In the case of some primitives, effects may apply themselves across the bounding box of the entire object (such as you saw with feTurbulence). In most cases, this will not be a desirable effect; you will want your effects to take the shape of the objects they are applied to. Thus, you'll need to use a primitive such as feComposite to merge certain filter effects with the shape of the original object.
At a common level of usage, feComposite's syntax is simple, as shown in the following:
<feComposite in=”id1” in2=”id2” operator=”value”/>.
As feComposite involves the compositing of two sets of content, it needs the capabilities for two inputs, hence the addition of the in2 attribute. How these ...