[Public WebGL] determining depth of z-buffer

Steve Baker [email protected]
Wed Feb 2 17:38:53 PST 2011

Oopsie!  That should have been 10x bigger and 10x further...I'm getting
confused...too many simulators, too many years and far, FAR too many
"dirty tricks"!

On 02/02/2011 07:24 PM, Steve Baker wrote:
> Well, you just have to know when this kind of thing can't happen.  It's
> definitely not perfect for all applications - but in the case of
> spacecraft and distant planets - or a Predator flying at 30,000' and
> looking at far off terrain - you know that you can make the object some
> number of times larger without it interfering.
> Another simulator we built was a manipulator arm simulator for the space
> station.   The ISS flies at an altitude of around 150km - and it's about
> 100m across.  So you could comfortably draw it 100 times too big (1km)
> and move it 100 times further from the camera.  That gives you a 100
> times bigger near-clip plane.   Alternatively (and equivalently) you
> could leave the near clip plane alone and just draw the Earth 100 times
> smaller and bring it 100 times closer to where the Z precision is better.
> We had to be careful when we wanted to draw a 100x too-big shuttle
> coming in to dock because when it's 1.5km away and below the ISS, it
> would end up buried into the Earth and giving away the whole trick...but
> so long as you are moderately in control of the situation, it can be
> done.  We just scaled the shuttle smoothly from 1x size and "true"
> distance to 100x size and 100x distance as it approached close enough to
> the ISS to matter.
> Where these tricks get messy is when you want to do things like shadow
> casting...because the Z buffer contents are essentially garbage, you
> can't do many of the things you'd like to do with it.
>   -- Steve
> On 02/02/2011 06:38 PM, Giles Thomas wrote:
>> Steve,
>> Thanks for the naming and the critique of depth-strip[ep]ing! 
>> The trick of drawing objects up close as being larger but further away
>> sounds neat, but how do you stop them from being obscured by objects
>> that are further away than the objects' real depth but closer than the
>> depth at which you draw them?  I can imagine that it might not be an
>> issue with the Predator specifically (because most things are either
>> very far away from it -- targets -- or very close -- the drone itself)
>> but could it work in a more general case?
>> Also:
>> On 3 February 2011 00:20, Steve Baker <[email protected]
>> <mailto:[email protected]>> wrote:
>>     The alternative is just to turn off Z-testing and Z-writing and
>>     draw the
>>     sky before anything else on the screen.
>> That sounds way better than my idea of clearing the Z-buffer, but if
>> you're drawing the skysphere first, why do you switch off Z-testing?
>> Cheers,
>> Giles
>> -- 
>> Giles Thomas
>> [email protected] <mailto:[email protected]>
>> http://www.gilesthomas.com/
>> http://projectdirigible.com/
>> http://learningwebgl.com/
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