OpenGL cards and multiple windows / WinNT

[John Biddiscombe]

Lisa,

>What do you mean by "the latest changes to the clipping planes in nightly
>releases"?  The clipping range changes were made a couple of months ago -
>is it something more recent that is causing you a problem?

I've had the problem for several months.


>Do you
>always see this problem? Even when looking at just a simple sphere with a
>radius of 1.0?

No, as per usual the fault is all mine. I'm viewing terrain data, the
terrain points are stored in UK ordnance survey coordinates which are
typically
{123442,812343} - 6 figure numbers. Now, because I'm only using terrain
points sampled at 10 metres resolution in x,y I do not feel the need to
switch to doubles, so in fact I'm using float data at the very edge of its
numerical accuracy. When zooming in on data, clearly the numerical accuracy
of the depth buffer calculation is not enough to display the fine change in
depth of polygons ranging over several kilometres in depth when the actual
coordinates are so large.

The thing is that there SHOULD be enough resolution in the zbuffer to give
(say) 30km/65537 = 0.4metre resolution in depth - assuming a 16bit zbuffer
if the clipping planes are set to the closest and furthest extents of the
dataset. Where I'm viewing a max of 30km of data (in fact a bit less)...

The thing is that although I should be getting 0.4 metre zbuffer
resolution, in reality I'm not and I suspect strongly that its because the
coordinate are 6 figure numbers and float numbers have only 7 digits of
precision or thereabouts. However, this shouldn't matter. The zbuffer
calculation should be independent of the absolute coordinates and only
matter about the difference. 

Anyway. I blamed diamond not vtk so apologies if I sounded like I thought
it was vtks fault.

John B

PS. I try to report as many bugs as I can find, but I already think I post
far too much to the list and if I'm not sure who to blame I keep my mouth
shut until someone else notices something! - even then I seem to get it
wrong most of the time.






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