LDraw Loader: Include building step count in model (assemble multiple parts per step)

[sbgib]

Adds building steps to the LDraw Loader.

numBuildingSteps and buildingStep properties are included in the model to count and index the STEP commands in the LDraw model file. This allows 'assembling' multiple parts together in one step, as you would see in an instruction booklet. The visibility of objects could be changed this way instead of with construction steps, which seem to represent each object as a separate step.

Example for car.ldr_Packed.mpd:

userData: {
   buildingStep: 0
   category: null
   constructionStep: 0
   keywords: null
   numBuildingSteps: 8
   numConstructionSteps: 57
}

In CAD software for editing LDraw models, 'building step' appears to be the common name for this. Sources:
http://www.holly-wood.it/mlcad/basic6-en.html
http://www.melkert.net/LDCad/docs/basicEdit
https://www.leocad.org/docs/tutorial1.html

[yomboprime]

I can review this in 3 or 4 days.

[yomboprime]

We should check if model has children, just for safety.

[yomboprime]

I think we should stay with "building" or "construction" steps, but not both (it would be confusing). I was a bit simplistic when implementing construction steps, so your approach could be better.

Please modify the loader example to use building steps and see better how they work. A live link of the example would be appreciated. After that, construction steps can be removed.

Finally, the LDrawLoader docs should be updated.

[sbgib]

Thanks for reviewing! That makes sense. The changes have been made and here are some live examples:

• Original: https://threejs.org/examples/?q=ldraw#webgl_loader_ldraw
• Assembly by part: https://codesandbox.io/s/magical-varahamihira-oefoug
• Assembly by building step: https://codesandbox.io/s/optimistic-pond-d4plmg

Compare the effect of the Construction step / Building step slider in each case to see the intended difference. Having the flexibility to use the steps or not would be ideal, so the Assembly by part example shows how the latter can be achieved even without them. This way, no functionality is lost.

It became clear that model.traverse is preferable to model.children where sub-models (.ldr) have also been used. So, the original approach is maintained here, except that startingBuildingStep is reset to false again after pushing to subobjects. After some testing, it seems that's what is needed to respect the steps throughout the entire model.

constructionSteps are renamed to buildingSteps here as well, though that's not strictly necessary.

[yomboprime]

Thanks! Looks good.