From: Angus Dorbie (dorbie++at++sgi.com)
Date: 10/27/2000 09:43:48
ihawkes2++at++csc.com.au wrote:
>
> Thanks for all the replies on this topic. We have now confirmed that the
> problem is definitely wobble rather than jello (we added a high-res inset
> around the texture coord origin & observed that the wobble was not apparent
> in this region). Reducing the number of effective levels all the way back
> to 7 does not help.
Based on what your observations I agree this is probably 'wobble'
related.
>
> To clarify the solution, is the following what you are suggesting we do? ::
> 1) Divide the geometry into a grid of N by N regions, where N is chosen
> such that there is no wobble observed within the region closest to the tex
> coord origin with no (s,t) offset applied.
>
> 2) For each region, pre-process the texture coordinates (presumably using
> double precision calculations) so that the appropriate offset is applied to
> each region.
> eg if regions are labelled (1,1) to (N,N), then for region (X,Y) the offset
> for s and t would be (X-1)/N and (Y-1)/N respectively.
Yes, the offset is a subtraction from the coordinates which must be
undone later.
>
> 3) At runtime, apply the appropriate clipcentre offset, depending on which
> region we are currently in.
Yes, you also adjust the texture matrix being applied per tile to shift
the smaller coordinate tiles into clipcentre space for most of the other
tiles. I suppose specifying a different clipcentre per tile would also
do it but would probably break under certain transformations.
>
> If this is correct, then presumably this means that we will get odd effects
> when flying over region boundaries? The application we are developing is a
> helicopter simulator and our cliptexture covers the whole of Australia. We
> really want to be able to fly between any two points without any region
> boundary effects.
No strange effects, when you change clip centre you change the texture
matrices, this is an important part of the algorithm.
>
> The cliptexture has 26 levels with high-res insets at 15cm resolution,
> which means that we have only 2.0e-7 tex-coords per metre of geometry.
> Does this mean we will need a lot of regions?
How many regions do you need based on empirical observation see stage 1)
above? I have no set numbers for when wobble is a factor it's fairly
complex and has a subjective component too.
>
> I guess the alternative is to adjust the tex coords at runtime - but
> without using a texture matrix this is a daunting prospect!
See above :-)
>
> Regards,
> Ian Hawkes
-- For Performer+OpenGL tutorials http://www.dorbie.com/"In the middle of difficulty lies opportunity." --Albert Einstein
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