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Quasi-random, more or less unbiased blog about real-time photorealistic GPU rendering

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    The following video accompanies the screenshots from the previous post:

     

    It shows 1024 dynamic instances of a Stanford dragon model of 100k triangles each, for a total of 100 million triangles, path traced with Brigade in real-time. Brigade easily allows path tracing of several billion triangles in real-time (we tried 4096 randomly spinning instances of the Asian city model which contains over 400k triangles, and the framerate was surprisingly fluent). This is impossible with OpenGL or DirectX. This is the prime reason why games must switch to a pure ray tracing or path tracing approach (hybrid raster/ray tracing will just not work) if game developers want to have scenes with several hundred million triangles filling the screen. And that's not even taking the perfect soft shadows, depth of field, reflections, refractions, arbitrary BRDFs and color bleeding into account.

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    A new test with the Brigade path tracer, showing 1024 physics driven dynamic cubes in a street scene. It's incredibly fun to fly through a photoreal scene in real-time while tons of cubes are falling from the sky in slow motion and being able to change all the lighting and materials at the same time. The beauty of how instancing works in Brigade is that moving hundreds or even thousands of rigid objects happens for free, with almost no impact on the rendering performance. Brigade also doesn't care much about how many polygons these rigid objects contain: a 100K poly Stanford dragon will render nearly as fast as 3K poly Utah teapot. This opens up a lot of possibilities: you could for example render scenes with hundreds of spaceships flying around in an extremely detailed procedurally generated landscape. Dealing with non-rigid meshes like animated characters is a bit harder for a path tracer, because acceleration structures need to be updated every frame, but we have a solution to that problem as well (that's something for another post :) In the meantime, enjoy the video and screenshots below:

    720p video:



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    Not that many updates lately, during the last two months we've been working on a kick-ass Brigade tech demo that will be shown on a conference in the second half of this month. I'll post screenshots and videos posted after the event, but in the meantime we're still doing some smaller tests, such as this as animated character instance test. The video and screenshots are rendered with Brigade's superfast path tracing kernel (maxdepth 6) and show 12 instanced characters animated in real-time. 

     
     
                 
    Update: Some teaser images of a new scene, rendered in real-time with very nice color bleeding and real-time post processing:


    More to come soon

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    After the successful presentation at Siggraph 2012, Brigade will rear its ugly ahead again in public next week. This time it's Nvidia's GTC conference, where OTOY will have a 50 minute talk on Tuesday, showing off Brigade, Octane Render and LightStage. Make sure to be there if you want to see the scene below running in real-time:


    It's going to simultaneously blow your mind and socks off your feet (I'm still not completely recovered from the breath taking beauty) and you will regret it for the rest of your life if you won't be there. However, in the unlikely case that you can't make it to the conference, do not despair, because I'll post tons of screenshots and videos when the event is over. 



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    With the GTC almost over, we can finally unveil what we've been working on for the past seven months. Brigade has made massive progress during that time in all areas: performance, quality, sampling efficiency and tremendously improved support for dynamic scenes and multi-GPU setups.
    The level of realism in Brigade is just absurd and playing with it often feels like you're watching a real life movie.

    We love playing GTA, so we set out to make a real-time path traced GTA like demo to be shown at Nvidia's GTC conference (and also next week at the GDC). The plan was to have hundreds of cars and pedestrians populating a living and breathing city, all path traced in real-time. A rather ambitious goal, since path tracing this kind of highly dynamic scenes in real-time was never done before, but do we love a challenge! After doing successful tests with hundreds of moving cars and characters in a city environment, we added physics, which slowed things down massively so we had to settle on just one car. Brigade was blowing our minds, time and time again, it renders monstrously fast.

    The video below made by Hayssam Keilany shows some of our tests rendered at 1280x72:

    - the city scene has 750 instanced animated characters (30k triangles each, in total 22.5 million animated triangles), all of them physics driven with Bullet physics in a 600k triangle city

    - the Piazza scene is fantastic to test color bleeding, there are 16384 instances of a 846k triangle city, 13.8 billion triangles in total, rendered in real-time

    - interior scene from Octane Render, created by Enrico Cerica, 1 million triangles rendered in real-time



    We will post screenshots and a lot more videos after the GDC.

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    To celebrate Siggraph, here's a new video of Brigade for your enjoyment, showing ananimated character mesh consisting of 125k dynamic triangles rendered in real-time at 35 fps with path tracing (the static background contains 600k triangles).

