Category Archives: Miscellaneous

HPG 2009 – a closer look

When discussing things to do and see at SIGGRAPH, it is important to note the co-located conferences. This year, SIGGRAPH is co-located with the Eurographics Symposium on Sketch-Based Interfaces and Modeling (SBIM), the Symposium on Computer Animation (SCA), Non-Photorealistic Animation and Rendering (NPAR), and High-Performance Graphics (HPG). SCA has had good animation papers over the years, and is of interest to many game graphics programmers. NPAR is also a good conference for anyone interested in stylized rendering. In this post I will focus on HPG, which is a new conference formed out of the merger of the venerable Graphics Hardware conference, and the newcomer Symposium on Interactive Ray Tracing.

HPG is a three-day conference; the first two days are just before SIGGRAPH, and the third overlaps the first day of SIGGRAPH (unfortunately conflicting with the excellent SIGGRAPH course, Advances in Real-Time Rendering in 3D Graphics and Games).

HPG has managed to line up two pretty amazing keynotes. The first one is by Larry Gritz on film production rendering. Larry is a legend in the field; he was with Pixar from the Toy Story days, and co-wrote one of the most well-regarded books on Renderman. He since worked on several important renderers (BMRT, Gelato), and is now at Sony Pictures Imageworks. The second keynote is by Tim Sweeney, on the future of GPUs. As the outspoken chief architect of Epic’s Unreal Engine, Tim should need no introduction.

At the end of the conference, the two keynote speakers are joined by Steve Parker (NVIDIA) and Aaron Lefohn (Intel) for a panel on high-performance graphics in 7 year’s time.

HPG also has posters and “Hot 3D” systems presentations (hardware manufacturers talking about their latest designs). Inexplicably, although the acceptance deadline for both has long since passed, the content of neither of these is listed on the conference website yet.

I briefly discussed HPG papers in a previous post, but then only paper titles were available, making it hard to judge relevance; now many of the papers have preprints linked from Ke-Sen Huang‘s HPG 2009 papers page.

Some of the papers look relevant to current or near-future work. There are two interesting antialiasing papers: Morphological Antialiasing was covered by Eric in a recent post. The other antialiasing paper (A Directionally Adaptive Edge Anti-Aliasing Filter) does not have a preprint, but the title is promising. It is notable that one of the authors on this paper (Jason Yang) is listed as a speaker at the SIGGRAPH Advances in Real-Time Rendering in 3D Graphics and Games course; perhaps he will discuss the paper there. Although the NVIDIA paper Image Space Gathering has no preprint (yet), some information on this technique was disclosed at GDC: it involves rendering reflections and shadows into 2D buffers and then performing cross bilateral filters to mimic glossy reflections and soft shadows. I have seen similar techniques used in games, so it will be interesting to hear NVIDIA’s take on this concept. Another promising paper title: Scaling of 3D Game Engine Workloads on Modern Multi-GPU Systems.

The paper Parallel View-Dependent Tessellation of Catmull-Clark Subdivision Surfaces deals with tessellation using GPGPU methods rather than the DX11 tessellation pipeline; I’m not an expert in this area so it’s hard for me to judge, but it might be of interest for people working in this field.

I’m a bit skeptical of depth peeling techniques in general, but recent work in this area has shown some promise. The paper Bucket Depth Peeling lacks a preprint at this moment, but I look forward to learning more about it at the conference.

I found the title Data-Parallel Rasterization of Micropolygons With Defocus and Motion Blur promising because I am interested in the REYES micropolygon algorithm, and particularly in the way it handles defocus and motion blur effects. The technique presented in this paper appears to be less efficient than the REYES method, except for cases with very high velocity and/or defocus. The paper presents a GPU-efficient version of the REYES algorithm as well as an alternative algorithm which is faster in some cases. One of the authors has a blog post that gives some interesting context for the paper.

The amount of actual graphics hardware papers at the Graphics Hardware conference has been declining for years, which is probably one of the factors that precipitated the conference merger with IRT. This year there is only one paper which is clearly about hardware design: PFU: Programmable Filtering Unit for Mobile Multimedia Applications on Graphics Hardware. It has a fairly self-explanatory title, which is fortunate since it has no preprint available. Texture filtering is the last unassailable bastion of fixed-function hardware; for example, it is the only fixed-function unit in Larrabee. Programmable filtering is an intriguing concept; I look forward to the paper presentation. There is one more paper that may be about hardware (Embedded Function Composition); but the title is a bit opaque and it also has no preprint, so it is hard to be sure.

