Anton's Research Ramblings

Converting Industry Foundation Classes for 3d Graphics

European public buildings have adopted IFC files as a new standard for architectural plans. They're basically an indexed format that has tags for lots of different types of structural, electrical, etc. plans within a single file. I wrote a couple of parsers to read structural data from the format directly, but it's really not efficient to do this as part of specialised viewing programme [read: grossly inefficient and not at all practical], especially if you want a light-weight browser-driven web script to do it. There are specialised libraries for doing this, but once again I find that it's really bloated when all you want to do is display some geometry. This is an order of magnitude more 'heavy' than the similar "do everything for all purposes" XML COLLADA graphics format.

Why IFC is Not Appropritate for Modern Visualisation Hardware

Modern graphics processors only draw triangles. IFC meshes are not arranged in triangles. Nor are they lists of quads. What you get is a jumble of different 'template' pieces like 'WallA', and 'WallSectionB2' that are reused here and there like LEGO prefab bricks. You can get vertex points out of each prefab, which means that you can construct your triangulated mesh eventually. Each prefab can call on other prefabs. That's okay, but each prefab has it's own coordinate space. This creates a small nightmare for parsing, as you have to 1. do a lot of store-fetch-lookup-store-fetch type of operations to construct the final mesh, and 2. as you do it recurse a matrix hierarchy to assemble the mesh into a single coordinate space. Now, assuming that you've triangulated it all you have a reconstructed mesh you can draw. This can take up to a minute on a modern CPU, for a moderately complex building model [and generate a lot of unnecessary carbon] which is really not acceptible if you're making a user-friendly interface.

I suspect that the original file format design did consider 3d rendering, but not modern 3d rendering. It would have lent itself to OpenGL 1.1/2.1 rendering, which had fixed-function blocks that could render quads. That might have been okay for rendering all the different prefab bits separately with begin() and end() commands, with the vertices pumped in one-by-one, and told to draw as quads. Neither of these concepts are available now; and in fact, it is impractically slow to draw a whole building like this in real-time (it has been tried). We need to pack the whole mesh into a single array of triangles, and draw the whole thing in parallel on the GPU. Hence why all of the parsing and assembly is a problem. Putting it all together means that we've lost a lot of the ability to individually highlight or tinker with the separate pieces in a flexible way. I have some ideas for how to do this, but it must really be tailored to the individual end-use scenario - unfortunately it's just not efficient to do this in a fully-flexible way. Modern GPU design just isn't good at that sort of thing.


My initial software didn't use the IFC files. It turns out that these are usually not the primary format being worked on my architects, but rather they'll use a typical CAD design programme, and convert into IFC later. What we did was grab the original CAD files and export an XML file, then parse that. Considerably faster, but still not ideal. On a range of devices you really want something that can be interpreted without XML; a JSON format for WebGL, and a plain-text C-ready format for everything else, so a better solution would be to convert to a specialised graphics format. Most CAD software should be able to generate a Wavefront .obj static mesh, which is fairly trivial to parse, but an ideal solution would treat the IFC file as the source, because it's becoming a standard sort of thing, and convert that to a custom graphics format. A pre-processing stage is okay, and then you don't need to keep a second file with the IFC file just for graphics - it can be generated as needed.

Converter Attempt 1

A couple of posts ago I made a general purpose converter that took just about any format, and output to my new custom format. It turns out that this should be able to do the painful job of constructing a mesh from IFC as well, and stripping out all the non-graphics bits. I gave it a go with one of Karlsruhe IT's sample IFC meshes:

Converting a sample IFC mesh into a custom format sort-of worked.

The output mesh is massively simplified into a long array of points forming what should be triangles. The holes and incorrect link-ups in the viewer look like some of the mesh might have been exported in quads of 4 points (hence the holes because it was expecting triangles), and some in triangles (the cars are all correct). I guess the triangulation wasn't consistent. Perhaps I can load it into Blender first to check that out.

The nice thing about having a custom converter is that you have the option of adding your own annotations to different parts of the mesh, for example:

An observation about 3d architectural plans is that they aren't very efficient, as a rule - the architects will add more polygons to minutely detail things like stair rails, or bayonette lamp fittings, that basically aren't at all important in the context of the whole scene. You can see in the image above that some completely flat areas have been sub-divided into large numbers of polygons for no reason at all. This means that we have been doing a lot of manual clean-up-and-tidy work on architectural plans before we visualise them. Some of this could potentially be automated, but any automation will inevitably introduce some error. Identifying highly-detailed components and separating them from the main mesh might be an idea worth considering. Then the user could tick check-boxes to display these things i.e. be lazy and let the user decide if it's important to display stair rails or not. I suspect that a better idea would be some sort of indication to the actual designers at the CAD level, or a "can I flatten this area into a single quad?" prompt-based optimisation script that can be run before exporting.

I'm investigating this topic as part of a wider range of visualisation topics for the EU Knoholem FP7 research project.