Invisible collision surfaces


Hi everyone.

The image below attempts to explain what I am on about.

I create an obect with enclosed holes. For example, a raised walkway around a square. Or a wall with a big window in the middle. In theory an object moving in the square or through the window should not have any interaction with the walkway or wall. However on importing into an mmo the empty spaces turn out to be not holes at all, but invisible surfaces. So what looks like a hole or emptiness acts like a solid, and other objects attempting to move through the “empty” space collide and stop.

Also, using any modelling program (I am new to all this, regardless of software) I cannot see anything (polygons, whatever) in the emptiness.

So … how can I make actual holes using sketchup? Remove things or enclose spaces that really have nothing actually there?



I’m not knowledgeable about mmo’s, but this sounds to me like an issue with whatever process you are using to import the model into the mmo or perhaps with how the mmo interprets the geometry.

In your figure A, SketchUp will not have any geometry in the open space unless you created it there and subsequently hid it. Yes, as in figure B, when you close a boundary SketchUp will deduce that you meant to close a face and will create it for you. But this does not mean that some invisible geometry was already there; it did not previously exist. The blue line in figure C marks the bounding box of the group. It is a temporary display item, not real geometry.


Hi slbaumgartner.

Thanks for your answer :smile: I never considered that aspect of it and will see if I can do something on the upload side. That would explain why the other modelling software doesnt show anything in the holes! :smiley:

Thanks again :slight_smile: And cross fingers for me! If I can find a way to upload without artifacts or invisible fills I will come back and mark this as solved.


what file format does your mmo require?

some game engines would need four separate objects to depict your SU model…


Hi John.

Thanks for your response. :smile:

The format uploaded is collada, so .dae - I am importing to simonastick (it is a virtual world) using a client called firestorm. I am going to try importing using other clients and see if that makes a difference.

I have exported as dae. I have also exported as obj and converted the obj to dae using blender.

Thanks :slight_smile:


SU works by assuming that if you draw lines to enclose a shape, then you want the shape to be filed and become a surface. The bounding lines become edges. If you want to delete the surface, then simply select it and delete it.

Any edge is still treated as a line that can be used in combination with other lines to form another enclosed shape unless it’s within a group or component - then it will only interact with other lines within that entity.

A single click on an edge/line or face will select it - it highlights blue. A single click on a group/component will select it - a blue bounding box will surround all the geometry within the group.
(Double clicks select all geometry attached to an edge/face or will enter a group/component. Triple clicking will select everything within the current group/component)
Hitting [del] will delete a selection, or you can use the eraser tool.

So with your model in mind, when you draw a line to enclose the inner shape, it encloses the inner shape and forms a surface. If you don’t want it, then you can delete the surface and leave the line. And as slbaumgartner said, the blue is simply the bounding box when you select a group.

Edit: the problems might also be the fact that it looks like you are modeling a 2D plane for a 3D environment?


it’s how it is modelled that the physic’s engine is having trouble with…
each side needs to be it’s own self contained group, created on axis and then moved into position…

download some of the “Sketchy Physic’s” examples from 3d Warehouse and inspect how they are made…

For a doughnut shaped enclosure, you’ll need something like this…

there are plugins that will split existing models, but it’s best to understand the requirements and model for them …



Hi Gadget, thanks for the input :slight_smile: That is 3d, just relatively thin/high (0.5m) compared to the width and length (250m). It was a simplified build as I tried to work out what was wrong.

Hey John, I will do as you suggest and I think I can mark this as solved now, it being not one simple thing (well not simple to me, lol) but an overall conceptual understanding.

Thanks everyone!


happy to help…

have a look at ‘collision detection’ type topics at SketchUcation and you should find ‘Fredo’s plugin’ along with lots of useful modelling tips…