To simulate naturally occurring highlight glints, render your specular
as a separate pass from Cinema4D so you can composite it multiple
times. In After Effects composite the specular pass over the image
using an Add transfer mode. Then duplicate the layer and apply a
standard Gaussian blur to it (Effect > Blur & Sharpen >
Gaussian Blur). Adjust the layer opacities to match the intensity of
the background footage.

In After Effects, apply Light Wrap (Effect > WalkerFX 2.2 > wfx
Light Wrap) to your main scene layer and select the background image as
your source of light. This mimics the effect of light overexposing on
the film and the tendency of really bright light sources to bloom.

Render a grayscale fog or depth pass of your scene from Cinema4D to
mimic atmospheric haze. You can use WalkerFX Color Fill in After
Effects to tint the fog and then vary the amount of haze by simply
using the layer opacity, or a Levels filter.

Match the footage grain using WalkerFX Channel Noise and/or Add Grain
(Effect > Noise & Grain > Add Grain) applying it to your main
CG layer. Look at each of the RGB channels individually to help dial in
the grain. To make the grain easier to see, apply a Hue/Saturation
effect to an adjustment layer and set the Master Saturation to 75%.

Render shadows that fall on the background as a separate pass from
Cinema4D. In After Effects composite the shadow twice over the image.
The first (bottom) shadow layer should be lighter and softer using the
Darken transfer mode to bring down highlights in the footage. The
second (top) shadow layer should be darker and harder with a Multiply
transfer mode. Use WalkerFX Color Fill to set the color of each layer
to match colors in the footage.

With your shadow pass rendered as a separate element, you can use it to
prevent Light Wrap from working in the wrong areas. In the Light Wrap
controls, enable the Obscuration Layer and select the Shadow layer as
its source and turn on Invert.

To help sell that your 3D object is in the space of the footage,
accentuate areas where your model intersects the scene with some subtle
details. One way to do this is to create a texture map on the ground
plane with detail painted around the model's footprint. Render this
from Cinema4D as a separate pass and composite it into your After
Effects scene.

In After Effects use an adjustment layer at the top of your comp to
test your final composite by putting it through a series of color
permutations. By adjusting the gamma, saturation or hue extremely one
way or the other, you will get a good sense of whether your composite
is holding up. If your CG element starts looking like a cutout at any
point, then you still have some work to do.

Steven J Walker
President
WalkerFX
www.walkerfx.com
steve@walkerfx.com

Steven J. Walker is president and chief software developer of WalkerFX,
a software company specializing in the development of visual effects
and motion graphics tools, and is also a seasoned visual effects
animator with nearly 10 years professional experience in film and
broadcast graphics.

There are other critical aspects to photorealism that cannot be fixed
by compositing. Good lighting is the first and foremost. Flat,
unrealistic or wrong lighting is a dead-giveaway that an element is
fake. Using lighting techniques such as global illumination help
tremendously. The level and type of detail is another crucial aspect of
photorealism. When you can, use high-resolution, photo-based texture
maps.

When you find yourself in a pinch, or you are unable to control the
elements given to you, then often the best you can do is just try to
match the color levels, so the darks, lights and mid-tones are in the
same range as the footage.