Shadow catcher surface

Shadow catcher surface

Post by mike » Fri, 05 Jan 2001 08:16:11



How do you make a object that ONLY catches shadows from othe objects.... To
allow CG objects to cast shadows on a background plate.

I've tried many combinations of unseen by rays and unseen by camera.

How is this possible...

mike.

 
 
 

Shadow catcher surface

Post by John Gros » Fri, 05 Jan 2001 17:21:32


Only geometry can receive shadows from other geometry. CG shadows can never
be cast onto a background plate if you are using the Background Image in
Layout. Instead, you need to apply the image to an object (you can use a
flat plane that is the same frame aspect as your resolution and place it so
it just fills camera)

If you are using geometry, make sure that the surface normals of the objects
are facing each other (turning on double sided will do this)

jg


Quote:> How do you make a object that ONLY catches shadows from othe objects....
To
> allow CG objects to cast shadows on a background plate.

> I've tried many combinations of unseen by rays and unseen by camera.

> How is this possible...

> mike.


 
 
 

Shadow catcher surface

Post by CH » Fri, 05 Jan 2001 23:51:48


if your using LW5.6 (don't know if it works on 6x) you could download
the facecam plugin from http://www.gfxprod.com/ (free) its a nifty
motion plugin that keeps a flat plane, or any object, facing the camera,
might be useful for background images too if the camera moves position
using the method suggested by John (which unless you want to model basic
shapes and map sections onto them) is the best way. Just a thought
 
 
 

Shadow catcher surface

Post by Ma3r » Fri, 05 Jan 2001 15:32:19



Quote:> How do you make a object that ONLY catches shadows from othe objects....
To
> allow CG objects to cast shadows on a background plate.

> I've tried many combinations of unseen by rays and unseen by camera.

> How is this possible...

> mike.

For compositing such as this, you'll need to use a combination of stand-in
geometry and front projection using your BG imagery.  Read through the user
Guide section on Front Projection mapping (4-27).

However, w/o a plug-in such as Worley's Gaffer, it then becomes a delicate
task of finding the right balance of object luminosity and lighting
intensity to get and invisible blend.  It's doable w/o it, but becomes
pretty much a no-brainer with it.

M.

 
 
 

Shadow catcher surface

Post by Will » Sat, 06 Jan 2001 07:04:50




>Only geometry can receive shadows from other geometry. CG shadows can never
>be cast onto a background plate if you are using the Background Image in
>Layout. Instead, you need to apply the image to an object (you can use a
>flat plane that is the same frame aspect as your resolution and place it so
>it just fills camera)

>If you are using geometry, make sure that the surface normals of the objects
>are facing each other (turning on double sided will do this)

>jg

I only use 5.6, dunno about 6...
There is a setting under surface editor -> advanced tab for alpha channel.
Set alpha channel to shadow density, and you can record just the shadows.

In order to record them, you need to build some geometry that matches teh
background plate to recieve the shadows.

Once you have your alpha chennel set up, and your geometry matching, save the
alpha images separately, and load them into your favorite compositor.  From
here, you can diddle endlessly with the darkness of the shadows, and coloring,
etc. to match the real shadows in the plate.

You can just map the scene onto a polygon the recieve shadow, but I don't
especially like this method because then the lighting of the scene and the
lighting of the CG scene can interact in annoying ways, and it can be harder to
match the shadows.

I still haven't figured out a good way to cast shadows on things like people
standing in the scene, or grass very realistically just yet.  I'm thinking that
for people you would need to shoot them on a bluescreen, and stuff...  That or
do a lot of hand roto-ing and do the shadow by hand...  Ugh.

---------------------------
"I /want/ Moofeus!"
-The V E C T O R-

 
 
 

Shadow catcher surface

Post by mike » Sat, 06 Jan 2001 13:06:08


I cant see any option for alpha>shadow intensity.

does anyone know if 6.5 can do this?

mike




> >Only geometry can receive shadows from other geometry. CG shadows can
never
> >be cast onto a background plate if you are using the Background Image in
> >Layout. Instead, you need to apply the image to an object (you can use a
> >flat plane that is the same frame aspect as your resolution and place it
so
> >it just fills camera)

> >If you are using geometry, make sure that the surface normals of the
objects
> >are facing each other (turning on double sided will do this)

> >jg

> I only use 5.6, dunno about 6...
> There is a setting under surface editor -> advanced tab for alpha channel.
> Set alpha channel to shadow density, and you can record just the shadows.

> In order to record them, you need to build some geometry that matches teh
> background plate to recieve the shadows.

> Once you have your alpha chennel set up, and your geometry matching, save
the
> alpha images separately, and load them into your favorite compositor.
From
> here, you can diddle endlessly with the darkness of the shadows, and
coloring,
> etc. to match the real shadows in the plate.

> You can just map the scene onto a polygon the recieve shadow, but I don't
> especially like this method because then the lighting of the scene and the
> lighting of the CG scene can interact in annoying ways, and it can be
harder to
> match the shadows.

> I still haven't figured out a good way to cast shadows on things like
people
> standing in the scene, or grass very realistically just yet.  I'm thinking
that
> for people you would need to shoot them on a bluescreen, and stuff...
That or
> do a lot of hand roto-ing and do the shadow by hand...  Ugh.

> ---------------------------
> "I /want/ Moofeus!"
> -The V E C T O R-

 
 
 

Shadow catcher surface

Post by CRS Defian » Sun, 07 Jan 2001 07:54:49




Quote:>How do you make a object that ONLY catches shadows from othe objects.... To
>allow CG objects to cast shadows on a background plate.

>I've tried many combinations of unseen by rays and unseen by camera.

>How is this possible...

I think you can use a special buffers saver to save the shadows. You
build some surfaces that match your background (like a plane for the
ground, etc.) and then render your objects in that scene, using
special buffers saver plugin to save out frames for the shadows. Then
composite the shadows onto the background. Then you render again, but
this time, with the objects actually on top of your new background.
 
 
 

1. Shadow Catcher shader

: Anyone have a shadow catching surface shader that will render the shadows
: to the alpha channel, without rendering the surface itself.  How could
: this be done?

: Marshall

In the surface shader, do something like:

  Ci = 0;
  illuminance(P, N, ((backfaces>0)?(PI):(PI2)))
  {
    Ci += Cl;
  }

  Ci /= num_lights;
  Ci = 1-Ci; /* invert shadows */

  Oi = Os;
  if (match_alpha_to_rgb == 1.0)
    Oi = Ci;

I haven't had the best results with more than one light
source, though.

-- Antoine
_______________________________________________________________________________


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