Blender 3D: Noob to Pro/Hacking Blender

Blender is an Open Source project. That doesn’t just mean you get to use it for free, you also get to see how it works, and you can even make your own changes and share them with others.

Blender is also a large software project (well over a million lines of code), with a great many active contributors over a lifespan of more than a decade, and it continues to be developed at a rapid rate. This can make things somewhat intimidating for less-experienced programmers.

This unit assumes you have some decent programming experience under your belt. Blender is mainly programmed in the C, C++ and Python programming languages. It can be built using either the CMake or SCons build systems.

Getting the Blender Source Code
The official Blender source is kept in Git repositories located at developer.blender.org. There are actually several separate repositories:
 * blender-main — the main part of the Blender source, excluding most Python addons.
 * blender-addons — the Python addons included in the standard Blender distribution.
 * blender-addons-contrib — additional useful Python addons.
 * blender-translations — localized language translations for text messages.
 * blender-tests — some interesting example .blend files used for testing and demonstrating Blender functionality.
 * blender-dev-tools — tools that are useful for performing maintenance tasks on the Blender source, but are not actually needed for building Blender.
 * blender-cloud — looks like a framework for offering a new cloud-based Blender service.
 * blender-cloud — looks like a framework for offering a new cloud-based Blender service.

Layout of the Blender Source
Say you’ve checked out a copy of the main Blender source tree. The top level looks like this:
 * — files used during the build process
 * — top-level control file for CMake
 * — note about Blender licensing (GPLv2)
 * — documentation files, among them:
 * — the format of .blend files
 * — how to build Blender, and hack the build system
 * — the man page
 * — non-Blender-specific libraries, primarily developed elsewhere, included in the source
 * — simple build script for those who can’t be bothered to go through the CMake setup process
 * — libraries which are non-Blender-specific but primarily developed here, also glue code for interfacing to external libraries not included in the Blender source. Notable subdirectories:
 * — Constructive Solid Geometry routines
 * — the Cycles renderer
 * — the fluid simulator
 * — Blender’s platform-independent GUI (including platform-dependent implementations). See for a more detailed description.
 * — thread-safe memory management with consistency checking
 * — the Inverse Kinematics library
 * — the library for simulating smoke and flames
 * — additional files to be included in the Blender distribution, including GUI icons and fonts
 * — top-level control file for SCons
 * — the main part of the Blender source, further divided (ignoring the CMakeLists.txt and SConscript files which you will find just about everywhere) into
 * — the bulk of the source, of which some useful parts are
 * — low-level stuff for file management, geometry algorithms, sorting and suchlike
 * — core Blender-specific code (no UI stuff)
 * — code for reading and writing .blend files
 * — some additional files for building the Blender Player executable
 * — the Blender mainline
 * — the Blender Game Engine
 * — some files specific to the Windows build
 * — some test scripts

Common Subdirectory Layout
Within many of the subdirectories in and, you will see the following pattern: The .h files define the functionality exported to other modules, while the intern subdirectory contains the <TT>.c</TT> or <TT>.cpp</TT> files that actually implement the module. Sometimes the <TT>.h</TT> files are put into an <TT>extern</TT> subdirectory instead of the upper directory level.
 * A bunch of <TT>.h</TT> files in the directory, and
 * an <TT>intern</TT> subdirectory.

(And yes, these meanings of <TT>intern</TT> and <TT>extern</TT> are not the same as the meanings of <TT>intern</TT> and <TT>extern</TT> at the top directory level.)

Blender’s “Genetic Code”: “DNA” and “RNA”
You will find references to “DNA” (or “SDNA”) and “RNA” throughout Blender’s source code. The analogy to the terms from genetics is weak at best, but what they refer to is:
 * “DNA” or “SDNA” (“structure” DNA?) is the system for mapping Blender’s in-memory data structures to the on-disk <TT>.blend</TT> file format. A <TT>.blend</TT> file is little more than a memory dump, which makes it quick to write. However, the file needs to be readable by Blender builds on machines with different endianness, 32 versus 64-bit pointers, etc, not to mention future versions with different capabilities. Thus, the saved <TT>.blend</TT> file includes a detailed description of the layout of all the data structures saved in the file, and this description is stored in the “DNA1” block at the end. This block is generated by the <TT>makesdna</TT> tool, which is built and run automatically as part of the overall build process, so its output can be included directly into the Blender executable. It parses all the <TT>.h</TT> files in the directory, so all data structures that could get saved into <TT>.blend</TT> files must be defined here, and they must be careful to use a limited subset of C that the parsing tool can handle.
 * “RNA” defines the Python interface to Blender’s internal data structures and routine calls.

Here is Ton “Mr Blender” Roosendaal explaining DNA and RNA.

Special Globals: “G” and “U”
There is a frequently-referenced global variable named “G”, of type <TT>struct Global</TT>, declared in. The variable is defined in. This same file also defines the global “U”, of type <TT>struct UserDef</TT>, declared in.

Naming Conventions
Some (but not all) global symbols have prefixes on their names indicating (roughly) where they come from. Here is a partial list.

User-Interface Implementation
The UI code is structured into several layers. This code also handles finding of user-preference files and shared application data. Starting from the lowest, the layers are:
 * (see for an overview)
 * (see for an overview)