Despite my claim in the previous blog post, there do indeed appear to be quite a few papers about ray tracing this year: Efficient Ray Traced Soft Shadows using Multi-Frusta Traversal, Understanding the Efficiency of Ray Traversal on GPUs, Faster Incoherent Rays: Multi-BVH Ray Stream Tracing, Accelerating Monte Carlo Shadows Using Volumetric Occluders and Modified kd-Tree Traversal, Selective and Adaptive Supersampling for Real-Time Ray Tracing, Spatial Splits in Bounding Volume Hierarchies, Object Partitioning Considered Harmful: Space Subdivision for BVHs, and A Parallel Algorithm for Construction of Uniform Grids. Another paper, Hardware-Accelerated Global Illumination by Image Space Photon Mapping, combines image-space, GPU-accelerated methods for the initial bounce and final gather with ray-tracing for a complete photon mapping solution.

There are only three “GPGPU” papers this year; two on GPU stream compaction (copying selected elements of an array into a smaller array): Efficient Stream Compaction on Wide SIMD Many-Core Architectures and Stream Compaction for Deferred Shading, and one on computing minimum spanning trees for graphs (Fast Minimum Spanning Tree for Large Graphs on the GPU).

Bokode – clever!

Not directly relevant to real-time rendering (although there might be some tangentially related applications in areas like augmented reality), this SIGGRAPH 2009 paper is just painfully clever. It exploits the phenomenon of bokeh (the large circular blobs that small intense light sources generate in out-of-focus images) to create tiny barcodes that can be seen from a distance by cameras. They put a lenslet over the barcode, so that when viewed in a defocused manner you see a large circular blob – with a sharp image of the barcode in the center!

7 Things for July 26th

Books, books, and more books: I received review copies of two books. Best of Game Programming Gems is as it sounds, certainly cheaper than buying the seven books in this series, no real review needed (look inside). Game Engine Architecture is about just that, how to make a professional-grade game rendering system, from soup to nuts (you can look inside). Eberly’s two books are the previous notable works in this area, but are quite different than this new volume. While they focus almost exclusively on algorithms, this book attempts to cover the whole task of developing an engine: what to use for source control, dealing with memory management and in-game profiling, input devices, SIMD, and many other practical topics. There is also algorithmic coverage of rendering, animation, collision detection and physics, among other areas. Naturally, the amount of information on each area is limited by page count (the book’s a solid 860 pages), but in my brief skim it looks like most of the critical areas and concepts are touched on. You won’t become an expert in any one area from this volume, but it looks like you’ll have some reasonably deep understanding of the elements that go into making a game engine. Quite an impressive work, and I know of nothing else in this area that is so detailed. I hope I get a chance to read it (who am I fooling? Though I do wish I had the time…) – well, at the least, it’s a place I’ll first go if I want to learn about a topic in game development that I know little about. If you’d rather wax nostalgic about great game engines you have known, as well as what the state of the are is, this article is for you (oh, yeah, the author of this new book works at the company that made #3).
Looking around for titles I’d like to look over at SIGGRAPH, I found these: Game Graphics Programming, Programming the Cell Processor: For Games, Graphics, and Computation, Introduction to 3D Game Programming with DirectX 10, Ultimate Game Programming with DirectX, 2nd Ed., Advanced Game Programming, Game Coding Complete. Which all sort of sound the same (except for the Cell book), but I’d be happy to page through each and see if it looks promising.
There’s a worthwhile comparison of average vertex normal computation methods on the MeshLab blog. He gives the nod to Thürmer and Wüthrich’s method. You can try each of the three using MeshLab itself.
Sure, Spore didn’t light the world on fire as many of us hoped, but a lot of cool technology was explored. Chris Hecker has a worthwhile rundown of some of the great stuff they worked on.
There are some surprisingly affordable 3D stereolithography objects available on Shapeways. I bought Spiral Cage (tiny, but impressive, and so cheap), Clematis (looks delicate, but is quite springy), and Gyroid (pricier, but more sizeable and a fun form). It’s great to see so many people exploring such areas; here’s a detailed summary of resources. Even if you never plan on getting involved, the Flickr area dedicated to such techniques is worth a browse.
This one amused me: a cloud computing company had a contest that was meant to show off Ruby and cloud computing strengths. It was won by people brute-forcing the problem with GPUs: 16 used by the first-place winner (plus 117 CPU cores, which had less performance total than the 16 GPUs), 4 by the second. Steve Worley and others talk about the GPU approach on the CUDA forum (his program, shared with the community there, was used to win second place).
I admire the dedication.

