NOTE: I have been promising some people to upload this for ages. I postponed it because I did not have good documentation. I still don't. I need to make some screen shots and demo renders but I haven't had time. So I'm posting this with only textual instructions. I'll do a better job explaining things some day. :) Baggins.
NOTE: Jan 9, 2012 I added an updated shadow catcher specifically designed to work right in Poser 8+/PP2010+ where you want to use IDL. That is Shadow Catcher 2012-R.mt5 below. This new one is using refraction and is capable of very accurate shadow tinting.
A shadow catcher is a special material used on props to "catch" shadows from your 3D lighting.
It is useful when you are trying to compose a photo on the Poser background, or a one-sided square, or an environment sphere with some 3D objects in a scene, directly within Poser, not in postwork. In order to make the characters and props seem like they're really part of the scene, you want their shadows to fall on the "ground" of the background image. But the geometry of the background object doesn't permit these shadows to fall at the right angles.
Also, a photograph of real life has shadows that are still "lit" by the ambient lighting. Other non-shadowed areas are lit by the ambient lighting as well as the direct lighting. For example, outdoors, if sunlight falls on the street it is gray, but if there is a shadow, there's a blue tint to the shadow because of the ambient light from the sky.
So this shadow catcher allows you to "tint" the shadows to match the photograph.
The material works by examining the incoming light to the surface. If it is above a certain threshold brightness, the material is completely transparent. Below this threshold the material becomes progressively opaque. Thus it darkens your view of the photo behind it. This is what produces the shadow. How much it darkens the photo is adjustable. In order to control this, you must "calibrate" the shadow catcher. This is so it knows what your 3D lighting is producing. The actual color of the shadow will be determined by nodes that you can adjust. Using the Poser color picker, you can "pick" the lit and shadow colors out of your photo, and drop them right into the material. Once you have calibrated the shadow catcher by identifying your 3D lit color, your 3D shadow color, your photo lit color, and your photo shadow color, then the shadow catcher will be able to correctly map almost any color in the scene to the corresponding color in the photo.
To use the shadow catcher, first you must choose which one you want. There are three versions:
ShadowCatcher-D operates on the principles of direct lighting - the shadows produced by infinite, spot, and point lights, i.e. the shadows produced by your Poser lights.
ShadowCatcher-AO operates on the principles of ambient occlusion - the shadows produced by taking into account ambient light being blocked in all directions by nearby objects, not just light coming from a particular light.
ShadowCatcher-DAO does both.
If you are using an IBL without AO, choose one of the AO versions to perform the AO calculation in the material. If you already have AO on your IBL use the D version, since the AO effect will already be produced by your Poser light.
Having chosen which to use, you must load it on a prop. You can load it on the Poser ground plane if you like. However, there are a couple reasons not to do that. One is that the entire ground plane will be involved in the calculation and it is often possible to speed things up by using something smaller, like a one-sided square. The other disadvantage of the ground plane is you cannot tilt it. If you have a need to make the shadow angles correspond to a sloped or curved ground surface, you want to use some other prop.
Whatever prop you choose for your shadow catcher, simply load the desired shadow catcher material on it.
Now you must calibrate it.
Select your shadow catcher prop and go into the material room. Load the desired shadow catcher material. Make sure you're in Advanced mode so you can see the material nodes. On the left side are the key controls for the shadow catcher.
The first control is the "Calibrate" node. This should be set to 1 initially. When Calibrate=1, the material renders completely opaque - no transparency. It renders as if it is a solid white surface. Once you have your lighting set up as you like, do a render with shadows enabled.
The shadow catcher prop will show what is produced by your lighting in directly lit areas as well as shadowed areas. Make sure you arrange your windows so that you have the shadow catcher material nodes on the left visible, as well as your render. The render doesn't have to be big or high quality. It just needs to show some lit areas and shadow areas.
Now click on the "Calibrate Lit" node Color value. This activates the Poser color picker. Click on your render in a spot where the prop is fully lit. This will copy that value into the material. It now knows what "fully lit" is for your scene.
Now click on the "Calibrate Shadow" node Color value. This actives the Poser color picker again. Click on your render in a spot where the prop is fully in shadow. This will copy that value into the material. It now knows what "fully shadowed" is for you scene.
Now set Calibrate Value_1 = 0 and render again. The shadow catcher should activate and darken the areas of your photo that are behind any part of the shadow catcher that is in shadow.
The shadows probably don't match at this point. You can now teach the material what colors should be produced.
Still with your scene on the screen, click on the Lit Color node, activating the color picker, and then click on the render on part of the scene ground that is fully lit. Now your material knows what "fully lit" is for your photo.
Finally, click on the Shadow Color node, activating the color picker, and click on the render on part of the scene ground that is fully in shadow. Now your material knows what "fully shadowed" is for your photo.
Render again. The results should be closer to the photo.
You may have to try sampling the photo colors a couple times. Try to find two spots very near each other that are lit and shadowed. You're trying to find colors that are actually the same ground color, but showing differently only because of lighting.
If you pick two spots that in real life were different colors to begin with, the calculations will be messed up. For example, if you clicked on a lit area of grass, and a shadowed area of dirt, this is not useful information as those colors are different even when lit the same.
Sometimes you have a photo where the ground is so noisy with different colors all over the place it's impossible to find the same color twice even when you click in the same area. In such cases, it can be helpful to load the photo into an image editor and blur the heck out of it. Calibrate your shadow catcher by sampling the blurred image instead of the original. This will smooth out the colors in the photo, allowing you to get more consistent results.