grosporina opened this issue on Mar 06, 2014 · 47 posts
obm890 posted Fri, 07 March 2014 at 5:01 PM
Quote - I'm confused now. Is it arbitrary or not?
Sorry, I was wrong, it is in fact arbitrary, not metres. I did a simple test: A 1m cube exported from Modo imports into Wings as 1 unit high, but I was wrong to conclude that it means Wings's internal unit is a metre, it's just a Wings Unit.
The obj file format contains no scale units, the size of models moving from one app to another depends on the internal units of the 2 applications (and the export/import scale options if there are any). This is the root of all the confusion about importing and exporting objs.
Modo's internal unit is the metre, so if you make a model 1 metre high and then export an obj the uppermost vert in the obj will have a Y value of 1. When that model comes into Wings where the internal unit is a Wings Unit, the obj will have a Y height of 1. The same model imported into sketchup where the internal unit is the inch would arrive there 1" high. Imported into Poser it would be about 8' high because that's the height of one poser internal unit, 96" or 103", I can't remember which.
If you make a model 1 inch high in SketchUp (where the internal unit is the inch) and then export an obj the uppermost vert also has a Y value of 1, and coming into wings that model would also be 1 unit high, the same height as the 1m modo model. So wings units are whatever is convenient to you, metres for big things, inches or centimetres for small things, but always decimal.
When apps allow you to set units on export, it means you can tell it what units the target application will be expecting them to be and the XYZ values wil be multiplied by a conversion factor before they are written to the obj file
Similarly, when apps allow you to set units on import, it means you can tell it what units the mesh was written in and the XYZ values will be multiplied by a conversion factor after they are read from the obj file.
Getting models to transfer at the correct size is just a matter of understanding this business of internal units and setting the right scale options where they are available.