Render wireframe on cut section of polygons

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Hi, after researching for a few days and trying everything free I came across I'm giving in and asking here. I'm looking for a way to render wireframe so that it'll complete the lines where geometry is cut off, either because of a cross section or from the near plane on a camera. I've found that the Source Engine does this which you can see in this picture and the video. Unlike source however I'd like it to be double sided, and though unlikely, if only the clipping edge could be isolated that'd be even better.

wireframe clipping with camera

The reason I want this behavior is that I've been looking for any way to scan a models shape in layers from the top-down using an orthographic camera and reconstruct it using layers of textures. I've tried many ways, had a pretty good one but it relied on the model being well made, my system has to account for any model from the user. I've also tried using highlight shaders, which worked well for the most part, but if a polygon is perfectly vertical (like on a building wall) there will be no highlight since it'll be too thin.

alt text

I've come to think this wireframe method is going to be the cheapest way to obtain an outline of a models shape at any point, by sending an orthographic camera through the model with the near and far plane extremely close to eachother. I'll take any solution that isn't too heavy tho, needs to be able to make a scan relatively quick for models with millions of polygons.
Thanks for reading, hope there's a way to do something like this.

Video demonstration

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Answer by Bunny83 · Oct 30, 2017 at 03:31 AM

I quickly created a script that will create a wireframe mesh (which consists of lines) based on a given object. In addition it will clip the edges against a plane.

The actual MonoBehaviour in this script is ment for demonstration. Modify it as you need it or just use the "WireframeClipMesh" to create / update the wireframe mesh on the fly. Unfortunately Unity still has no SetIndices variant that takes a generic List. So if the number of edges that has to be drawn changes we have to create a new index array. Though even when moving through an object that shouldn't happen too often.

When you attach this script to an object it will initialize the "WireframeClipMesh" with the mesh of the object it's attached to. It will create a child object "wireframe" which will actually draw the clipped wireframe mesh. The clipping plane is defined by a single Transform reference. The positive z-axis (forward) of that transform is the plane's normal vector. Note things in front of the plane get cut away. So it's not an "allow" plane (like the viewing frustum where the normals point inwards) but a cutting plane.

It's probably easier to look at this animated gif

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func_Mathias · Oct 30, 2017 at 04:06 PM 0

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Wow thank you very much for this, it works pretty darn well! There are some problems though, most importantly that on models that reach the max poly count of unity it gets errors. I don't expect you to do more for me for free here so I would like to contact you and offer some pay to do some fixes, additions and changes, if you're interested that is. My main specialty isn't programming and I don't think anyone on my team has much experience with this exact thing (or time to spend on it), so if you have time I'd be happy to give you something for your effort. If not thanks again for what you have done, didn't expect anyone to come with something so functional, or anything at all really. Thanks!

Bunny83 func_Mathias · Oct 31, 2017 at 01:44 AM 1

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Unity doesn't have a max poly count, just a max vertex count as Unity only uses 16 bit index buffers so it's not possible to address more than 65k vertices.

Well, yes if the original mesh has many shared vertices my current method does duplicate those for each edge. When i started writing the script i actually thought if i may actually create triangles as output which was the main reason to keep the vertices for each triangle.

This problem can probably be solved by simply sharing all vertices possible. Since we only deal with lines we actually don't care about vertex colors, normals, uv, ... so sharing should be no issue. The easiest would probably be a mapping dictionary to automatically merge vertices with the same position.It's probably the best to do the merging as the last step. It could be done right in the beginning, however as new vertices are generated by the clipping they would need to be merged as well.

Currently my family is around for the next few days. I will have a look at it. If it can be fixed easily I'll do it right away ^^

Bunny83 func_Mathias · Oct 31, 2017 at 02:02 AM 1

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It was faster than i expected ^^. Just added the merging code which is essentially just a few additional lines. I've updated the file on my dropbox. If i check the created mesh for the default sphere it actually ends up with less vertices than the original sphere.

func_Mathias Bunny83 · Oct 31, 2017 at 06:52 AM 0

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Man you're awesome, works nicely ^^. But sadly I still have a list of a few things that are needed before I can use it :c. Again I'd really like to reward you for what you have done and what I still need, and I'm all out of reputation points.

I'll put the list here, not because I want you to fix it for free but to give you an idea of what I need, not that I can stop you it seems :P. Show only the clip line (would probably fix the performance too), apply script to top parent only, size and orientation problems when not default I think, and nice to have would be to change line thickness and color.

Since you seem interested in helping please do give me any way to contact you so we can talk and maybe to give some back, even steam will do, assume you have a steam account to play Black Mesa at least, my name and avatar are the same on steam as here. Sorry if I'm pushy about rewarding you, just think you deserve it ^^.

Anyway thanks for everything you're awsome, glad you stumbled upon my question :D.

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Render wireframe on cut section of polygons

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