RE: [Algorithms] Current state of shadow maps?

[Tom F. <tom...@ee...>]

I've updated the description of the algorithm and included some =
pictures.
Hopefully it's a bit clearer, but this stuff can be tough to explain. =
The
odd toroidal topology of StarTopia doesn't help :-)

http://www.eelpi.gotdns.org/papers/shadowbuffer_pseudocode.html

> > The army-with-lots-of-short-range-torches example is an=20
> interesting one for
> > shadowbuffers.
>=20
> Just include it in your demo *g*

I did this - just hacked in a light floating above every person's head. =
It
works pretty well. It's slow, but not absurdly slow, considering what =
it's
doing! I'll try to take some pics some time - it looks pretty goofy.

You're right that to get "perfect" precision you need to render twice as
many shadowbuffer texels as pixels, but in practice you need a lot less =
than
this, even with the horrible alpha-test shadows I'm using here (I find =
that
half as many texels as pixels works well). With PCF, you can drop it a =
bit
more, and if you put in soft-edged shadows with something like Smoothies =
or
Willem's smooth-shadows method (http://www.whdeboer.com/writings.html), =
then
you need even fewer texels.

TomF.


> From: Christian Sch=FCler
>=20
> > Creating a lot of frustrums, but not necessarily 1 per reciever per
> > light - it's very likely you could merge quite a few of those
> > frustrums together, given an army is usually walking in close
> > formation
>=20
> That'd be lossy compression then ... but this opportunity to=20
> short-cut is not restricted to shadowbuffers, is applies to=20
> stencil too (Each unique "frustum" translates to an extrusion=20
> center). Besides I can see the danger of popping if the=20
> merger is inconsitent between frames.
>=20
> > The army-with-lots-of-short-range-torches example is an=20
> interesting one for
> > shadowbuffers.
>=20
> Just include it in your demo *g*
>=20
>=20
> > Here's some really really rough back-of-the-envelope=20
> figures to compare the
> > two. Warning - lots of assumptions ahead!
>=20
> I don't want to start a war. I just would not equate the=20
> overall performance to the # of Z reads/writes.
> I have experience with the "army of torches" scenario with=20
> stencils, and you can get decent performance if the average=20
> screen space area was just small enough.=20
> So there is little cost associated "per light" and large=20
> costs for "screen space covered" and "vertices touched". In=20
> the dynamic environment where all the recievers / casters=20
> were moving, guess the limiting factor for the CPU work was=20
> (for me) ---> the scene database queries to just get the=20
> objects for each light! With shadow buffers I can see=20
> shifting the cost more towards per light while per pixel and=20
> per vertex costs may be smaller, with added penalties of=20
> constant costs, like this:
>=20
>=20
> stencil:
> n lights =3D n passes=20
> where n being the # of scene database queries=20
>=20
> shadow buffers
> n lights =3D 2 * n passes (minimum) + n / c * ( render target=20
> switches + stall penalty for leaving the framebuffer / coming=20
> back to the framebuffer, etc etc)
> where c being how much buffers you can pack into a shadowbuffer atlas
>=20
>=20
> My experience also says that in order to over a 100^2 pixel=20
> screen area, you need a 200^2 shadow buffer, because on=20
> average the projected texels are stretched out due to the=20
> light hitting at grazing angles. A 1024'er screen would need=20
> a 2048'er shadow map. But that's a minor issue.=20
>
>=20
> -----Original Message-----
> From: gda...@li...=20
> [mailto:gda...@li...] On=20
> Behalf Of Tom Forsyth
> Sent: Friday, September 09, 2005 7:38 AM
> To: gda...@li...
> Subject: RE: [Algorithms] Current state of shadow maps?
>=20
>=20
> Interestingly:
>=20
> >Stencil volumes win the indoor/urban, night scenarios=20
> >(think doom3, or neverwinter nights for the record)
> >- shadows from vegetation can be neglected.
> >- many omnidirectional light sources, or lightsources with
> > large frustra, for which shadowbuffer is unoptimal (too many render
> targets)
> >- most light sources have small screen space extent=20
> >and world extent, so stencil is not expensive
>=20
> ...actually describes your average StarTopia scene moderately well :-)
> http://www.eelpi.gotdns.org/startopia/startopia_pictures.html
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> (yes, I will get the demo version done soon, I promise!)
>=20
>=20
>=20
> The army-with-lots-of-short-range-torches example is an=20
> interesting one for
> shadowbuffers. When the range of a light is small compared to the view
> frustum (as will be the case with >90% of the torches), then=20
> my scheme will
> just reduce to essentially a cube map per light. Actually, it=20
> gets slightly
> better - if there's nothing above the torch in range of it=20
> (likely), then
> that face never gets created, and also the face view angles=20
> can be opened up
> to about 120 degrees and still remain efficient - this=20
> typically means you
> lose another face and only need four frustums per light=20
> rather a cube-map's
> six.
>=20
>=20
> Here's some really really rough back-of-the-envelope figures=20
> to compare the
> two. Warning - lots of assumptions ahead!
>=20
> Assume the shadowbuffers are the type that only write to a Z/stencil
> surface, not a colour buffer as well. Remember that my scheme=20
> allocates
> shadowbuffer texels so that you get 1 texel per screen pixel=20
> for the area it
> covers, if you turn the detail to "max", i.e. pixel-perfect.
>=20
