Alain <ele### [at] netscapenet> wrote:
> JeffBTX nous illumina en ce 2009-05-29 21:55 -->
> > I am using POVRay version 3.6.1.msvc8.win64, OS is WinVista Home Premium 64 bit.
> >
> > I am getting back into POVRay after a LONG hiatus. Its been maybe ... 4 or 5
> > years at least since I last touched POVRay.
> >
> > Yesterday I started experimenting with .DF3 files. I've had SOME experience with
> > them under POVRay 3.5 (but I think I fully understand them). I've used Pod's
> > tga2df3 utility, and also experimented with using a program that I wrote to
> > *create* blank DF3s (just the header, plus filled with zeros). I've
> > experimented and figured out how to calculate the offset into a DF3 given x,y,z
> > coords. My results arwe what I expect, IE I'm not doing anything wrong, voxels
> > are where they are supposed to be.
> >
> > I've noticed something odd. I THINK I know what is happening, but could use
> > input from a "Guru".
> >
> > Assume you have a "blank" DF3 (just the header, plus filled with zeros). Assume
> > an 8 bit DF3, of say, dimensions 10x10x10 or 100x100x100 (because that is what
> > I've been experimenting with so far). Now... "activate" a *corner* of the DF3
> > by filling it (with, say, a density value of 255). The offset (for a 10x10x10)
> > (assuming zero-base coords of 0-9) would be x + (y*10) + (z*10*10) + 6. Or
> > substitute 100 for 10, for a 100x100x100 DF3. Okay... if you fill x,y,z = 0
> > with 255, you have filled the lower left forward corner of the cube.
> >
> > If you use interpolate 0 with the DF3, the result is exactly as expected,
> > nothing strange... just a blocky "pixellated" voxel.
> >
> > If you use interpolate 1 with the DF3, there are "artifacts" at other corners
> > and occasionally in other unexpected places (even in the center (?) of the cube
> > sometimes? huh?).
> >
> > If you use interpolate 2 with the DF3, the artifacting becomes stranger and
> > extremely "wacky".
> >
> > Okay, I guess the interpolation is ... "averaging" (kind of like antialiasing)
> > the region around that corner, and is unexpectedly "wrapping around" to the
> > other corners? And why are artifacts showing uop in odd places, like the middle
> > of the cube?
> >
> > Okay ... so I say to myself, no problem, just make sure that you stay at least
> > one voxel away from all corners, edges and faces to avoid this "wrap-around" /
> > "bleeding" effect.
> >
> > No good - if you are one voxel away from the corner, using interpolate 2, there
> > is still some "bleeding". Probably there is a "magic number" (number of voxels)
> > as a "safety zone"... before I try to find it by experimenting, I thought I
> > would ask for input, and maybe an explanation of what is happening.
> >
> > -Thanks
> > -jeff
> >
>
> Interpolate 2 have problems when the density reatch zero, it seems that it tries
>   to create some negative values that wrap around to be large positive values.
>
> The common solution is to not reatch zero. For your "empty" DF3, it can mean
> filling it with 1's instead of 0's.
>
> Alain

Thanks, Alain - I will experiment with that today.

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