Steve Baker (steve++at++mred.bgm.link.com)
Fri, 23 Aug 96 09:37:59 -0500
> Brian Cabral rhowed me a real-time bump mapping implementation
> a year ago running on RE2, this was also explained on a recent
> SGI graphics course. He's now published a paper on a more
> general shading approach which I haven't seen this).
OK, I think I follow this explanation - but it's a typical
'rigged demo' approach. You end up having people believe that
the machine can do something that is never going to work in
most 'real' applications.
The approach suggested appears to need three rendering passes
(two for the texture and one for the final object?)
My favorite use for bump mapping is super-realistic oceans
scenes - with sunlight glittering off the water - and fan-shaped
sun reflections as the sun sets over the horizon. VERY COOL!
Proper bump mapping can do this with a single polygon and a
teeny-tiny bump map with some interfering sinusoids.
The problem in using the approach that Angus describes (if I
understand it - which I may not) is that my bump map is maybe
a 128x128 map, but it is repeated hundreds of times in each
direction in order to texture the water from horizon to horizon.
Hence when I peel the skin off my ocean - I need a 12800x12800
texture map - rendered twice and then pasted back onto my ocean.
Hardly a realtime solution.
Some of the really cool SGI demos really piss me off because they
lead people to believe that you have a usable technique when in
fact, you don't.
A classic (and very costly) example of this was a project that
my company has been working on for several years. It was a fairly
conventional RE2 flight simulator. Before we sold the system to the
customer, we did many demos of the system flying around some
nice looking terrain, during the day - and at night. But we didn't
show a night time landing. During night time landings, the pilot
turns on his landing lights - which just like car headlights,
illuminate a little puddle of light ahead of the aircraft.
So, can the RE2 do proper landing lights? Well, there is
a really cool demo on the RE2 of a room being illuminated with
a really beautiful spotlight. YOu can even change the spotlight to
look as though its shining through a photographic texture.
Well, that certainly looked (to our customer) like the machine
had the ability to do that - so he purchased a large number
of these machines.
Of course, if you look at the spotlight demo, it's doing lots of
special-case stuff, multi-pass rendering, etc, etc. SOme of the
techniques it uses are incompatable with other features of the
machine - some of the coplanarity kludges don't work with it, etc.
There was no way we could deliver a reasonable frame rate on a
complex database with a general purpose scene using that technique
without nearly doubling the amount of hardware required.
So - can the iR do bump mapping? The answer is emphatically *NO*
it can't - not at the same time as coming even close to
the pixel and polygon rates that the spec sheet say.
So unless you are building a rigged demo to try to sucker
some unfortunate customer into buying something that won't do
what he wants - don't bother with bump mapping.
Steve Baker 817-323-1361 (Vox-Lab)
Hughes Training Inc. 817-695-8776 (Vox-Office/vMail)
2200 Arlington Downs Road 817-695-4028 (Fax)
Arlington, Texas. TX 76005-6171 steve++at++mred.bgm.link.com (eMail)
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