> Let's also assume that each light's radius sphere covers 10k pixels (a
> 100x100 pixel area - not unreasonable). Also approximate the=20
> shadowbuffer
> coverage - in practice many pixels in that area won't have=20
> receivers, and
> many others will have multiple receivers. Let's call it even=20
> for the sake of
> argument. Also assume that in any rendering pass, all the=20
> pixels get tested,
> and half get rejected because of overdraw (an entire scene=20
> will have more
> overdraw, but my experience is that shadowbuffer/volume=20
> shadows, because of
> their limited range, get lower overdraw, and 2x is reasonable).
>=20
> Shadowbuffers:
>=20
> Per light, rendering shadowbuffers: 10,000 Z tests + 5,000 Z=20
> writes =3D 15k
> reads/writes.
>=20
> Per light, rendering actual scene: 10,000 shadowbuffer reads.
>=20
> Total =3D 25k reads/writes.
>=20
>=20
> Volume shadows:
>=20
> Per light, rendering volumes (remembering that volumes have=20
> two sides!):
> 2*10,000 Z tests + 2*5,000 Z writes =3D 30k reads/writes.
>=20
> Per light, rendering actual scene: the stencil tests come=20
> free with the Z
> reads. No extra cost.
>=20
> Total =3D 30k reads/writes.
>=20
>=20
> So in terms of fillrate, it's pretty close - shadowbuffering=20
> slightly ahead,
> but I made a lot of assumptions. But shadowbuffering has some=20
> big aces up
> its sleeve:
>=20
> The first is that I said the quality slider was on "best" -=20
> one texel per
> screen pixel. But you can turn that down - you can easily=20
> halve it without
> any quality loss. In fact, if you have a soft-edged shadow shader, you
> _want_ to turn it down lots! So that dramatically reduces the fillrate
> required for shadowbuffers.
>=20
> The second is that you can render a single receiver with multiple
> shadowbuffers in one pass - because you're just sampling a=20
> texture and doing
> a comparison. So you can do more than one of these per=20
> shader. Let's say you
> can do two - that's totally realistic for PS2.0 hardware. So=20
> you've now
> halved the number of passes you do when rendering the scene=20
> (I didn't list
> those reads/writes in the above). This can't be done with=20
> volume shadows
> (that I know of) - it can only reject the pixel or accept it, it can't
> half-shade it. That's a huge win!
>=20
>=20
> Also, the process of extruding volume shadows is far more=20
> expensive than the
> equivalent shadowbuffer thing, which is just rendering the=20
> object from a
> different POV. I believe most people using VS-driven=20
> extrusion find that
> they are frequently limited by triangle throughput rather=20
> than fillrate. And
> people using CPU-driven extrusion wish they were doing=20
> VS-driven extrusion
> :-)
>=20
>=20
>=20
> TomF.
>=20
>=20
>=20
> > -----Original Message-----
> > From: gda...@li...=20
> > [mailto:gda...@li...] On=20
> > Behalf Of Megan Fox
> > Sent: 08 September 2005 13:07
> > To: gda...@li...
> > Subject: Re: [Algorithms] Current state of shadow maps?
> >=20
> >=20
> > Well, let's take the army with torches but apply stencil shadows
> > instead (and let's say they're on a field of battle, a heightmap) -
> > how is that still not a nightmare scenario?
> >=20
> > With shadow buffers (using Tom's method), you'd end up:
> >=20
> > - Creating a lot of frustrums, but not necessarily 1 per=20
> reciever per
> > light - it's very likely you could merge quite a few of those
> > frustrums together, given an army is usually walking in close
> > formation
> >=20
> > With stencil, you'd end up:
> >=20
> > - Casting your extrusions back for every light/occluder pair.  You
> > can't really merge (I don't think?), so that's "it."
> >=20
> >=20
> > Especially after using Tom's handy-dandy frustum=20
> merge-o-matic method,
> > it seems like the two methods would be comperable - mind, both would
> > probably keel over and die in a slurry of render passes (and in both
> > cases, you'd probably enable your "oh god we're in trouble start
> > merging nearby lights into single lights" optimization code), but it
> > seems like neither does terribly well.
> >=20
> >=20
> > I'd thought the "big" win scenario for stencil over buffers was more
> > scenes with few occluders and many recievers (that is, your average
> > FPS environment)?
> >=20
> > > Stencil volumes win the indoor/urban, night scenarios=20
> > (think doom3, or neverwinter nights for the record)
> > > - shadows from vegetation can be neglected.
> > > - many omnidirectional light sources, or lightsources with=20
> > large frustra, for which shadowbuffer is unoptimal (too many=20
> > render targets)
> > > - most light sources have small screen space extent and=20
> > world extent, so stencil is not expensive
> > >=20
> > >=20
> > > However shadowbuffers have other qualities that make them=20
> > attractive (image based, soft edges), so it would be=20
> > desireable to use them for all purposes. It's just a pity=20
> > that they are so unfeasible for omni lights (I imagine an=20
> > army with torches here ...).
> >=20
> >=20
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> September 19-22, 2005 * San Francisco, CA * Development=20
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> SF.Net email is Sponsored by the Better Software Conference & EXPO
> September 19-22, 2005 * San Francisco, CA * Development=20
> Lifecycle Practices
> Agile & Plan-Driven Development * Managing Projects & Teams *=20
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> Security * Process Improvement & Measurement *=20
> http://www.sqe.com/bsce5sf
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