    To give some background: one of the main reasons why ray tracing has not been considered a viable alternative for rasterization as a rendering technique for  games is because ray tracing requires an acceleration structure to achieve real-time performance and dynamic scene support requires that acceleration structure to be rebuilt or updated every frame which has been a long standing and often revisited problem in the ray tracing research community.

    Until a few weeks ago, Brigade was capable of handling about 50k (non-instanced) dynamic triangles at 30 frames per second. Recently however, the dynamic triangle budget was tripled and we can now do around 150k triangles at 30 frames per second (and this will soon increase further to a dazzling 1 milllion dynamic triangles at 30 fps), which allows for some extremely detailed deformable meshes like characters. VFX houses doing previs of real-time motion captured characters will love this.

    UPDATE: Updated the post with a fresh batch of screenshots to show the extreme texture detail on the LightStage model.

    HD video (rendered at 720p):http://www.youtube.com/watch?&v=EgMy5dqAl_U


    Note the huge difference diffuse color bleeding makes on the character's body when the floor is matte in the next two screenshots: 

    The entire movie industry is going down the physically based rendering path with path tracers like Arnold. Recently even Pixar/Disney went with full path tracing for Monsters University and completely reworked their old Renderman renderer by adding a path tracing mode. The benefits of progressive rendering with physically based global illumination and materials without having to rely on time consuming point cloud baking has entirely revolutionized the way artists work as it's a game changer for the creative process. Games will eventually follow this path as well as game developers keep striving for cinema quality graphics as they've been doing since the introduction of the first OpenGL accelerator boards. And if you're still not convinced of the undeniable superiority of path tracing after all this fluff, you can talk to this nicely textured hand:

     

    Btw, in case you haven't noticed yet, we dramatically improved the lighting quality and sky model in Brigade over the past months and it's now almost up to Octane standards.

    More tests to come soon.

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    For the first time at QuakeCon, John Carmack showed off his talents as a lecturer as he gave a one hour talk on the Physics of Light and Rendering, which covered everything from rasterization to ray tracing and also path tracing. As the pioneer and godfather of 3D first person shooter games and the creator of the most popular 3D game engine of all time (the Quake 3 engine which is still being used in heavily modified form in all of the Call of Duty games) Carmack is still regarded as the highest authority on real-time game graphics, which makes this lecture on physically based rendering (a rendering method which until recently was strictly reserved for the academic research community and for offline production rendering) both surprising and remarkable. And when he divulges his view on the future of game graphics, you can be sure his ideas will be copied by every other game developer.

    The recorded talk can be seen here: http://www.youtube.com/watch?v=P6UKhR0T6cs


    Some interesting tidbits:

    - while Carmack believed that voxels were the way forward to do efficient ray tracing in the past (see Jon Olick's SVO research for id Software at Siggraph 2008), but today he thinks ray tracing triangles is the most efficient way and all ray tracing will be against triangles, because most content creation tools are built around triangle meshes

    - Carmack thinks that path tracing will be the way forward for all kinds of rendering, offline and real-time and the whole industry is moving towards it, because it's easier and much more accurate to create immersive lifelike graphics for movies and games with accurate global illumination, shadows and reflections

    - Brigade gets briefly mentioned at around 1:04:00. Did a quick transcript of the Brigade fragment:
    "[...] It does seem likely that the path forward is lots and lots of rays, physically accurate material definitions and approaches that are approximations of the sampling of path tracing. We can do, there are some neat demos going around today, like the Brigade path tracing demo which is real-time and it's doing simple path tracing from a parallel outdoor light and it's noisy and fizzly as it comes in, but you can stop and watch it come in more crisply. And eventually this is going to be the way things go, this is going to be the way we're gonna be rendering, but we still have maybe a couple of magnitudes before it's really competitive. I think one more order of magnitude in performance and you'll start seeing it used for some real things, but still, you have to have a good reason to step away from rasterization. But probably when we get two orders of magnitude, then you start seeing it as one of the more general tools. And the reason that it's winning in the offline world, even though it's still slower, people still care about how long their renderings take even if you're making a feature film or tv commercial, it matters for your iteration time. But the sense is that you get more out of this being understandable. [...]"
    - Carmack is pushing the artists at id Software to adopt a more physically accurate material rendering system

    While I don't fully agree that we need another magnitude of performance (Brigade can path trace outdoor scenes in real-time today with very little noise), it is great to see that path tracing is acknowledged by Carmack as the eventual future not only for offline rendering, but also for real-time game graphics. The future of GPU path tracers like Octane/Brigade has never been brighter.

