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We currently use the model derivatives API to upload 3D models and later visualize them in a browser with the Autodesk Viewer v7 (previously Forge Viewer).

For different reasons we have been trying to hide and show parts of the model, but to do so in some cases we need to be able to hide geometric pieces of some elements while still being able to see the rest. For example if we had an element in gray (see image below) we would like to make the part of the element in red transparent.

Gray block element with a red section indicating the piece to cut out

I have looked into the Viewer API docs and there are functions that allow showing and hiding complete elements, but not cutting elements along a custom path.

While the docs mention the overlay functionality which enables us to add custom geometries by interacting with the THREE library that the Viewer runs on, drawing a copy of the whole model seems inefficient and prone to giving bad results.

Searching in SO I found that the THREE v71 library (which is the version the Viewer uses) has the possibility of occluding elements with the colorWrite option. See these links for reference:

  1. three.js transparent object occlusion
  2. Occlusion of real-world objects using three.js

I tried implementing something similar by using the colorWrite key when creating a new Three Material in an overlay, but nothing happens. This makes me think that either the Autodesk Viewer overwrites the render order (which makes sense) or openly ignores the colorWrite option. Does anyone know why and if there is a workaround for this? Or does anyone know if there is another way to cut an element that I'm not considering?

Please note that we have to do this with multiple elements at the same time and that reuploading the model periodically is not an option for us.

M -
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CRuizH
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1 Answers1

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I got the following feedback:

The approach linked in the SO thread doesn't work, because we ignore both the colorWrite material property and the renderOrder mesh property. I also think that the approach would not work as expected in general, because using an invisible object to occlude a part of another object would always be view-dependent, and also occlude other objects in the background (so it wouldn't just 'cut out' a part of a single object). Generally, this is not really a use-case for LMV, because it's a viewing tool, not a solid modelling tool.

Cut planes might help to get some of what you want, but they also have limitations:

It's possible to define multiple section planes to construct a 'shape' (that's also what the section box does internally). But since they are infinite, you wouldn't be able to cut away parts of one object, but not other objects (or other parts of the same object that intersect the cutplane). They're also designed to be 'outward' facing. The section box, for example, can only be used to narrow the scene down to some elements of interest. It's not possible to do the opposite, i.e. cut out some inner part of the design and keep everything around it.

This is what the section box could offer: section box

Adam Nagy
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  • Thanks Adam, even though I was hoping someone would have a workaround, this answers my question about the viewer ignoring the colorWrite option and if there was an established way to cut elements, so I'll accept it as the answer. – CRuizH Feb 02 '23 at 12:04
  • Just for future reference, could you add who gave you this feedback? – CRuizH Feb 02 '23 at 12:06
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    The Viewer engineering team gave the feedback – Adam Nagy Feb 02 '23 at 14:27