On Feb 19, 2015, at 12:14 AM, Vasco Alexandre da Silva Costa <vasco.co...@gmail.com> wrote:
> I was reading your mailing-list archives and noticed you guys are interested > in a real-time RT visualization mode. I am currently doing my PhD on that > topic and it just so happens that I ended up bumping into BRL-CAD. I have not > made a code review to see what needs to be done. Hello and welcome! Glad to see the interest. If you need help finding your way around the code, please don’t hesitate to ask here or on our IRC channel (if you’re willing to wait for a reply). The ray tracing is almost entirely concentrated into two directories: src/rt for the front-end ray dispatcher and src/librt containing the rest. Digging down into src/librt/primitives is where you’ll find individual entity logic (the familiar shot/intersection functions for example). > AFAIK BRL-CAD uses CSG. So I assume you guys use a classical RT approach > where you have a bunch of primitive types and support complex object > hierarchies. It’s a hybrid representation system with different intersection behavior based on the geometry representation type (implicit, triangle, polygonal, NURBS, volumetric, etc), but you basically have it right. We do support Boolean combinations of any solid entity type against any other. This involves calculating ray intersections and then “weaving” segment partitions together based on the CSG operations. All highly expressive with modern coherency and vectorization techniques, but a rather woefully unexplored area of research within the academic community. > Most modern real-time RT systems only support triangles as a primitive for > code simplification and performance reasons. It is not impossible to support > multiple primitive types but I do not know how good the performance would be > in a system like that. Is your OpenGL backend (I'm assuming you have one for > the modeling stage) totally different? Ever since real-time RT really hit the research scene, circa 2002-2004 with Ingo’s thesis and Reschetov’s tracer, I’ve strongly asserted that the exact same techniques can apply to a primitive-based system like we have in BRL-CAD and that the results would be even more impressive. We’ve just not had anyone try. The downside is as you describe — that you have to tune a dispatcher, a dozen intersection algorithms, a lighting model, and a boolean weaver instead of just a dispatcher, one intersection routine, and a lighting model. This is not intractable difference though. In fact, one of the tenants of real-time is actually encouraged by using implicit geometry with CSG operations — data coherency. Implicit CSG geometry requires approximately two orders of magnitude less data to express the same shape compared to a roughly equivalent triangle mesh. CSG implicits are also, on average, spatially larger than your average triangle and often with fewer intersection calculations required (e.g., all the quadratics — cones, cylinders, ellipsoids, etc). Cumulatively, this means that not only should the same real-time performance improvements be realizable, but the results will likely exceed triangle tracing. For what it’s worth, as a mildly related aside, it’s fun to note that BRL-CAD’s ray tracing library is actually credited [1] as being the world’s *first* real time ray tracing implementation. Circa 1987, librt and our remrt/rtsrv tools achieved 5-10fps rendering across a network of ridiculously slow machines by today’s standards. :) Cheers! Sean [1] http://en.wikipedia.org/wiki/Ray_tracing_(graphics)#In_real_time ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=190641631&iu=/4140/ostg.clktrk _______________________________________________ BRL-CAD Developer mailing list brlcad-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/brlcad-devel