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    Another real-time path traced test of Brigade with HDR environment lighting, featuring the Stanford bunny and the dancing dude from one of the previous posts. Jeroen van Schijndel (Brigade's lead dev), had the outrageous idea to create an animated instance for every triangle of the Stanford bunny, resulting in 4968 instances. Each jolly dancer contains over 60k triangles, totalling over 300 million dynamic triangles in real-time. 

    720p video: http://www.youtube.com/watch?v=huvbQuQnlq8


    Some screenshots below, notice the real-time color bleeding, reflections, soft shadows and soft realistic lighting from the HDR environment.


    Soft shadows coming only from the HDR environment (no additional lights used):


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  • 10/01/13--17:55: Soon...
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  • 10/08/13--20:22: Sooner...

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    Time for an update on Brigade 3 and what we've been working on: until now, we have mostly shown scenes with limited materials, i.e. either perfectly diffuse or perfectly specular surfaces. The reason we didn't show any glossy (blurry) reflections so far, is because these generate a lot of extra noise and fireflies (overbright pixels) and because the glossy material from Brigade 2 was far from perfect. Over the past months, we have reworked the material system from Brigade and replaced it with the one from OctaneRender, which contains an extraordinary fast converging and high quality glossy material. The sky system was also replaced with a custom physical sky where sky and sun color vary with the sun position.And there's a bunch of brand new custom post effects, tone mapping filters and real camera effects like fish eye lens distortion (without the need for image warping).

    We've had a lot of trouble finding a good way to present the face melting awesomeness that is Brigade 3 in video form and we've tried both youtube and Vimeo at different upload resolutions and samplecounts (samples per pixel). Suffice to say that both sites have ultra shitty video compression, turning all our videos in a blocky mess (although Vimeo is still much better than YT). We also decided to go nuts on glossy materials and fresnel on every surface in this scene, which makes everything look much a lot more realistic (in particular fresnel, which causes surfaces to look more or less reflective depending on the viewing angle), but the downside of this extra realism is a lot of extra noise.

    So feast your eyes on the first videos of Brigade 3 (1280x720 render resolution):


    Another one using an Xbox controller:

    The scene in the video is the very reason why I started this blog five years ago and is depicted in one of my very first blog posts from 2008 (see http://raytracey.blogspot.co.nz/2008/08/ruby-demo.html). The scene was created by Big Lazy Robot to be used in a real-time tech demo for ATI's Radeon HD 4870 GPU. Back then, the scene used baked lightmaps rendered with V-Ray for the diffuse lighting and an approximate real-time ray tracing technique for all reflective surfaces like cars and building windows. Today, more than five years later, we can render the same scene noise free using brute force path tracing on the GPU in less than half a second and we can navigate through the entire scene at 30 fps with a bit of noise (mostly apparent in shadowy areas). When I started this blog my dream was to be able to render that specific scene fully in real-time in photoreal quality and I'm really glad I've come very close to that goal. 

    UPDATE: Screenshot bonanza! No less than 32 screenshots, each of them rendered for 0.5 - 1 second. The problem with Brigade 3 is that it's so much fun mucking around with the lighting, the time of day, depth of field and field of view with lens distortion. Moreover, everything looks so photoreal that it's extremely hard to stop playing and taking screenshots. It feels like you're holding a camcorder.


    We plan to show more videos of Brigade 3 soon, so stay tuned... 

    Update: I've uploaded the direct feed version of the second video to MEGA (a New Zealand based cloud storage service, completely anonymous, fast, no registration required and free, just excellent :). You can grab the file here: brigade3_purely_random_osumness (it's 2.40 GB)

    Update 2: The direct feed version of the first video can be downloade here: brigade3_launch_vid_HD.avi (2.90 GB). This video has a higher samplecount per pixel per frame (and thus less noise and lower framerate).

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    Jacco Bikker just announced a new CUDA based path tracer called Arauna 2 on the ompf forum. There's also a demo version available that you can grab from this post. Seems like the era of real-time path tracers has just begun!

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    This looks incredible, raymarching with GI, glossy road, glossy car: https://www.shadertoy.com/view/ldsGWB

    The future of real-time graphics is noisy!

