This covers the open source application blender (BlenderWiki).

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The major portion of development discussion is at:

http://www.blender.org/mailman/listinfo/bf-committers

Another place to go is the tuhopuu "evil tree" discussion list:

http://www.blender.org/mailman/listinfo/tuhopuu-dev

Tuhopuu is mostly a prooving ground for new features.

Try

http://www.blender.org/mailman/listinfo/

for other aspects of blender development, or try

http://www.blender.org/

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The blender source follows a general sort of naming convention: the directory 'intern' is module-level stuff, 'extern' is for the API files.  Some of the times 'extern' is missing and the API is defined in the root directory.

eg:

* blender/source/blender/blenkernel - contains API
* blender/source/blender/blenkernel/intern - contains implementation

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The blender source is broken into several directories:

* intern
* extern
* source

Extern, for now, is exclusively for the physics engines such as Solid and ODE.  Qhull is part of Solid.

Intern is where a lot of the (mostly) independent modules are, such as GHOST, memory managment routines, the sound system, ik, etc. '''WARNING''':  the python in this directory is not where the current development is taking place!

Source is where the main action takes place.
The 'creator' directory is where the main() function lives.  The 'blender' directory is where the majority of the GUI, python, mesh stuff, etc lives.  Not sure what the other directories are used for, beyond the obvious.

Okay, that's the basic directory structure layout.  Now for the source layout.  This part of the discussion will focus on the 'blender/source/blender' directory.

* 'src' is a jumble of various stuff.
* 'python' is the current python development tree, 'bpython' is obsolete.
* 'imbuf' contains image manipulation and IO functions
* 'makesdna' is the heart of the .blend file format.
* 'yafray' is the native support for the YAFRAY renderer
* 'ftfont' is the Free Type Font interface
* 'blenkernel' contains various stuff, from mesh manipulation to fonts to textures.
* 'deflate, encrypt, decrypt, streamglue, sign' will probably be removed someday

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This is the extent of my knowledge.  I will be adding more as time permits