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18 June, 2016

Learning to teach

This past couple of months I've been teaching once a week a rendering course at private art school here in Vancouver. It took quite a bit of my time for course preparation (which in turn left this blog dry), but it's a quite nice experience and a small way to contribute to our rendering community, if you can, I wouldn't discourage people into getting involved with teaching.

This is not the first time I held a class, I actually worked for a small professional school in Italy teaching the very basics of computers while I was going to university. But it's the first time I teach rendering in a systematic way, outside sporadic presentations at work and conferences. 

So, still lots to learn...

What to teach?

The first obstacle actually is to decide what to teach. I had a curriculum for this course as it was taught in previous years, it went through the motions of building an engine in OpenGL, and from what I can tell was fairly extensive, albeit a bit oldschool.
I decided pretty early on to scrap it as I didn't think I could provide enough value that way.

I have to premise that this course takes place over 7x3=21 hours, and it's part of what I imagine is a very intense, packed year of courses in game programming. 
It's one of the last courses that the students take, so there is an expectation of a certain level of proficiency with programming and math (I don't cover linear algebra, for example). So you really have to pick your battles and fight for student's attention.

It might be my own bias (as I started programming extremely young and went on towards more theoretical studies in my higher education), but I still think that APIs and frameworks can (and probably should) be self-learned. These are not complicated topics, and while you might pick some bad advice if you mindlessly accept everything the web throws at you, I didn't think going through such things would be the best use of (rather limited) time.

Ok, so I started with a clean slate. What now? This is truly challenging. You can easily talk to people at work, on in a conference, when you have a quite precise idea of what can be assumed to be "common knowledge". But here? Not so much.
So my first decision was to keep it agile... I had from the get go a rough conceptual plan of how to structure the seven lessons and how they could relate to each other, and I discussed it with the head of the faculty, but I did everything one lesson at a time, with a good dose of improvisation.

This is my strategy. I would think and jot down notes (pretty much as I do when I have some thoughts that I think might one day go on the blog...) on where I wanted to go with a lesson, starting with the overall objective (what I wanted the students to achieve). 
Then, the day before I would scramble and start laying down some slides, trying to keep them minimal (which is against my instincts), I just make sure I have enough supporting material, diagrams, illustrations, to cover what I wanted. 
I post the lesson materials online after the class, by actually pruning out things I didn't end up covering, writing a summary to cover the main points of the lesson and giving links to further resources.

The way I tried to adapt is to slightly over-prepare. Not as in to be very prepared and rehearsed (I don't rehearse), but to have a good amount of options for the class, as it's easier to cut corners in class than to have to scramble to find materials that you didn't expect to need. I never walk in the class thinking that I -have- to go through everything, and I don't have the lesson after the one I'm teaching prepared at all, so I can always adjust.

How to assess?

Improvising (to an extent) the lessons gives the freedom to adapt, but you can't adapt if you don't have the pulse of how things are going. And this is probably the hardest thing, ever. It's especially hard when you are very versed in a field, because you totally lose sight of what means not to understand certain concepts. 

The more basic the course the harder the task. If you ever teach an introductory class to programming, and have your students problem-solve, you'll see what I mean. It's actually hard to articulate why certain things are solved in a certain way, when for your brain it's just obviously what you need to do...

I'm not great at this yet actually. I waited with anticipation the student's solutions for my first assignment, because I had not a solid idea of it was going to be trivial or daunting, and if really things where coming through (they ended up doing great). Assignments are for teachers! I really don't care about assigning grades, I care about getting feedback!

That said, a few things I can share.

First. Generic "checkpoints" are worse than useless. Asking to an audience how things are going, whenever things are understood, or to ask questions freely, is at best a waste of time, and at worst a way to get a false sense of confidence. Questions have to be real. 

Problem solving in class is great. I tried to mix problem solving with in-class assignments, and the reason I think you need both is that if you do only questions you risk engaging always the same people (and pointed questions are a bit more aggressive), while assignments let you walk through desks and see how people are doing individually. 

