How to use Blender with OSG/VTP

This page describes how to use the free modelling tool Blender to produce 3D models which look correct in realtime 3D software.  Although the emphasis is on the VTP software built on OSG, most of these instructions are also relevant for other OSG users and even other 3D runtimes.

1. Beware Blender Tutorials

There are lots of Blender tutorials which teach you how to build models and apply materials.  However, they assume you will be rendering your scene inside Blender, using the non-realtime raytracer.  Applying materials for realtime use is very different!  Hence, this page.

2. Link to Object vs. Link to Mesh

Let's say you want to make a blue cube.  You select your cube, bring up the Material buttons, and see something like this:

See the OB and ME?  If the OB button is highlighted, it's showing you the material that's mapped to the Cube Object.  If the ME button is highlighted (as it is in the picture), it's showing you the material that's mapped to the Cube Mesh.

This is an important distinction.  You must map colored materials to the mesh (ME).  Anything mapped to the object is ignored when you export your object.

You may see Blender tutorials and documentation describing how to map more than one material to a mesh (that 1 Mat 1 up there).  Don't do it.  Stick to one material.  For example, with one blue material linked to your mesh, it will look like this:

That goes for textures as well.  Use no more than one texture per mesh!  Which brings us to:

3. Applying a Texture

Making a textured object is surprisingly very different from a simple colored object.  To start with, you do not make a material.  Instead, you use a very different Blender method of applying a texture.

Here is an example of texturing a cube.  Select your object and change your mode to UV Face Select:

 

In this mode, you will map a texture onto your object using UV coordinates.  There are several good tutorials on how to do this operation in Blender (BioRust UV Mapping, Blender 3D: Noob to Pro/UV Map Basics).  What's important is to use the UV/Image Editor to apply the texture, using the command Image: Open:

Once the image is open, and you've done some UV mapping, then the texture should appear on your object in the 3D View, when you choose Draw type: Textured:

If you want to change the texture image later, go back to UV Face Select mode and use the UV/Image Editor to open another image.

4. Removing a Texture

Removing a texture might seem as simple as pressing that little X in the UV/Image Editor while UV editing your object.  But there is another step!  Select your object and use the buttons for Editing (F9).  In the Mesh area, you will see the word TexFace and a button next to it which says Delete.  Press Delete.  Now the texture is really gone:

5. Exporting: File Format

Blender comes with lots of export options.  I count export to 27 different file formats in the current version!  However, only a few of these work well as a way to transfer models to realtime rendering systems.  In my testing, the following work the best with OSG:

• Collada 1.4 (.dae)
• OpenSceneGraph (.osg)
• Wavefront (.obj)

If you don't see .osg listed on your File: Export menu, you can get it from the osgexport page.  Unzip it into your .blender/scripts folder, and the next time you run Blender it should be there.

Each of the three formats has pros and cons.  Here are a few issues to consider with each format.

• .dae:
  • It's a new and complex file format, so not much software supports it yet.
  • Standard OSG 1.1 doesn't read it by default, but there is a Collada reader available as a separate project.
  • It's an XML format, so the files are rather big.
•  .osg:
  • Naturally, it's OSG-specific, which is fine if OSG/VTP is your only destination.
  • It's also quite a bulky text format, although you can use CManager (VTP) or osgconv (OSG) to convert your .osg to a smaller, binary .ive file.
• .obj:
  • It's a simple, ancient format, fairly limited, but widely supported.
  • It doesn't support scene hierarchy, so all your coordinates will be "flattened" into a single space.
  • Blender (in which the Z axis points up, or Z-up for short) writes OBJ files which are Z-up.  OSG (by default) assumes .obj files are Y-up, so unless you take steps to deal with it, they will be turned 90 degrees.

6. Using Blender's osgexport with the VTP Software

Blender was primarily designed for making detailed, non-realtime scenes, rendered with a raytracer.  It has a different way of thinking about material properties and lights, which doesn't quite fit with OpenGL (and hence OSG/VTP).  However, osgexport has some options you can set to compensate for the difference.

Here is a quick list of what to do:

1. When creating a material (for any normal colored surface - remember, textures don't use materials), in Blender's Material controls, set the color to 2/3 of the fully illuminated color you want.  For example, if an object should be pure blue (0.0 0.0 1.0) on its lit surface, use (0.0 0.0 0.666).
2. In the osgexport UI, turn on Ambient and TexMats:
   
3. You can leave the values at the default (Ambient ration 0.5, TexMats diffuse 0.7 and ambient 0.3)
4. Export!

Why these values?

For those interested, here is an explanation of why the above values produce good results, and how ambient lighting works in OpenGL, and the VTP software in particular.

7. Dealing with existing models

Sometimes you get a whole model from someone else instead of creating a model from scratch.  In that case, see How to fix Blender models for use with OSG/VTP.