Re: [Algorithms] Current state of shadow maps?

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I wonder - have you seen this? -
http://www.gamedev.net/community/forums/topic.asp?topic_id=3D346786

Seems this approach might be applied to your problem with great success.

On 10/4/05, Tom Forsyth <tom...@ee...> wrote:
> Yep - that's the tricky part. One method is to partition the large object=
s
> with user clip planes. Another is to simply decimate them into smaller an=
d
> smaller chunks of triangles as needed (rather expensive, but it should on=
ly
> happen to a few objects - hopefully!). Another is to use cube-map
> shadowbuffers, if hardware is available. But I haven't actually tried any=
 of
> these yet :-) There are some good papers on cube-map shadowbuffers around
> though - I recall one from some nVidians.
>
> TomF.
>
> -----Original Message-----
> From: gda...@li...
> [mailto:gda...@li...] On Behalf Of David
> Whatley
> Sent: 03 October 2005 23:30
> To: gda...@li...
> Subject: Re: [Algorithms] Current state of shadow maps?
>
>
> Tom,
>
> That stuff is great!  Just wish it didn't have the one limitation that ma=
kes
> it not work for me... "The only places that do not currently work well ar=
e
> large objects with lights close to or inside them."  Any dude carrying a
> torch on my terrain would describe that... the terrain is a "large object=
"
> in that sense.  Ah well.  But stencil shadows are looking great so far!  =
I
> hope to combine them with some form of what you are doing for a nice hybr=
id
> shadowing approach.
>
> -- David
>
>
> Tom Forsyth wrote:
> 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
>
> interesting one for
>
> shadowbuffers.
>
> Just include it in your demo *g*
>
>
> I did this - just hacked in a light floating above every person's head. I=
t
> 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 t=
han
> this, even with the horrible alpha-test shadows I'm using here (I find th=
at
> half as many texels as pixels works well). With PCF, you can drop it a bi=
t
> 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), t=
hen
> you need even fewer texels.
>
> TomF.
>
>
>
> From: Christian Sch=FCler
>
>
> 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
>
> That'd be lossy compression then ... but this opportunity to
> short-cut is not restricted to shadowbuffers, is applies to
> stencil too (Each unique "frustum" translates to an extrusion
> center). Besides I can see the danger of popping if the
> merger is inconsitent between frames.
>
>
> The army-with-lots-of-short-range-torches example is an
>
> interesting one for
>
> shadowbuffers.
>
> Just include it in your demo *g*
>
>
>
> Here's some really really rough back-of-the-envelope
>
> figures to compare the
>
> two. Warning - lots of assumptions ahead!
>
> I don't want to start a war. I just would not equate the
> overall performance to the # of Z reads/writes.
> I have experience with the "army of torches" scenario with
> stencils, and you can get decent performance if the average
> screen space area was just small enough.
> So there is little cost associated "per light" and large
> costs for "screen space covered" and "vertices touched". In
> the dynamic environment where all the recievers / casters
> were moving, guess the limiting factor for the CPU work was
> (for me) ---> the scene database queries to just get the
> objects for each light! With shadow buffers I can see
> shifting the cost more towards per light while per pixel and
> per vertex costs may be smaller, with added penalties of
> constant costs, like this:
>
>
> stencil:
> n lights =3D n passes
> where n being the # of scene database queries
>
> shadow buffers
> n lights =3D 2 * n passes (minimum) + n / c * ( render target
> switches + stall penalty for leaving the framebuffer / coming
> back to the framebuffer, etc etc)
> where c being how much buffers you can pack into a shadowbuffer atlas
>
>
> My experience also says that in order to over a 100^2 pixel
> screen area, you need a 200^2 shadow buffer, because on
> average the projected texels are stretched out due to the
> light hitting at grazing angles. A 1024'er screen would need
> a 2048'er shadow map. But that's a minor issue.
>
>
> -----Original Message-----
> From: gda...@li...
> [mailto:gda...@li...] On
> Behalf Of Tom Forsyth
> Sent: Friday, September 09, 2005 7:38 AM
> To: gda...@li...
> Subject: RE: [Algorithms] Current state of shadow maps?
>
>
> Interestingly:
>
>
> Stencil volumes win the indoor/urban, night scenarios
> (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
> and world extent, so stencil is not expensive
>
> ...actually describes your average StarTopia scene moderately well :-)
> http://www.eelpi.gotdns.org/startopia/startopia_pictures.html