The evils of fps

I completely agree with this blog post by Humus on the uselessness of the performance numbers in most rendering papers. This is something that often comes up when reviewing papers. Frames-per-second (fps) numbers are less than useless, since they include extraneous information (the time taken to render parts of the scene not using the technique in question) and make it very difficult to do meaningful comparisons. The performance measurement game developers care about is the time to execute the technique in milliseconds.

Some papers do get it right, for example this one. The authors use milliseconds for detailed performance comparisons, only using fps to show how overall performance varies with camera and light position (which is a rare legitimate use of fps).

7 Things for July 20th

While at SIGGRAPH I like to look at new books at the booths. One you may wish to check out is Graphics Shaders: Theory and Practice, from AK Peters (or just use “Look Inside” on Amazon). I received a review copy and skimmed through it. If you’re interested in programming in GLSL 1.2 (part of OpenGL 2.1), consider looking at this one. A minor problem is that it’s not quite as up-to-date as the Orange Book (now on OpenGL 3.1), but the difference in core concepts between language versions is not large. The Graphics Shaders book is full color and comes with a lot of GLSL code examples. It has a bias towards scientific visualization, though not so much that it neglects the basics. I particularly enjoyed the chapter on noise, as it gave one of the clearest explanations I’ve seen on the differences between various types of basic interactive noise functions. One or two elements in the book are a little weak – the flowcharts for pipelines are often too small and difficult to read, for example – but all in all this looks like a solid contribution to the field. Don’t expect more elaborate effects, e.g., shadows are not touched upon. It does cover the basics, plus some additional topics like image post-processing (not normally covered in texts I’ve seen). One of the authors wrote a nice learning tool for GLSL, glman, free for download. If you find you like this tool, definitely consider the book.

Another book I noticed recently is Fluid Simulation for Computer Graphics. This is a topic I know little about, I was just interested to see that there’s any book at all. It looks pretty equation-filled, so is definitely for the serious practitioner.

Speaking of fluid simulation, Intel has an article on this topic for games. One of the chief strengths of any publication is that its staff makes a decision based on merit as to what is published and what is culled. So, I have to admit to being leery of anything that says, “Sponsored Feature”, as that means editorial review and decision-making are gone. I tend to err on the side of ignoring such articles (there’s plenty to read already). That said, Intel’s had quite a number of these articles recently, including such topics as instancing, ocean fog, FFT’s for image processing, and quite a few on parallelism.

In the “clearing the queue” category of links, I don’t think I ever pointed out this handy page, which presents all AMD/ATI and NVIDIA presentations at GDC 2009.

There’s now a (not very active, but at least it exists) Microsoft DirectX blog.

On the OpenGL front, NVIDIA has introduced bindless graphics to help avoid L2 cache misses. I will be interested to see how APIs evolve, as the elements in the current APIs that are bottlenecks are not so much CPU or GPU limitations as due to the API constructs themselves.

Thing for the day: an advertisement with interesting stippling.

7 Things for July 19th

Seven more:

Michael Abrash has an in-depth article on rasterization on Larrabee. Perhaps a little too in-depth at times; just skim past the assembly instructions. I also found myself asking, “why do that?” – the key is to just keep reading. He tries to make his examples simple and comprehensible, but at the cost of sometimes feeling like they’re oversolving the problem. They aren’t, it’s just that the solution is in fact used in different circumstances in order to be efficient.
SIGGRAPH has an interactive rendering event summary page. This page is more for the art production side of things, though; Naty’s course, talks, and production sessions summaries are more comprehensive and more useful for programmer attendees.
NVIDIA has a number of events they’re involved in at SIGGRAPH 2009. Here’s the list.
I love this sort of madness: a business-card ray tracer that does depth of field.
Accumulated SSAO: the idea of reprojection, of using previous results by finding where they lie on this frame’s view, is one that seems a tad expensive for interactive rendering. It’s hard to know anything about performance and quality from this page, but I thought it was interesting to see.
I mentioned Processing in the last post. Another language-related resource for graphics and game programming is pygame, a set of Python modules for writing games. A friend said he found this system to be pretty great, that he could whip up a fairly involved game idea in a few hours.
Scribblenauts sounds like the coolest game that will ever come out, period. Even if it’s only 1/10th as good as the previews read, it looks to be pretty darn entertaining.

