Rigify is an add-on for Blender designed to create modular based rigs for 3d character animation. This add-on ships with Blender by default and includes some "Rig Types" to start creating a usable biped or quadruped rig with.
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Pros
Pro Build a complex rig in minutes
This is the main reason to use any rig construction tool, including Rigify. Doing so manually can take much longer and may inhibit the ability to rig many characters in a short timespan.
Pro Extendable via custom rig types
One of Rigify's strengths is that due to how it builds a rig using rig types, you can create your own and reuse them later on for nearly any time custom rig setup.
Pro Includes starting metarig template and rig types
You can use the built in content to create a biped or quadruped right away with little tweaking required.
Pro Comes with Blender out-of-the-box
You get this add-on when you download Blender and all you need to do is activate it in the add-on menu.
Cons
Con Customization features are vague
Unless you know where to look, this is not exactly beginner friendly as it's not obvious how to customize a metarig or where the settings even are. A number of Rigify properties are split between edit and pose mode as well as bone and armature properties.
Con Not Game Engine friendly
Due to how a Rigify rig is built, the deformation bones are not all parented together as a single hierarchy which is what game engines such as Unity and Unreal are designed to work with. You need to create a duplicate of the bones and constrain them to the ones manipulated by the created Rigify rig before using it for such applications.
Con Starting rig types are not very flexible
While you can create biped and quadruped rigs, you're limited to what features you get. For example, in Blender 2.79 you can only add neck and tail through the super_spine rig type and not through a seperate one. This can cause an undesirable setup in a metarig with tails for example.
Con Rig types subject to change
Due to the fact the authors of Rigify can change/modify rig types and other functionalities, how rig types are built and function can change without notice.
Con You need to know Python to add new rig types
If you want to extend the number of rig types, you MUST know Python scripting in order to do so. While there are some options to generate Python code for what Rigify calls Samples (preset set of bones for a specific rig type), adding a rig type itself is not so simple.
Con Sensitive to naming conventions
Be careful when naming bones in the metarig as if they're similarly named, it can cause the generated rig to be incorrectly assembled with bones being in places they're not suppose to be.
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