Re: [Algorithms] Stopping shadow volumes from going through geometry

[John P. <daw...@at...>]

MessageHmm, if you must do darkening stencil shadows, and you don't want =
them to go through walls, maybe you could do a four-pass scheme, where =
you render two types of shadow volumes.=20

First, clear stencil to 128 or something.  Then render the shadowvolumes =
to the stencil buffer.   At this point, the stencil buffer will contain =
values where you should darken the screen.  Draw a full screen invisible =
quad that sets this shadow region to a certain value other than 128, say =
129.  At this point, you have values of 128, and 129 in the stencil =
buffer.  129 is currently where the screen should be shadowed.  Now, for =
all the polygons that you want the shadow volume to be blocked by, =
extrude those (call them anti-shadowvolumes) and render the =
anti-shadowvolumes into the stencil buffer.  They will invalidate the =
parts of the screen that are blocked by the anti-shadowvolumes.  =
Finally, draw another fullscreen quad, and darken areas that remain 129 =
in the stencil buffer.

Would this work?  Sounds like alot of passes, but it would allow you to =
keep an old school renderpipe, and use the darken stencil shadow volume =
method, and still allow geometry to block the shadows.  Your probably =
going to still see funny anomalies though, if your not careful about =
choosing the anti-shadowvolumes.  You'd want to make sure that the =
blocking object in question truly blocked the entire shadow, or the =
shadow will have "holes" in it.

-John
  ----- Original Message -----=20
  From: Joris Mans=20
  To: gda...@li...=20
  Sent: Thursday, November 21, 2002 7:11 PM
  Subject: RE: [Algorithms] Stopping shadow volumes from going through =
geometry


  this is what i mean, in "real-life" this is not possible. Usually the =
static geom uses some prebaked form of shadowing (lightmap, vertex =
darkening, whatever) and it will clash with the dynamic shadowcaster. We =
only want the character to cast shadows on the vicinity using stencil =
and leave the rest as-is. Ofcourse when your initials are JC and you are =
making a game called D III things change a bit ;)


  Joris
    -----Original Message-----
    From: gda...@li... =
[mailto:gda...@li...] On Behalf Of John =
Pollard
    Sent: Friday, November 22, 2002 1:31 AM
    To: gda...@li...
    Subject: Re: [Algorithms] Stopping shadow volumes from going through =
geometry


    Hit reply too fast :-O

    >>You don't even need to make the entire scene cast shadows
    Actually, the character's shadow will still go through the wall.  =
But you can simply make the wall cast shadows as well.  Since it's =
blocking imcoming light, it works.  And no double darkening.

      ----- Original Message -----=20
      From: John Pollard=20
      To: gda...@li...=20
      Sent: Thursday, November 21, 2002 6:23 PM
      Subject: Re: [Algorithms] Stopping shadow volumes from going =
through geometry


      You won't get the double darkening if you do all your =
shadowvolumes at once at the end, and do the fullscreen quad to darken, =
but it still doesn't solve the problem completely, as the ledge will now =
go through walls, and it's a never ending process, unless like you said, =
the whole scene cast shadows.  But this method is such a pain, and you =
never really fix all the problems.

      Our shadowvolumes actually block imcoming light, so the problem =
goes away (which is how most people are doing it these days I'm =
assuming).  The drawback, is you have to interleve your light rendering =
code between the shadowvolume rendering.  The advantage, is all those =
problems go away.  You don't even need to make the entire scene cast =
shadows.  Everything just works.

        ----- Original Message -----=20
        From: Joris Mans=20
        To: gda...@li...=20
        Sent: Thursday, November 21, 2002 6:04 PM
        Subject: RE: [Algorithms] Stopping shadow volumes from going =
through geometry


        this is the answer i see all the time when ppl ask this question =
(please dont take this personally), but as we all know this doesnt solve =
it, even with the ledge casting a shadow you will get "double shadowing" =
under the ledge.
        So if anyone has a way of solving this which does not require =
stencil-shadowing the entire scene ;) please reply as i dont know it =
either :|


        Joris
          -----Original Message-----
          From: gda...@li... =
[mailto:gda...@li...] On Behalf Of =
Gilles Marion
          Sent: Thursday, November 21, 2002 10:17 PM
          To: gda...@li...
          Subject: Re: [Algorithms] Stopping shadow volumes from going =
through geometry


          Hi,

          I think the ledge should also cast its shadow onto the ground, =
no ?
          If so, it doesn't matter if the shadow volume is extruded past =
the ledge.
          If fillrate is your bottleneck, think about clipping (cpu =
intensive), or try setting the far side=20
          of the shadow volume nearer from the character.
          If it's a sun-like light, radiosity of the environment should =
make look that=20
          correct to your eyes.

          Gilles.

          ----- Original Message -----=20
            From: Fong, Jay=20
            To: 'gda...@li...'=20
            Sent: Thursday, November 21, 2002 9:29 PM
            Subject: [Algorithms] Stopping shadow volumes from going =
through geometry


            Hello,

            I was wondering if there is some way to prevent shadows, =
generated by the stencil shadow volume technique, from being extruded =
through other objects in the scene?  e.g. a character standing on an =
overhanging ledge above ground casts a shadow on the ledge but also onto =
the ground below the ledge.

            I thought about clipping the extruded shadow polys against =
the world geometry but I'm hoping for a more elegant (i.e. simpler) =
solution since our world geometry is quite dense.

            Any solutions or links/pointers to papers related to this =
issue would be much appreciated!

            Thanks,
            Jay