JGT is now the Journal of Graphics, GPU, and Game Tools

The name change for the journal formerly known as Journal of Graphics Tools was announced today. This does not reflect a change in focus; rather, the new name more accurately reflects the existing focus of the journal. The Journal of Graphics Tools was originally conceived as an ongoing successor to the Graphics Gems book series, and it has since been a great place for practical “nuggets” of graphics tech. Many of the best articles were also collected in a recent book.

I recommend subscribing to this journal, but contributing to it is even better. JGT is an excellent choice for any game developer who would like to publish a bit of tech they have created; while it is a fully peer-reviewed and citable publication, the focus is squarely on practical applications. Authors are not expected to spend a lot of time writing up previous research, summary, conclusion, future work, etc. For more information see the online author’s guide.

In the interests of full disclosure, I should note that all three authors of Real-Time Rendering are on the JGT editorial board.

Game Engine Gems CFP

As I mentioned in a previous post, Eric Lengyel is heading up a new project, a book series called “Game Engine Gems”. It turns out that we ran across the website before it was announced (moral: there’s no hiding on the internet). He’s sent out an official call for papers today – see the book’s website for basic information.

I’m posting today to mention a few dates not currently shown on the website (though I expect this will change soon):

August 1 – Final day to submit article proposals
August 15 – Authors notified of acceptance
October 15 – Final day to submit completed articles

Contact Eric for more information.

Computer-modeled stop motion in “Coraline”

This is a bit old, but still cool: the film Coraline (based on a great book by one of my favorite authors) was stop-motion, not CG. However, the facial animation was extremely smooth, due to a unique process. The expressions were modeled by computer, exactly as they would have been for a CG animated feature. Then for each frame, the faces were “printed” out on a “3D printer” (rapid prototyping machine). These tweened “frame faces” were swapped in for each stop-motion frame. CGSociety has an article describing the production pipeline for this film, an intriguing combination of CG and stop-motion animation.

Left-Handed vs. Right-Handed Viewing

In my previous post I talked about how I think about left-handed vs. right-handed world coordinate systems. The basic idea is simply that there is an underlying reality, and the coordinate system choice is yours.

I compared notes with Jeff Weeks, a topologist friend (who wrote this cool book The Shape of Space that is not full of math symbols, but is just the opposite – approachable and fun, and what you should read after Flatland and Sphereland). Happily, he agrees. He also introduced me to another word for handedness: chirality, coined by Lord Kelvin. Jeff notes an interesting distinction:

You can ask whether the object’s own intrinsic coordinate system agrees with the ambient world space coordinate system (same chirality) or disagrees with the ambient world space coordinate system (different chirality), but you can’t meaningfully call either one of them “left-handed” or “right-handed”.

In other words, the only time you can meaningfully introduce the words “left-handed” and “right-handed” into the discussion is when a human being enters the picture. Once an object (or a space) gets drawn into the human’s physical space, then “left-handed” means “same chirality as the human’s left thumb-index finger-middle finger” and “right-handed” means “same chirality as the human’s right thumb-index finger-middle finger”.

So in particular, data sitting in a file on disk has no intrinsic chirality. What it has is the author’s intention that it be drawn into the human’s physical space with one chirality or the other (so that, for example, the steering wheel in a car model appears on the left-hand side of the vehicle, as perceived by the human viewing it).

OK, so only for viewing you must also know the handedness of the data. We also know it’s fine to have local coordinates that are RH or LH inside a world that is LH or RH, e.g., you create a frog model by mirroring the right half of your frog model with a mirror matrix, but the world itself is one or the other. So far so good.

Where things get tricky in computer graphics is when we talk about something like a right-handed coordinate world and a left-handed viewing system. Right-handed coordinates are pretty common: all Autodesk applications I know use them. It’s a natural extension of a 2D Cartesian plane to make the Z axis point upwards – a 2D floor-plan that is extruded would be thought to extend upwards from the ground, for example. Also, the determinant of a right-handed viewing matrix is positive; left, negative.

