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There are few things as important to compositing (and, indeed, painting) as the artist having a good understanding of shadows. The image below sums up a few things that you should know about them:

Pictorial 29.png

1 Cast shadow
Cast by one object on another, a cast shadow is (generally) a hard-edge thing.
2 Form shadow
The part of the form hidden from the light, a form shadow is (generally) a soft-edge thing.
3 The start of a cast shadow... darker than...
4 ...the end of a cast shadow...
...except in the case when the object that casts the shadow is...
5 ...parallel to the object that receives it.
6 The foot of the shadow is a 'super dark'
In 3D rendering this is supplied by the ambient occlusion render pass.
7 A shadow is a darkness, not a 'thing'. A color in shadow is still a color.

In the image below I have taken a detail by the painter Raphael (left) and added some shadow (right).

Pictorial 52.png

Note the sharp edges of the cast shadow (right) and the soft edges of the form shadow (left).

Pictorial 53.png

Uses of shadows

  • Cast shadows are placed underneath an object in order to integrate it with a new background. Shadows are one of the key things that are needed when moving material from one location to another and gettingnthem to match.
  • Improvments to existing shadows can be painted-in using a Multiply blend mode. In this way existing lighting can be made to look far more dramatic and interesting. This can be used in conjunction with a Color Dodge or Screen Layer Blend to bring out the highlights.
  • Existing shadows can be added to using a simple layer in Normal blend mode. Multiply blend mode will not work if you wish, for example, to soften the edge of an existing cast shadow.
  • Large shadow masses can be used to add dram to a scene. This is talked about in the page on Lightness Organisation.