[GRASSLIST:5370] NVIZ and vectors -- bug?

[Nick Cahill]

Dear all,

I posted a while ago about NVIZ making "spiky" vectors when draping 
over a surface. Hamish Bowman suggested that it might be simply because 
I'm draping a circle over an irregular surface, so that seen from the 
side it is distorted, but I don't think that's it. I remain baffled by 
this behavior, and wonder if someone might see what the problem is by 
looking at some images. I've posted what I'm trying to do at:

http://www.wisc.edu/arth/sardis/nviz.spiky.jpg

There should be no linear features on this -- only circles. The linear 
"spikes" radiating out from the circles are just not there in the data, 
but are a bug or something in nviz.

I think the issue has to do with resolution of the DEM vs. length of 
vector. I'm using a 90 meter DEM, draping over that a 30 meter 
resolution TM image, and then draping vectors which consist of 
"circles" -- i.e. 360 short lines in a circle. The mean diameter of the 
circles is about 43 meters, so the lines are much shorter than the cell 
size of the DEM. I'm setting the region resolution to 90 before running 
NVIZ. When I set the polygon resolution within NVIZ to 1 or 2 for a 
high-res image, the vectors get all spiked out. When I set the polygon 
resolution to something much higher, however, like 100 or 500, the 
vectors are drawn correctly as circles.

These are illustrated here:

polygon resolution 2: http://www.wisc.edu/arth/sardis/test.2res.jpg
polygon resolution 5: http://www.wisc.edu/arth/sardis/test.5res.jpg
polygon resolution 100: http://www.wisc.edu/arth/sardis/test.100res.jpg


My temporary solution was to draw the image twice, once with the dem 
and satellite overlay, and then a second time with the polygon 
resolution at 500 and drawing only the vectors, and then combining the 
two images in photoshop. This isn't a  perfect, solution, though, 
because the vectors drawn with the high polygon resolution are actually 
drawn in different places - apparently changing the polygon resolution 
changes the effective surface that the vectors are draped on, so they 
appear sometimes above and sometimes below where they were in the 
high-res rendition, so the circles end up wrongly placed. I overlaid 
two images made at 2 and 500 resolution to check out what was happening 
here -- the blue vectors are at 500, the red at 2

http://www.wisc.edu/arth/sardis/ vectors.2and500.jpg

This doesn't happen with all data -- I have another, 2 meter DEM of a 
smaller area, and draping even very detailed vectors over that causes 
no problem. So I'm bamboozled. Does anyone have any idea of how to 
achieve this?

I did try  creating a raster from the vector map, overlaying that over 
the TM image, and then simply draping that over the DEM. The results 
just aren't as clean, though, and if possible I'd like to do it with a 
vector overlay. However, if there's some unfixable bug in NVIZ or some 
factor I'm forgetting, I'll try it the other way.

I'm running GRASS 5.0 on Mac OSX.

many thanks,

Nick Cahill
Dept. of Art History
UW-Madison