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When an IFC model is uploaded and translated by Autodesk Forge, some weird "shadow lines" are generated and appear in the Forge viewer. These lines mostly are IFC wall elements and correspond to real walls in the model, but they are drawn under the actual model. Here is an example of such lines:

IFC model with weird "shadow" lines

Now, if I try to add another model, which should align with the first one and I set both Global Offset Zs to 0, the added model fits into the shadow lines at the bottom and not the actual model. Thus, it seems that the shadow lines are the considered to be a part of the model. Here is a visualisation of the problem:

Added model aligns with shadow lines instead of actual model

When I load the model with other IFC viewers, these shadow lines don't appear:

Other IFC viewer with no shadow lines

Thus, I guess these lines are generated during the Forge translation process. Where are these lines coming from and how can I get rid of these? And even more important, how can I align the models correctly?

Thanks for any kind of help!

EDIT: When I use legacy conversion, the alignment works! Here is an image:

legacy conversion causes no alignment issue

The problem is, that I need to have spaces and opening elements, thus I need to use the modern conversion.

Toni Maccheroni
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2 Answers2

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The "shadow lines" probably are 2D representations of your building elements, and they are located on z=0.

The 3D representations seem to be located at a higher level, maybe the origin of this model isn't set correctly.

So what you see is basically a plan of your model at the correct z value, while the 3D elements are at a wrong z value. This also explains why other models are places at the z level of the 2D representations.

The IFC files would be helpful to take a closer look.

wkrt
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  • Thanks for your comment! I checked with legacy conversion, and there I also have some lines on the floor, but the alignment works fine (see my edit for an example). Thus, I guess the 3D represantations seem to be located correctly. I also checked with other models with the same result: legacy => correct alignment, modern => wrong z alignment – Toni Maccheroni Jan 27 '22 at 14:39
  • Do you have any non-confident IFC file that shows the described behavior in non-legacy conversion that you can share? – wkrt Jan 27 '22 at 22:50
  • I believe @wkrt is right. The shadows in Forge Viewer are always heavily blurred, so what we see in the screenshot is more likely some 2D geometry from the original IFC design, offset from the rest of the 3D geometry. – Petr Broz Jan 31 '22 at 13:19
  • If you could share the IFC file with us (confidentially - we would not share the design with anyone outside of Autodesk) via `forge (dot) help (at) autodesk (dot) com` we could investigate it on our end. – Petr Broz Jan 31 '22 at 13:20
  • Hi Petr, thanks for answering. Yes, the lines seem to be part of the model. They are clickable and when I open the property browser, I can see that they mostly are walls. When I select such a line, the wall in the model on top gets selected as well. I sent you an email with the models attached. – Toni Maccheroni Jan 31 '22 at 13:30
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The lines shown are actual geometries. Some viewers draw them, others don't. Unfortunately, our customer generates the data and isn't able to remove these line geometries.

The main problem of aligning the models was fixed by manipulating the IFC file. We figured out, that one model has a geographic reference point defined in the IfcSite and the other doesn't. When we replaced the longitude, latitude and elevation attribues by '$', the alignment worked again. According to iFC definition, the geographic reference point is only given for informational purposes, but still, it affects the global offset, at least when using 'modern' conversion.

Toni Maccheroni
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