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TUTORIAL: Compositing: Shadows

Posted: Tue, 11th May 2004, 10:51am

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Simon K Jones

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Chromanator Tutorials: Compositing


A problem often encountered when using greenscreens is that the compositing process tends to remove all the shadows from the footage. Of course, this is generally a good thing – after all, you wouldn't want your composite covered in dirty shadows cast by your equipment and surroundings. However, occasionally you might want your subject to cast a shadow – after all, if your actor casts a shadow onto the artificial set, it will help to sell the illusion.

With very careful lighting and keying you might be able to retain specific shadows. Otherwise you will want to create an artificial shadow, which is a technique I will cover below.

The initial composite
The greenscreen shot in question features an actor flying around near the ceiling of a warehouse. Of course, in reality he was just standing on a stool in front of a greenscreen, as you can see here:

There is clearly no usable shadow, so I will have to create it artificially.

After compositing, even with a key, the actor still looks very super-imposed and the special effect is obvious:

With a shot like this the audience will always be aware of some kind of special effect, as actor's don't tend to have ‘ability to fly' on their resume. The trick is to add small details so that the shot fools them for the brief time that it is on-screen, or at least so that they cannot immediately see the seams.

In this case I'll add an artificial shadow.

Artificial shadows
The easiest way to create a fake shadow is to use a copy of the actual subject and grade it to appear shadow-like. This way the movement of the shadow will match the movement of the subject.

To begin, I place another copy of the greenscreen shot onto the timeline, just underneath the main shot:

First the shadow needs to be animated correctly. For this example I shrink it slightly and move it to the side, so that the shadow is being cast on the back wall. It still looks like two versions of the actor though:

The crucial element is the grading.

First I quickly apply a transparency (under the Effects tab) of 53:

I don't want the colours of the face and hands in the shadow, so lowering the brightness to -255 will fix that:

The edge of the shadow is still far too harsh, but it's nothing that a quick application of the Box Blur RGBA can't fix:

Shadows are different depending on the light source, and tweaking the blur and transparency tools can achieve different appearances. It is very important to make sure the shadow looks the same as the surrounding shadows.