Support outputting a STEP file with per-face colors

[mkovaxx]

I need to set per-face colors on programmatically created shapes, then export into STEP format. Please help me find a way to do this. I'm somewhat familiar with the underlying C++ OCCT, and I'm more than happy to contribute code. I'm filing this issue so we can discuss and align on the best approach here.

Here's a self-contained demo of the approach I found so far: https://github.com/mkovaxx/cadquery_fused_assembly/tree/4572a63616a614f3920c859b5224c8d4aaed3d64

I create each face as a separate shape, then pass it to the Assembly.add method along with its desired color. Saving the assembly as STEP then contains the colors as expected.

For details, see this section: https://github.com/mkovaxx/cadquery_fused_assembly/blob/4572a63616a614f3920c859b5224c8d4aaed3d64/src/tubes.py#L22-L37

However, this approach has two severe limitations.

First, the output contains topologically disconnected shells. I expected this to be resolved by setting ExportModes.FUSED on the Assembly.save method, unfortunately that does not help.

Second, the output contains no volumes. There doesn't seem to be a way to get around this without extending CadQuery in some way.

[adam-urbanczyk]

So on OCCT level you have to use sub-shapes (see fused assy export). This could be added to the CQ assy class via an optional field mapping sub-shapes to color:

colors: Dict[Shape, Color]

Additionally, assy export would have to be updated to actually make use of this property.

[mkovaxx]

@adam-urbanczyk Thanks for proposing an approach!

I'll write down what I understood just to make sure that it is correct.

1. Extend Assembly with a subshape_colors: Dict[Shape, Color] field.
2. Extend exportAssembly to encode subshape_colors into the TDocStd_Document before it is written as a STEP file.

Is that correct?

Based on that, I think these are the places that require modifications:

1. Add subshape_colors: Dict[Shape, Color] in here: https://github.com/CadQuery/cadquery/blob/7bc8c00dc1a81633daea4810e04762d61a2abd5a/cadquery/assembly.py#L82-L90

2. Propagate subshape_colors into the call to exportAssembly here: https://github.com/CadQuery/cadquery/blob/7bc8c00dc1a81633daea4810e04762d61a2abd5a/cadquery/assembly.py#L486-L487

3. Encode subshape_colors into doc here, between line 86 and 88: https://github.com/CadQuery/cadquery/blob/7bc8c00dc1a81633daea4810e04762d61a2abd5a/cadquery/occ_impl/exporters/assembly.py#L82-L95

Did I miss anything?

[adam-urbanczyk]

So (1) sounds fine. There should be (1.5) which is adding some kind of method (methods?) to set colors e.g. based on selectors. (2) is not required - you don't need to propagate anything, it is already a field. (3) actually requires modifying toCAF() and is likely going to be more complicated than you wrote.