Convert DAE to OBJ

COLLADA tells you its unit, and we listen

Near the top of every .dae there is a line stating how many metres one unit is. A file from SketchUp is often in inches, one from Blender in metres, one from a CAD exporter in millimetres. Most converters ignore that line and assume, which is why models from this format so often turn up at absurd sizes.

Here it is read and used. The first unit box shows what the file actually said. If the file states a unit we have no name for — a yard, say — the coordinates are converted into millimetres instead, so the size shown is still the real one.

And which way is up

COLLADA also states its up axis, and files disagree: Z-up from CAD and SketchUp, Y-up from Blender and Maya. That statement is read too, so the model is understood the way the file meant it rather than being tipped onto its side.

The output keeps that orientation unless you change it. OBJ is conventionally Y-up, so if the .dae was Z-up and the OBJ is going into Blender, setting the output up axis to Y saves you a rotation.

The scene is flattened

A .dae describes a tree of nodes with transforms, and geometry can be instanced — the same shape placed several times in different positions. Every instance is expanded and every transform applied, so a model built out of repeated parts arrives assembled and complete.

Faces with more than three corners are split into triangles, material colours are followed through COLLADA’s chain of material, effect and shader, and normals are rebuilt from the geometry with a thirty degree crease angle.

SketchUp exports are the common case, and they have a shape of their own

Most .dae files that reach a converter came out of SketchUp, and SketchUp models have characteristics worth knowing before you convert one. They are built from flat faces with no thickness, because that is how SketchUp works — a wall is a single surface, not a solid. They use components heavily, so the same window appears forty times as forty instances of one definition. And they are frequently in inches, because the default template is.

The instancing is handled: every placement is expanded into real geometry, so the OBJ contains forty windows rather than one. That is also why a SketchUp export can turn into a much larger OBJ than the .dae suggested — the .dae stored the window once and the OBJ has to store it forty times.

The single-surface walls are not something a converter can fix. They come across exactly as they are, which is correct for rendering and useless for printing. If the model has to be printed, it needs thickness added in SketchUp first, using the Solid Tools or a push-pull on every face.