Moreover, as this was a very intensive course, I could not expect really a lot of at-home study from the students (my class ends at 21:30!), and I think working in class goes well with such programs because you need to have the time to come up with solutions by your own, and practice, so if you can't do it at home, at least some degree of that was done during the class itself. 
Arguably all the real learning happens not because one memorizes a lesson as a teacher taught it, but by practicing and thinking hard enough that the underlying concepts start to become apparent.

Finally, just asking for guesses and speculations before revealing the next bit of information also just helps keeping people engaged and in general, awake!

The course.

A few people after learning that I was trying this asked for the course program. I can't share the course materials themselves, both due to contractual restrictions and because they are not polished enough to be comprehensible without me explaining them, but I will go through what ended up being my ideas for the lessons (keeping in mind that if I am to do this again, I will probably change a lot).

Lesson 1: Introduction. My background, how I did things and why that doesn't matter. The course objectives: giving a basic understanding of the field to facilitate further study and practical, hand-on tinkering with GPUs and shaders. Introduction to the "rendering problem". Rendering in a videogame company, what it means to be a rendering engineer, what are the problems we face (spoiler alert: tech is the easier part).

Lesson 2: What's a GPU? Starting from a CPU (serial execution) going towards a GPU (latency, hiding it with data-parallel work...). Going from the rendering equation to rasterization: visibility and shading. Some hand-on time with executing kernels on the GPU: ShaderToy.

Lesson 3: Going from data-parallel processing to 3d scenes. What goes on when we draw an object? Vertex shaders, pixel shaders. Tinkering in Unity.

Lesson 4: "Advanced" shading. Texturing, projections, UV mapping. Some fun examples (reflection mapping with pre-integrated cubemaps in Unity). "Advanced" lighting and back to the rendering equation. Fundamentals of PBR.

Lesson 5: Going past a single draw. Engine and (generic) rendering API concepts. API: resource creation (memory), binding of state (registers), draws -> command buffer. Engine: visibility and sorting. Rendering algorithms: shadow mapping, post-effects, forward and deferred rendering.

Lesson 6: DirectX 11 API in-class "live coding". I downloaded SharpDX (an DX9-10-11-12 C# wrapper) and we went from drawing a single 2D triangle (the "hello world" of rendering) to drawing a 3D quad.

Lesson 7. I haven't actually taught this yet, but I settled now with the idea of going through an actual contemporary game, showing art and rendering choices, workflows, and more advanced rendering algorithms.

Closing remarks.

Teaching gives us an unique perspective we don't have writing articles or speaking at conferences. We can see (if we pay attention) what people understand from our words, and we can see if we are boring or engaging. I still have a lot to improve. Especially narrowing down topics is hard! Creating something truly beautiful and to the point.

To a degree I think trying to squeeze too much into a lesson (or as I do in this blog, an article), responds to the same urges that make programmer over-generalize: being concerned about covering all the possible bases even if it comes at a detriment to the overall quality. I'm aggressively against these practices in code, but still not great at exercise the same restraint in lectures.

So, I'd better stop here!

12 March, 2016

Beyond photographic realism

Service Note: if you're using motion-blur, please decouple rotational, translational and moving object amounts. And test the effect on a variety of screen sizes (fields of view). In general, motion blur shouldn't be something you notice, that registers consciously.
I've been recently playing Firewatch (really good!), which is annoyingly blurred when panning the view on my projector, and that's not an uncommon "mistake". The Order suffers from it as well, at least, to my eyes/my setup. When in doubt, provide a slider?

Ok, with that out of the way, I wanted to try to expand on the ideas of artistic control and potential of our medium I presented last time, beyond strict physical simulation with some color-grading slapped on top. But first, allow me another small digression...

This winter I was back in my hometown, visiting Naples with my girlfriend and her parents. It was their first time in Europe, so of course we went to explore some of the history and art of the surroundings (a task for which a lifetime won't be enough, but still). 

One of the landmarks we went to visit is the Capodimonte art museum, which hosts a quite vast collection of western paintings, from the middle ages up to the 18th century. 

