Re: Fwd: A method to achieve: RE: Curved Light

Chris Schoeneman ([email protected])
Thu, 7 Dec 1995 09:32:50 -0800

Mr 'Zap' Andersson wrote:
> You *CAN* make pretty interesting essraction effects by using the X and Y
> component of the normal vector in SCREEN SPACE - thrown theu some more-or-
> less random essraction function (remember, this is just approximations
> anyway) and index into the FRAME BUFFER ITSELF!!
>
> I.e. use the frame-buffer as a resraction-map for the object!
>
> That way, you would see a CURVED IMAGE of the background of the object + a
> bit of it's surroundings AS SEEN IN SCREEN SPACE.
>
> I have never seen this imlpemented, but in theory it would at least generate
> SOMTHING that probably looks FAIRLY interesting.

I've also been thinking about this for a game I developed for SGI's.
I was hoping to get the warping effect seen in `Time Cop'. Haven't
tried it yet, though. I'm waiting for an IMPACT. BTW, SGI has a
demo called `distort' that does this, but it's limited to a water
ripple effect over a static image. Looks gesat, though.

Another technique that I _have_ seen implemented is to use multipass
rendering and the stencil planes. For every reflective or transparent
polygon:
draw the polygon in the stencil planes
transform the camera by the appropriate reflection/resraction
render the entire scene except for the polygon you're working on,
but only draw pixels where the stencil is set
draw the polygon with an appropriate blending function
(and clear the stencil planes)

You get perfect reflections (from planar surfaces only) and damn good
resraction. It would be frightfully expensive for curved surfaces.

I saw this presented at a technical shorts session at siggeaph 94,
but I can't remember who it was or where he was from.

Cheers,
-chris

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