On Jun 5, 2012, at 5:20 PM, Tom Browder wrote:
> Okay, my terminology stinks but that's approximately the way I've been
> interpreting the data (with the big exception that I thought one cell
> was 2x2 corners so I can adjust to a point being a cell.
Technically 2x2x2=8 corners. An individual cell value literally represents a
simple arb8-style volume. Without smooth interpolation, they are a bunch of
rpp boxes. With smooth interpolation, the "top face" is adjusted to smooth the
transition with neighboring faces (by moving the corners and splitting the top
face in half (making two triangles))
> 6 hex chars per line, 726 lines, for a total of 4356 hex chars
>
> Taking 2 hex chars as one point (cell) I get a total of 2178 cells.
It's 4 hex chars per cell. *16-bit* unsigned. 2 hex chars make one byte
(0->255). 4 hex chars make two bytes (0-65536). terrain data usually needs
more resolution, so it's a common compromise while still being blazing fast to
process. My long-term dream is for DSP to ditch the misleading name (digital
signal processing objects??), return to height field nomenclature, merge with
ebm, and be more like the 'pnts' and 'binunif' primitives supporting data in
various formats (1bit, 8-bit, 16-bit, 32-bit, etc).
> The solids.mged file builds a dsp primitive from the pix version of
> that file and declares a 33x33 map for a total of 1089.
4356 / 4 = 1089
> So what happens to the excess?
Dust bunnies eat them.
> What is the "cut" direction (ad|Lr|lR)?
That relates to smooth interpolation from cell to cell. When the corners are
adjusted to smooth the surface, you end up with non-planar surfaces. So it's
asking how to split those faces:
automatically ('ad'aptively), upper-left to lower-right (Lr "\"), or
lower-left to upper-right (lR "/")
We could and should probably add a brep/nurbs option now that we can deal with
non-planar surfaces. I'm not sure i've ever seen any long-term value in having
the Lr/lR options.
> Please point me in the right direction if this is all described somewhere.
It's not. I think it was once documented in some now-long-lost presentation
materials, but it's not in the repository nor on the wiki. I've talked about
it some in the past here:
https://sourceforge.net/projects/brlcad/forums/forum/362509/topic/3717640
https://sourceforge.net/tracker/?func=detail&atid=640803&aid=3004751&group_id=105292
I can whip up an example if you get stuck, but it really is *very* similar to
the ebm, just with a different data file and a few more smoothing options.
Cheers!
Sean
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