between +1
and -1.  (You math folks, please correct me if I'm out of my depth here.) I
guess the only really "confusing" thing is that the slope pattern is called
that...though I can't think of a more appropriate term.

Ken

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"Trevor G Quayle" <Tin### [at] hotmailcom> wrote:

>
> Ah, figured something out.  You can use interior_texture, which applies a
> different texture to the interior of the object....

That's really brilliant!  It works as advertised. THANKS!

I'm attempting to use this trick to see if I can get my original, hoped-for
results. (So far, no luck.) But it's worth experimenting with.

Ken

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