Imporing blender models to XNA as FBX

This post was meant for release on April 28th. I've been very busy in May and hadn't had much chance to update until now. Sorry.

For Space Combat Sim I've been making game models using Blender. It's powerful, it's free and it's got an active development community churning out new versions regularly. Check it out.

Like most modelling programs Blender uses its own file format for model data. XNA projects can't read these files. You normally can only add .X and .FBX files to games using XNA (though there are ways of adding your own model file importers via content pipeline extensions.) Fortunately Blender provides exporters for both X and FBX. I chose FBX due to the readability of the format and the fact the exporter worked more consistently for me.

Exporting from Blender to XNA created two major issues for me. One was ensuring the right material shaders were used. That was handled by writing a simple custom content processor. I simply named materials using "shader-name" which worked nicely. The second issue was more annoying. Blender uses a different co-ordinate system from XNA.

Huh? All modelling programs express positions using three numbers representing the object's position on three axes called X, Y and Z. Each axis represents one direction an object can move. In XNA, the X axis goes left to right, the Y axis goes from upward and the Z axis goes out from the screen (backward in the game.) In Blender the Z axis goes upward and the Y axis goes into the screen.

The result is that if I export a model and use it in XNA as is, it faces downward. Obviously that's a problem. Fortunately XNA provides a nice simple solution.

Just rotate it!

As you can see here the content processor lets you specify a rotation to be applied to the model before it's used by the game. This takes care of things nicely.

To finish, here's a list of things to watch out for.

• Scale. If you don't make your models size consistent with each other you'll need to deal with this in the content processor.
• Shaders. By default the model processor in XNA will assign the model a basic shader to render it. Not much good for fancy lighting effects and such.
• Orientation. I tended to model my ships facing along the negative Y axis keeping rotation simple. If your models aren't facing straight along an axis importing can be more painful