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    Just want to share a couple of real-time rendered videos made with the upcoming OctaneRender 1.5. The scene used in the videos is the same one that was used for the Brigade 3 launch videos. The striking thing about Octane is that you can navigate through this scene in real-time while having an instant final quality preview image. It converges in just a few seconds to a noise free image, even with camera motion blur enabled. It's both baffling and extremely fun. 

    The scene geometry contains 3.4 million triangles without the Lamborghini model, and 7.4 million triangles with (the Lamborghini alone has over 4 million triangles). All videos below were rendered in real-time on 4 GTX 680 GPUs. Because of the 1080p video capture, the framerate you see in the videos is less than half the framerate you get in real life, it's incredibly smooth. 



    There are a bunch more real-time rendered videos and screenshots of the upcoming OctaneRender 1.5 in this thread on the Octane forum (e.g. on page 7).

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    Recently I was blown away by a video posted by niuq.cam on the Octane forum, called "Lamborghini Test Drive" as a tribute to celebrate the 50h anniversary of Lamborghini. The realism you can achieve with Octane is just batshit crazy as evidenced by the video.

    Try to spot the 7 differences with reality:


    Some specs:

    - the scene is 100% 3D, all rendered with Octane
    - rendered on 4x GTX Titan
    - render resolution 1280 x 538 Panavision format (2,39:1)
    - average rendertime per frame: from 1 minute for the large shots with the cars to 15 minute for the helmet shots by night
    - over 5.000.000 triangles for both cars
    - instances for the landscape.


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    OctaneRender 1.5 has some really powerful features like support for Alembic animations and a fully scriptable user interface. The Alembic file support allows for real-time rendered animations in the standalone version of Octane for scenes with both rigid and deformable animated geometry. Seeing your animations rendered in final quality in real-time with GI, glossy reflections and everything is a blast:


    You may remember the actor in the following video from blockbuster movies like "Ultra high detailed dynamic character test" and "4968 daftly dancing dudes on Stanford bunny". Even in Octane, his dancing prowess remains unrivalled. The actor is made up of 66k triangles and there are 730 clones of him (48 million triangles in total). For every frame of the animation, Octane loads the animated geometry from an Alembic file, builds the scene and renders the animation sequence with a script, all in real-time. 



    Some examples of rigid body animations:




    With support for Alembic animations and Lua scripting, Octane has now a very solid foundation for animation rendering in place, allowing for some very cool stuff (yet to be announced) that can be done fully in real-time on a bunch of GPUs (inspired and fueled by earlier Brigade experiments). In 2014, Octane will blow minds like never before.

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    Today, the GPU Pro blog posted a very interesting article about a novel technique that seemlessly unifies rasterization and ray tracing based rendering for fully dynamic scenes. The technique entitled "Object-order Ray Tracing for Fully Dynamic Scenes" will be described in the upcoming GPU Pro 5 book (to be released on March 25, 2014 during the GDC conference)  and was developed by Tobias Zirr, Hauke Rehfeld and Carsten Dachsbacher .  

    Abstract (taken from http://cg.ibds.kit.edu/ORTFDS.php)
    This article presents a method for tracing incoherent secondary rays that integrates well with existing rasterization-based real-time rendering engines. In particular, it requires only linear scene access and supports fully dynamic scene geometry. All parts of the method that work with scene geometry are implemented in the standard graphics pipeline. Thus, the ability to generate, transform and animate geometry via shaders is fully retained. Our method does not distinguish between static and dynamic geometry. Moreover, shading can share the same material system that is used in a deferred shading rasterizer. Consequently, our method allows for a unified rendering architecture that supports both rasterization and ray tracing. The more expensive ray tracing can easily be restricted to complex phenomena that require it, such as reflections and refractions on arbitrarily shaped scene geometry. Steps in rendering that do not require the tracing of incoherent rays with arbitrary origins can be dealt with using rasterization as usual.

