Advanced Blender to Unity 3D model importer
C#
Latest commit 9e8f284 Feb 16, 2016 @EdyJ Support for meshes without UVs
Thanks to Denis Opa for pointing it out!

README.md

Blender to Unity 3D importer

An advanced Blender model importer for Unity 3D

By Angel García "Edy" http://www.edy.es

Setting up

Copy the Editor folder to your project's Assets folder. No files will be touched by the Importer unless they are explictly configured to.

How to import models with the Importer

Rename the model to include the string [importer] in the name or path (case insensitive). Example:

Assets/Models/Scenery/Big House [importer].blend

The [importer] string can be used in a path. All contained 3D files are processed:

Assets/Models/Player Vehicles [IMPORTER]/Car1.blend

Right-click the file or folder, then choose Reimport to apply new settings after renaming.

How to use the advanced parameters

The [importer] string allows additional parameters as keywords separated by a dot (.):

[importer]                  (use default settings)
[importer.opt.zreverse]     (optional parameters are applied)

Parameters:

IMPORTER        Use importer. Otherwise bypass the file and let Unity import without changes.
                This must be the first parameter. All others are optional.

OPT             Optimize mesh usage. Ensure that the identical meshes are instanced instead of 
                duplicated. This optimization can also be applied later scene-wide (menu 
                GameObject > "Optimize mesh instances in this scene").
SKIPFIX         Do not fix the Blender file. This is automatically applied with any 3D format
                other than Blender. OPT and NOMODS can still be used separately in other 3D formats.
FORCEFIX        Forces to apply the fix even in non-Blender files. This is useful with FBX files
                exported from Blender.
ZREVERSE        Turn the model around 180º when importing so the Z axis points the opposite 
                direction. Use when the "forward" direction of the model points backwards (-Z).
                "Forward" should be +Z in Unity (-Y in Blender).

NOMODS          Do not apply selective commands to meshes (see below).  
NOANIMFIX       Do not fix animation clips in .blend files when imported.
NOFLOATFIX      Do not fix floating point precision errors (i.e. rounding 0.9999998 to 1)
FORCEFIXROOT    Fixes an specific case of Blender file consisting on a single Empty object
                as root of all other objects. See Known Issues below for details.

Selective commands to meshes

Some keywords can be specified in the name of the objects in the hierarchy. They will trigger specific commands unless the NOMODS parameter is specified above.

Example object name:

collider_hull --norend--coll

All commands must be grouped together at the end of the name, starting by -- each, without any other character or separator.

Mesh commands:

NOREND          Remove the Mesh Renderer
COLL            Add a Mesh Collider
COLLCONV        Add a Mesh Collider and mark it as Convex

Hints & Tips

Parenting in Blender

Always use Ctrl-Shift-P for parenting objects instead of Ctrl-P. Parenting with Ctrl-P makes Blender to store two transforms per parenthood relationship, direct and inverse. Only the direct transform is imported by Unity.

Alt-P clears the parent's inverse transform in a children when the parenthood was established with Ctrl-P.

Please contact me if you have experience developing Blender Add-ons. I'd like add a function (maybe triggered by Shift-Alt-P?) for parenting objects like Unity does, without inverse transform.

Rotations and transforms

Unity applies Euler rotations in this order: ZXY

In Blender, the euler rotation mode that matches Unity is: "YXZ Euler" (Z and Y axis are exchanged). Still, you can use any rotation mode in Blender. The result of the rotation will be preserved, with the Euler angles in Unity adjusted accordingly.

All transformations (position, rotation, scale) will look in Unity exactly as seen in Blender, provided the parenthood relationships have been established as speficied above. Only the specific values will be different as result of the coordinate change (axis exchanged, etc).

Known issues

Rotations are incorrectly imported when the Blender file contains a single Empty object as root for all other objects. Use the FORCEFIXROOT option in this case.

Animation clips are processed when imported in "Legacy - Store in Root" mode.