1. Many people use front projection image mapping to cast a
background onto a 3D object so that a shadow can be added to the image.
This does in fact work but its major limitation is that the object with
the shadow cast onto must be lit to exactly 100% using a mix of
luminousity and diffuse. Otherwise it is possible to see a change in
intensity at the edges of your object/background transition. Making this
lighting and intensity work visually is very difficult to do well, and
second, it is very limiting on how you want to light your objects that are
added to the scene.
Or you can...
2. Use a shadow alpha on the surface to make the surface add
only to the alpha information of your image and not the RGB portion.
Ok, to create the images above, make sure that the water surface had
shadow alpha enabled as you just did to the surface above. Then, make the
object receive shadows but do not cast shadows or self shadow.
Front projection map the background image on the stand-in objects but DO NOT make them 100% luminous. In this case make them 50% luminous
and 50% diffuse. That allows the shadow to cause a 50% decrease in alpha
intensity for the output. If you use less luminousity, your shadows will
be darker and this lets you completely control how you want your shadows
to look in the composite. Obviously, you need to match lighting on your
objects as well, but a shadow that is too dark or too light will easily be
seen. Remember, you want to do such a good job with the composite, that
noone will even know it was added and think you just filmed it. (So much
for getting much credit...)
So once your finished with your surfacing, lighting, motion and shadow
intensities, you need to render your scene saving the alpha maps and RGB
images. If all your stand-in objects that you don't want to see have
shadow alpha enabled in its surface settings, then they will only show
through your alpha map if a shadow is cast on it. This makes it easy to
now finish the composite in a new scene. Load the same background in as
you used before and set it as the backgound image. Then set your just
rendered RGB image sequence as the foreground and the just rendered alpha
sequence as the foreground alpha. Activate the foreground fader alpha and
your finished!
Seamless stand-in objects plus composited objects
with their shadows.
The water surface could also be bump mapped so that the shadow
isn't perfectly flat which would make it appear to cast across the waves
properly. Also, the lighting and surface shading needs to properly match the
background image. Which it doesn't do in this example. You should try to match
the environment as well such as the fog if you are flying the plane in from far
back in the scene.
(Originally printed as "Compositing" at Ethereal3D. Reprinted with permission from Mike King and Ethereal3D.com.)
