Problem with Drape Tool - unwanted hidden lines

Hi Folks

I’m using Drape on some fairly basic geometry (flat rectangles) and once the Drape operation has completed, I’m seeing a lot of unwanted ‘connecting’ edges. It’s as if everything has been triangulated.

Is there a way to avoid this happening?
I’ve tried various combinations of groups/loose, and trying very small areas at the same time. model origin isn’t too far away.


Images show Before( textured areas are floating over top of the green surface)
and After (showing hidden triangles etc.

I guess the answer might be yes because if the green areas are just plain faces it is possible to do this. The question might be what is special about your model that is causing that. You could post an example file. Once the lines are there, are they separating coplanar faces? In that case CleanUp plugin could get rid of them for you.

Sharing a portion of the model would help us far more than screenshots.


I’ve uploaded a file - thanks for taking the time to check it.
This is a SU Pro 2019 file and Im modelling in Metres (0.00 precision).

The geometry was 95% created by me using some Real-world site 2d boundaries (surveyed) which are generally rectangular, but not exactly (as realworld survey setouts never get it 100% right, only 99.9%)

In this file, I’ve already draped the group of geometry onto the loose geometry and begun the (painstaking) process of cleaning it up. But if you try Draping again you’ll see the issue.

Drape Issue.skp (4.6 MB)

Did you import geometry? I think it must be that. I tried simplifying the model as possible and it still did this. Using Vertex Tools I made each large plane co-planar and it did drape without making extra edges for most of them… but then the drape was not complete. Many faces didn’t separate out (which can happen with drape sometimes–usually not to this extent)
So I think the imported geometry isn’t quite planar and SketchUp can’t make it work.

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No faces were inported, they all were all created by push pull or drawing drom scratch.
The only imported geometey is a few lines (at zero elevation) which form the outer site bounday.

There is other imported geometry in the overall model but its in seperate components.

I had a very close look at the geometry, and found that the faces are all flat (though some parts of the model show 0.00001m of verticle deflection across each plane…not sure how that happened!).

In this model, I have used an extension Eneroth Face Creator and Eneroth Flatten to Plane to help keep geometry on the XY axis.

However there are a number of edges that are not quite 90deg to one another and this creates some small faces or slightly skew edges. This isn’t totally accidental, it’s a result of drawing perpendicular edges from existing edges (real, surveyed boundaries) that are not quite perfect.

I hope the solution to this issue isn’t “Don’t have geometry that isn’t 100% perfect”. In the real world, things aren’t always perfectly “on axis”.
Making rectangular shapes and placing them onto a (basically) flat surface shouldn’t be a difficult exercise, though somehow it often seems to result in hours of cleanup work.

You have Enable Length Snapping activated in Model Info/Units. In my experience this is often the cause of fractured faces due to tiny discrepancies in flat planes.

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Edit, I just checked and length snapping is Unticked.
Angle snapping is ticked (15deg,so not too small)

I checked length snapping too. I also cleaned up the drawing best I could. Front faces, eliminated other geometry. Co-planar faces in both groups. As I said I found that I could avoid the hidden lines being created but drape didn’t work so well.

I don’t think it has to do with having all your geometry being square or perfect in some sense. The sandbox drape tool CAN work on complex meshes, BUT it is not perfect, SketchUp is not perfect. Sometimes it doesn’t catch everything correctly.

I’ve seen other cases where SketchUp thinks a face is a face but NOT. It can close the face but only with manual “fixing” and lose it again easily. This creation of hidden lines is not that common, may have something to do with the other plugins you tried. I’d move on. (Or like the “real world”, this could be just another instance of the uncertainty principle. :wink:)

On an aside. I don’t use the tool much myself. It doesn’t really serve a purpose in my workflow. If I must cut terrain it is after I’ve worked out a foundation and use intersect faces on a portion of the terrain near the house, if I haven’t already replaced it with new grading.

I am also having this issue, and have never had it happen before after years of use. i regularly have imported dwg linework (with no elevation aka flat) and drape additional linework on the top and it retains a flat plane - until now. for example, i will drape road linemarking onto a road alignment. now it triangulates the sh! out of the face. i use 2018 although i have just obtained a new PC - the only difference