    This is by my knowledge the first practical implementation of the so-called hybrid rendering technique which mixes ray tracing and rasterization by plugging a ray tracer in an existing rasterization based rendering framework and sharing the traditional graphics pipeline. Since no game developer in his right mind will switch to pure ray tracing overnight, this seems to be the most sensible and commercially viable approach to introduce real ray traced high quality reflections of dynamic objects into game engines in the short term, without having to resort to complicated hacks like screen space raytracing for reflections (as seen in e.g. Killzone Shadow Fall, UE4 tech demos and CryEngine) or cubemap arrays, which never really look right and come with a lot of limitations and artifacts. For example, in this screenshot of the new technique you can see the reflection of the sky, which would simply be impossible with screen space reflections from this camera angle:  


    Probably the best thing about this technique is that it works with fully dynamic geometry (accelerating ray intersections by coarsely voxelizing the scene) and - judging from the abstract - with dynamically tesselated geometry as well, which is a huge advantage for DX11 based game engines. It's very likely that the PS4 is capable of real-time raytraced reflections using this technique and when optimized, it could not only be used for rendering reflections and refractions, but for very high quality soft shadows and ambient occlusion as well. 

    The ultimate next step would be global illumination with path tracing for dynamic scenes, which is a definite possibility on very high end hardware, especially when combined with another technique from a very freshly released paper (by Ulbrich, Novak, Rehfeld and Dachsbacher) entitled Progressive Visibility Caching for Fast Indirect Illumination which promises a 5x speedup for real-time progressively path traced GI by cleverly caching diffuse and glossy interreflections  (a video can be found here). Incredibly exciting if true!

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  • 10/19/14--17:10: Scratch-a-pixel and more
  • Having left Otoy some time ago and after enjoying  a sweet as holiday, it's time for things new and exciting. Lots of interesting rendering related stuff happened in the past months, below are some of the most fascinating developments in my opinion:

    - starting off, there's an excellent online tutorial series on computer graphics (mostly ray tracing) for both beginners and experts called Scratch-a-Pixel. The authors are veterans from the VFX, animation and game industry and have years of experience writing production rendering code like Renderman. The tutorials deal with all the features that are expected from a production renderer and contains a lot of background and insights into the science of light and tips and tricks on how to write performant and well optimized ray tracing code. Rendering concepts like CIE xyY colorspace and esoteric mathematical subjects like discrete Fourier transforms, harmonics and integration of orthonormal polynomials are explained in an easy-to-digest manner. Most tutorials also come with C++ source code. At the moment some sections are missing or incomplete, but the author told me there's a revamp of the website coming very soon... 

    - hybrid rendering (rasterization mixed with ray tracing) for games has finally arrived with the recent release of Unreal Engine 4.5 which supports ray traced soft shadows and ambient occlusion via signed distance fields (which can be faster to compute than traditional shadow mapping, but works only for static geometry): https://docs.unrealengine.com/latest/INT/Engine/Rendering/LightingAndShadows/RayTracedDistanceFieldShadowing/index.html 


    A nice video of the technique in action: http://www.youtube.com/watch?v=4249b94KtyA
    Like voxels or triangles, distance fields are another way to represent scene geometry. Just like voxels, distance fields approximate the scene geometry and are more efficient to trace than triangles to create low frequency effects like soft shadows, ambient occlusion and global illumination that don't require 100% geometric accuracy (and because they have inherent multiresolution characteristics by approximating the scene geometry). Inigo Quilez wrote a few interesting articles on rendering with distance fields (in 2008):

    Free penumbra shadows for raymarching distance fields

    More on distance fields:
    Distance fields in Unreal Engine
    Alex Evans from Media Molecule invented a neat trick to approximate AO and GI with distance fields in "Fast Approximations for Global Illumination for Dynamic Scenes"

    There's also  a very recent paper about speeding up sphere tracing for rendering of signed distance fields or path tracing: Enhanced sphere tracing 

    - one of the most interesting Siggraph 2014 surprises, must be the announcement from Weta (the New Zealand based visual effects studio that created the CG effects for blockbusters like the Lord of the Rings, King Kong, Avatar, Tintin and the Hobbit movies) that they are developing their own production path tracer called Manuka (the Maori name for New Zealand's healing tea tree) in conjunction with Gazebo, a physically plausible realtime GPU renderer. While Manuka has been used to render just a couple of shots in "The Hobbit: the Desolation of Smaug", it will be the main renderer for the next Hobbit film. More details are provided in this extensive fxguide article: http://www.fxguide.com/featured/manuka-weta-digitals-new-renderer/ Another surprise was Solid Angle (creators of Arnold) unveiling of an OpenCL accelerated renderer prototype running on the GPU. There's not much info to be found apart from a comment on BlenderArtists.org by Solid Angle's Mike Farnsworth ("This is a prototype written by another Solid Angle employee (not Brecht), and it is not Arnold core itself. It's pretty obvious we're experimenting, though. We've been keeping a close eye on GPUs and have active communication with both AMD and Nvidia (and also obviously Intel). I wouldn't speculate on what the prototype is, or what Brecht is up to, because you're almost certainly going to be wrong.")