However, a left-handed viewing system is used by default by DirectX: the viewer’s X axis goes to the right, the Y axis is up, and Z goes into the screen. This also feels natural, as you label the screen’s X and Y axes from the lower left corner of the display and Z increases with depth into the screen. OpenGL’s viewing system is right-handed by default, the difference being that +Z goes towards the viewer. This negation leads to a fair bit of confusion in documentation with what near and far mean, but it’s consistent with right-handed world coordinates.

So what if you want to use a left-handed viewing system with right-handed data, or vice versa? All it means: there must be a conversion from one to the other, or expect mirrored images. Like my first post notes, the world coordinate system chosen is arbitrary: your RH system or the mole men’s LH system are both fine, you just have to decide which one to use. However, once you choose, that’s it – the view itself ultimately has to be working in the same system, one way or another. It is honestly meaningless to say “I want to use LH viewing with RH world coordinates”, if you want to do so “without conversion”.

Some transform has to be done to go from RH to LH, but which one? Any mirroring transform can mirror through any arbitrary plane will convert from one chirality to the other. Mirroring along the world’s Z axis is one common solution, i.e., set Z’ = -Z for vertices, normals, light positions, etc. If the scene’s data is converted once to LH space, case closed, your LH camera will work fine.

However, say you don’t want to touch the data. Microsoft gives a way to convert from RH to LH, which boils down to again mirroring along the world’s Z axis as the first transform on the view matrix, i.e., done every frame. No time is lost, since the mirroring is simply part of a different view matrix. The funny bit is that you have to deal with the world in RH, the view in LH, as far as defining where the camera goes and how it is oriented. A common way to define a view is by giving a camera position, a target it is looking at, and an up direction. From this you can form a view basis, a matrix to transform from world to view. By Microsoft’s method, you have to make sure to use LH positions and vectors for forming your camera (with Z negated), vs. the RH coordinate you use for your data. Confusing.

There’s another somewhat sensible place to perform mirroring: go from RH to LH at the end of the transforms, after projection to clip space. You’re given a right-handed camera position, target, and up vector and you use these to make a left-handed view matrix – there’s nothing stopping you. None of these coordinates need to be transformed, they can be defined to be the same in RH and LH spaces. However, what will happen is that the image produced will be mirrored along the vertical axis. That is, left and right sides will be switched, since you didn’t compensate for the difference in chirality. Lots of models are symmetric, in fact, so this sort of mistake is not often immediately noticeable. By adding a simple mirror matrix with X’ = -X you can swap the left and right of the screen. This comes down to negating the upper-left value in the projection matrix.

By using this mirror matrix at the end, you’ve made your left-handed coordinate system into a right-handed one. Each time you move the camera, you’re essentially defining a new and (usually) different mirroring plane in world space, one that passes through your eye, target, and up vector. This mirror matrix will then not affect the eye, target, and up directions, since they lie in this plane. Maybe that’s fine for your system. However, this solution can also mess up basic operations and queries. Say you want to pan the camera to the left. So you query your camera for which direction it thinks is left. It hands you one, you move the camera that direction, but the view shifts the other way. This is because you have a mismatch in chirality: your camera’s basis is in LH, but you correct it at the back-end to be RH. The view matrix returned a left-vector which was also in LH, and needs to be converted to RH. Also confusing.

The whole point of the camera is to transform some chunk of visible space into clip coordinates, which then convert (by dividing by W) to NDC coordinates (visible space being -1 to +1 in X and Y, -1 to +1 in Z for OpenGL, 0 to +1 in Z for DirectX). Neither OpenGL nor DirectX in any way requires LH or RH coordinates, you can feed in whatever view transforms you like. Just make world and view chirality match and be done with it.

That’s about it – just make them match, and do so by setting the view (lookat) matrix to match the world’s chirality. At least, that’s my take, this week (which sounds flip, but I’ve been trying to get a good handle on this area for a long time; I like to think I now have one, as of today). If there are other ways to think about this area, and especially if I’ve made any mental errors, please do comment.

By the way, if you want an enjoyable book about handedness and symmetry, get Martin Gardner’s The New Ambidextrous Universe. It talks about some surprising stuff, such as experiments where parity in the universe is not conserved (so if you’re ever mirrored a few times through the fourth dimension on your way to a distant planet, you’ll know how to test which way you came out).

Oh, and also check out Jeff Weeks’ site – some fun and beautiful OpenGL math applications there for free, Mac and Windows, with source. I should note that he uses LH throughout for his OpenGL applications, the rebel. There’s no “correct” coordinate system, so choose your own.