>
> (yes, I will get the demo version done soon, I promise!)
>
>
>
> The army-with-lots-of-short-range-torches example is an
> 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
> my scheme will
> just reduce to essentially a cube map per light. Actually, it
> gets slightly
> better - if there's nothing above the torch in range of it
> (likely), then
> that face never gets created, and also the face view angles
> can be opened up
> to about 120 degrees and still remain efficient - this
> typically means you
> lose another face and only need four frustums per light
> rather a cube-map's
> six.
>
>
> Here's some really really rough back-of-the-envelope figures
> to compare the
> two. Warning - lots of assumptions ahead!
>
> Assume the shadowbuffers are the type that only write to a Z/stencil
> surface, not a colour buffer as well. Remember that my scheme
> allocates
> shadowbuffer texels so that you get 1 texel per screen pixel
> for the area it
> covers, if you turn the detail to "max", i.e. pixel-perfect.
>
> Let's also assume that each light's radius sphere covers 10k pixels (a
> 100x100 pixel area - not unreasonable). Also approximate the
> shadowbuffer
> coverage - in practice many pixels in that area won't have
> receivers, and
> many others will have multiple receivers. Let's call it even
> for the sake of
> argument. Also assume that in any rendering pass, all the
> pixels get tested,
> and half get rejected because of overdraw (an entire scene
> will have more
> overdraw, but my experience is that shadowbuffer/volume
> shadows, because of
> their limited range, get lower overdraw, and 2x is reasonable).
>
> Shadowbuffers:
>
> Per light, rendering shadowbuffers: 10,000 Z tests + 5,000 Z
> writes =3D 15k
> reads/writes.
>
> Per light, rendering actual scene: 10,000 shadowbuffer reads.
>
> Total =3D 25k reads/writes.
>
>
> Volume shadows:
>
> Per light, rendering volumes (remembering that volumes have
> two sides!):
> 2*10,000 Z tests + 2*5,000 Z writes =3D 30k reads/writes.
>
> Per light, rendering actual scene: the stencil tests come
> free with the Z
> reads. No extra cost.
>
> Total =3D 30k reads/writes.
>
>
> So in terms of fillrate, it's pretty close - shadowbuffering
> slightly ahead,
> but I made a lot of assumptions. But shadowbuffering has some
> big aces up
> its sleeve:
>
> The first is that I said the quality slider was on "best" -
> one texel per
> screen pixel. But you can turn that down - you can easily
> 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.
>
> The second is that you can render a single receiver with multiple
> shadowbuffers in one pass - because you're just sampling a
> texture and doing
> a comparison. So you can do more than one of these per
> shader. Let's say you
> can do two - that's totally realistic for PS2.0 hardware. So
> you've now
> halved the number of passes you do when rendering the scene
> (I didn't list
> those reads/writes in the above). This can't be done with
> 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!
>
>
> Also, the process of extruding volume shadows is far more
> expensive than the
> equivalent shadowbuffer thing, which is just rendering the
> object from a
> different POV. I believe most people using VS-driven
> extrusion find that
> they are frequently limited by triangle throughput rather
> than fillrate. And
> people using CPU-driven extrusion wish they were doing
> VS-driven extrusion
> :-)
>
>
>
> TomF.
>
>
>
>
> -----Original Message-----
> From: gda...@li...
> [mailto:gda...@li...] On
> Behalf Of Megan Fox
> Sent: 08 September 2005 13:07
> To: gda...@li...
> Subject: Re: [Algorithms] Current state of shadow maps?
>
>
> 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?
>
> With shadow buffers (using Tom's method), you'd end up:
>
> - 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
>
> With stencil, you'd end up:
>
> - Casting your extrusions back for every light/occluder pair.  You
> can't really merge (I don't think?), so that's "it."
>
>
> Especially after using Tom's handy-dandy frustum
>
> 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.
>
>
> 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)?
>
>
> Stencil volumes win the indoor/urban, night scenarios
>
> (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 and
>
> world extent, so stencil is not expensive
>
> However shadowbuffers have other qualities that make them
>
> attractive (image based, soft edges), so it would be
> desireable to use them for all purposes. It's just a pity
> that they are so unfeasible for omni lights (I imagine an
> army with torches here ...).
>
>
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--
-Megan Fox
Lead Developer, Elium Project
http://www.elium.tk