    - Alex St John, ex-Microsoft and one of the creators of DirectX API, has recently moved to New Zealand and aims to create the next standard API for real-time graphics rendering using CUDA GPGPU technology. More details on his blog  http://www.alexstjohn.com/WP/blog/. His post on his visit to Weta contains some great insights into the CUDA accelerated CG effects created for The Desolation of Smaug. 

    - Magic Leap, an augmented reality company founded by a biomedical engineer, recently got an enormous investment from Google and is working with a team at Weta in New Zealand to create imaginative experiences. Info available on the net suggests they are developing a wearable device that directly projects 3d images onto the viewer's retina that seemlessly integrate with the real-life scene via projecting multiple images with a depth offset. Combined with Google Glass it could create games that are grounded in the real world like this: http://vimeo.com/109214393 (augmented reality objects are rendered with Octane Render). 

    - the Lab for Animate Technologies at the University of Auckland in New Zealand is doing cutting edge research into the first real-time autonomously animated AI avatar: http://vimeo.com/97186687 
    The facial animation is driven in real-time by artificial intelligence using concepts from computational neuroscience and is based on a physiological simulation of the human brain which is incredibly deep and complex (I was lucky to get a behind the scenes look): it includes the information exchange pathways between the retina, the thalamic nuclei and the visual cortex including all the feedback loops and also mimics low level single neuron phenomena such as the release of neurotransmitters and hormones like dopamine, epinephrine and cortisol. All of these neurobiological processes together drive the avatar's thoughts, reactions and facial animation through a very detailed facial muscle system, which is probably the best in the industry (Mark Sagar, the person behind this project, was one of the original creators of the USC Lightstage and pioneered facial capturing and rendering for Weta in King Kong and Avatar). More info on http://thecreatorsproject.vice.com/blog/baby-x-the-intelligent-toddler-simulation-is-getting-smarter-every-day and https://www.youtube.com/watch?v=tfACJcgCGv0. One of the most impressive things I've ever seen and it's something that is actually happening now. 

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    With last month's unveiling of Microsoft's augmented reality glasses project dubbed "HoloLens", today's announcement of Sony's plans to release a similar AR device called the "SmartEyeGlass", and with more details surfacing on Magic Leap's retina projecting fiber optic AR glasses (cleverly reconstructed from publicly available patents by a Gizmodo journalist), the hype around augmented reality seems to be reaching a peak at the moment. Unfortunately, most of the use cases for these technologies that have been demonstrated so far, for example husbands assisting their wives with screwing a new syphon on a sink, projecting the weather forecast for Maui on the kitchen wall or casually investigating a suspicious rock on the surface of Mars, look either gimmicky, far-fetched or both. 

    The area where I see a real and immediate use for these high tech AR devices is in the operating room. In my previous life as a medical student, I've spent quite some time in the operating theatre watching surgeons frantically checking if they were cutting the right part of the brain by placing a sharp needle-like pointer (with motion capture dots) on or inside the brain of the patient. The position of the pointer was picked up by 3 infrared cameras and a monitor showed the position of the needle tip in real-time on three 2D views (front, top and side) of the brain reconstructed from CT or MRI scans. This 3D navigation technique is called stereotactic neurosurgery and is an invaluable tool to guide neurosurgical interventions.    

    Instruments for stereotactic surgery (from here)


    While I was amazed at the accuracy and usefulness of this high tech procedure, I was also imagining ways to improve it, because every time the surgeon checks the position of the pointer on the monitor, he or she loses visual contact with the operating field and "blindly" guiding instruments inside the body is not recommended. A real-time three-dimensional augmented reality overlay that can be viewed from any angle, showing the relative position of the organs of interest (which might be partially or fully covered by other organs and tissues like skin, muscle, fat or bone) would be tremendously helpful provided that the AR display device minimally interferes with the surgical intervention and the augmented 3D images are of such a quality that they seamlessly blend with the real world. The recently announced wearable AR glasses by MS, Sony and Magic Leap seem to take care of the former, but for the latter there is no readily available solution yet. This is where I think real-time ray tracing will play a major role: ray tracing is the highest quality method to visualise medical volumetric data reconstructed from CT and MRI scans. It's actually possible to extend a volume ray caster with physically accurate lighting (soft shadows, ambient occlusion and indirect lighting) to add visual depth cues and have it running in real-time on a machine with multiple high end GPUs. The results are frighteningly realistic. I for one can't wait to test it with one of these magical glasses.

    As an update to my previous post, the people behind Scratch-a-Pixel have launched a v2.0 website, featuring improved and better organised content (still work in progress, but the old website can still be accessed). It's by far the best resource to learn ray tracing programming for both novices (non engineers) and experts. Once you've conquered all the content on Scratch-a-Pixel, I recommend taking a look at the following ray tracing tutorials that come with source code:

    - smallpt from Kevin Beason: an impressively tiny path tracer in 100 lines of C++ code. Make sure to read David Cline's slides which explain the background details of this marvel. 

    - Rayito, by Mike Farnsworth from Renderspud (currently at Solid Angle working on Arnold Render): a very neatly coded ray/path tracer in C++, featuring path tracing, stratified sampling, lens aperture (depth of field), a simple BVH (with median split), Qt GUI, triangle meshes with obj parser, diffuse/glossy materials, motion blur and a transformation system. Not superfast because of code clarity, but a great way to get familiar with the architecture of a ray tracer

    -  Renderer 2.x: a CUDA and C++ ray tracer, featuring a SAH BVH (built with the surface area heuristic for better performance), triangle meshes, a simple GUI and ambient occlusion

    - Peter and Karl's GPU path tracer: a simple, but very fast open source GPU path tracer which supports sphere primitives, raytraced depth of field and subsurface scattering (SSS)
       
    If you're still not satisfied after that and want a deeper understanding, consider the following books:
      
    - "Realistic ray tracing" by Peter Shirley, 
    - "Ray tracing from the ground up" by Kevin Suffern, 
    - "Principles of Digital Image Synthesis" by Andrew Glassner, a fantastic and huge resource, freely available here, which also covers signal processing techniques like Fourier transforms and wavelets (if your calculus is a bit rusty, check out Khan academy, a great open online platform for engineering level mathematics)
    - "Advanced global illumination" by Philip Dutré, Kavita Bala and Philippe Bekaert, another superb resource, covering finite element radiosity and Monte Carlo rendering techniques (path tracing, bidirectional path tracing, Metropolis light transport, importance sampling, ...)

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    A very interesting paper called "Gradient domain path tracing" was just published by Nvidia researchers (coming from the same incredibly talented Helsinki university research group as Timo Aila, Samuli Laine and Tero Karras who developed highly optimized open source CUDA ray tracing kernels for Tesla, Fermi and Kepler GPUs), describing a new technique derived from the ideas in the paper Gradient domain Metropolis Light Transport, which drastically reduces noise without blurring details. 
    Abstract 
    We introduce gradient-domain rendering for Monte Carlo image synthesis. While previous gradient-domain Metropolis Light Transport sought to distribute more samples in areas of high gradients, we show, in contrast, that estimating image gradients is also possible using standard (non-Metropolis) Monte Carlo algorithms, and furthermore, that even without changing the sample distribution, this often leads to significant error reduction. This broadens the applicability of gradient rendering considerably. To gain insight into the conditions under which gradient-domain sampling is beneficial, we present a frequency analysis that compares Monte Carlo sampling of gradients followed by Poisson reconstruction to traditional Monte Carlo sampling. Finally, we describe Gradient-Domain Path Tracing (G-PT), a relatively simple modification of the standard path tracing algorithm that can yield far superior results. 
    This picture shows a noise comparison between gradient domain path tracing (GPT) and regular path tracing (PT). Computing a sample with the new technique is about 2.5x slower, but path tracing noise seems to clear up much faster, far outweighing the computational overhead: 

    More images and details of the technique can be found in https://mediatech.aalto.fi/publications/graphics/GPT/kettunen2015siggraph_paper.pdf

    Related to the previous post about using real-time ray tracing for augmented reality, a brand new Nvidia paper titled "Filtering Environment Illumination for Interactive Physically-Based Rendering in Mixed Reality" demonstrates the feasibility of real-time Monte Carlo path tracing for augmented or mixed reality: 
    Abstract 
    Physically correct rendering of environment illumination has been a long-standing challenge in interactive graphics, since Monte-Carlo ray-tracing requires thousands of rays per pixel. We propose accurate filtering of a noisy Monte-Carlo image using Fourier analysis. Our novel analysis extends previous works by showing that the shape of illumination spectra is not always a line or wedge, as in previous approximations, but rather an ellipsoid. Our primary contribution is an axis-aligned filtering scheme that preserves the frequency content of the illumination. We also propose a novel application of our technique to mixed reality scenes, in which virtual objects are inserted into a real video stream so as to become indistinguishable from the real objects. The virtual objects must be shaded with the real lighting conditions, and the mutual illumination between real and virtual objects must also be determined. For this, we demonstrate a novel two-mode path tracing approach that allows ray-tracing a scene with image-based real geometry and mesh-based virtual geometry. Finally, we are able to de-noise a sparsely sampled image and render physically correct mixed reality scenes at over 5 fps on the GPU.

    While Nvidia is certainly at the forefront of GPU path tracing research (with CUDA), AMD has recently begun venturing into GPU rendering as well and has previewed its own OpenCL based path tracer at the Siggraph 2014 conference. The path tracer is developed by Takahiro Harada, who is a bit of an OpenCL rendering genius. He recently published an article in GPU Pro 6 about rendering on-the-fly vector displacement mapping with OpenCL based GPU path tracing. Vector displacement mapping differs from regular displacement mapping in that it allows the extrusion of overlapping geometry (eg a mushroom), which is not possible with the heightfield-like displacement provided by traditional displacement (the Renderman vector displacement documentation explains this nicely with pictures).

    Slides from http://www.slideshare.net/takahiroharada/introduction-to-monte-carlo-ray-tracing-opencl-implementation-cedec-2014:


    This video shows off the new technique, rendering in near-realtime on the GPU:

    There's more info on Takahiro's personal page, along with some really interesting slideshow presentations about OpenCL based ray tracing. This guy also developed a new technique called "Foveated real-time ray tracing for virtual reality devices" (paper), progressively focusing more samples on the parts in the image where the eyes are looking (determined by eye/pupil tracking), "reducing the number of pixels to shade by 1/20, achieving 75 fps while preserving the same visual quality" (source: http://research.lighttransport.com/foveated-real-time-ray-tracing-for-virtual-reality-headset/asset/abstract.pdf). Foveated rendering takes advantage of the fact that the human retina is most sensitive in its center (the "fovea", which contains densely packed colour sensitive cones) where objects' contours and colours are sharply observed, while the periphery of the retina consists mostly of sparsely distributed, colour insensitive rods, which cause objects in the periphery of the visual field to be represented by the brain as blurry blobs (although we do not consciously perceive it like that, thinking that our entire visual field is sharply defined and has colour).
    This graph shows that the resolution of the retina is highest at the fovea and drops off quickly with increasing distance from the center. This is due to the fact that the fovea contains only cones which each send individual inputs over the optic fibre (maximizing resolution), while the inputs from several rods in the periphery of the retina are merged by the retinal nerve cells before reaching the optic nerve (image from www.telescope-optics.net/eye.htm):

    Foveated rendering has the potential to make high quality real-time raytraced imagery feasible on VR headsets that support eye tracking like the recently Kickstarted FOVE VR headset. Using ray tracing for foveated rendering is also much more efficient than using rasterisation: ray tracing allows for sparse loading and sampling of the scene geometry in the periphery of the visual field, while rasterisation needs to load and project all geometry in the viewplane, whether it's sampled or not.


    Slides from www.slideshare.net/takahiroharada/foveated-ray-tracing-for-vr-on-multiple-gpus

    This video shows a working prototype of the FOVE VR headset with a tracking beam to control which parts of the scene are in focus, so this type of real-time ray traced (or path traced) foveated rendering should be possible right now, (which is pretty exciting):


    It's good to finally see AMD stepping up its OpenCL game with its own GPU path tracer. Another example of this greater engagement is that AMD recently released a large patch to fix the OpenCL performance of Blender's Cycles renderer on AMD cards. Hopefully it will put some pressure on Nvidia and make GPU rendering as exciting as in 2010 with the release of the Fermi GPU, a GPGPU computing monster which effectively doubled the CUDA ray tracing performance compared to the previous generation. 

    Rendering stuff aside, today is a very important day: for the first time in their 115 year long existence, the Buffalo's from AA Gent, my hometown's football team, have won the title in the Belgian Premier League, giving them a direct ticket to the Champions League. This calls for a proper celebration!

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