Giotto. Nativity scene. The beginnings of the use of perspective (see also Cimabue).
I've toured this museum a few times with my father in the past, and we always follow a path which illustrates the progress of western art from sacred, symbolic illustrations, where everything is placed according to an hierarchy of importance, to the beginnings of study of the natural world, of perspective, sceneries, all the way to commissioned portraits, mythological figures and representation of common objects and people.

Renaissance painting by Raphael. Fully developed perspective.
What is incredibly interesting to me in this journey is to note how long does it take to develop techniques that nowadays we take for granted (the use of perspective, of lighting...), and how a single artist can influence generations of painters after.

Caravaggio. The calling of Saint Matthew.
When is the next wave coming, for realistic realtime rendering? When are we going to discover methods beyond the current strict adhesion to somewhat misunderstood, bastardized ideas borrowed from photography and cinematography?

Well, first of all, we ought to discuss why this question even matters, why there should be an expectation; couldn't photography be all there is to be in terms of realistic depiction? Maybe that's the best that can be done, and artistic expression should be limited to the kind of scene setups that are possible in such medium. 

In my view, there are two very important reasons to consider a language of realistic depiction than transcends physical simulation and physical acquisition devices:

1) Perception - Physical simulation is not enough to create perceived realism when we are constrained to sending a very limited stimuli (typically, LDR monitor output, without stereopsis or tracking). Studying physiology is important, but does not help much. A simple replica of what our vision system would do (or be able to perceive) when exposed to a real-world input is not necessarily perceived as real when such visual artefacts happen on a screen, instead of as part of the visual system. We are able to detect such difference quite easily, we need to trick the brain instead!

A tone-mapped image that aims to reproduce the detail perceived
by the human visual system does not look very realistic.
2) Psychology - Studying perception in conjunction with the limits of our output media could solve the issue of realism, but why perceptual realism matters in the first place? I'd say it's such a prominent style in games because it's a powerful tool for engagement, immersion. The actual goal is emotional, games are art. We could trick the brain with more powerful tools than what we can achieve by limiting ourselves to strict (perceptual) realism.

In other words the impression of seeing a realistic scene is in your brain, reproducing only at the physics of light transport on a monitor is not enough to make your brain fire in the same way as when it's looking at the real world...

So it's this all about art then? Why is this on a rendering technology blog? The truth is, artists are often scientists "in disguise", they discover powerful tools to exploit the human brain, but don't codify them in the language of science... Art and science are not disjointed, we have should understand art, serve it.

I've been very lucky to attend a lecture by professor
Margaret Livingstone recently, it's a must-see.

Classical artists understood the brain, if not in a scientific way, in an intuitive one. Painters don't reproduce a scene measuring it, they paint what it -feels- like to see a given scene.

Perceptual tricks are used in all artistic expressions, not only in painting but in architecture or in sculpture. Michlangelo's David has its proportions altered to look pleasing from a top-down viewing angle, enlarging the head and the right hand. And there is an interesting theory according to which Mona Lisa's "enigmatic" smile is a product of different frequencies and eye motions (remember that the retina can detect high-frequency details only in a small area near the center...)

Analyzing art through the lens of science enquiry can reveal some of these tricks, which is interesting to researchers as they can tell something about how our brain and visual system work, but should be even more interesting for us, if we can codify these artistic shortcuts in the language of science, turning talent into tools.

Edward Hopper understood light. And tone mapping!
(painting has a much more limited dynamic range than LCD monitors)
Cinematography has its own tricks. Photography, Set design, all arts develop their own specific languages. And real-time rendering has much more potential, because we control the entire pipeline, from scene to physics simulation to image transfer, and we can alter these dynamically, in reaction to player's inputs.
Moreover, we are an unique blend of science and art, we have talents with very different backgrounds working together on the same product. We should be able to create an incredibly rich language!
The most wonderful thing about working in lighting is the people that you encounter. Scientists and artists; engineers and designers; architects and psychologists; optometrists and ergonomists; are all concerned about how people interact with light. It is a topic that is virtually without boundaries, and it has brought me into contact with an extraordinary variety of people from whom I have gathered so much that I know that I cannot properly acknowledge all of them. - From "Lighting by Design", Christopher Cuttle.
We have full control over the worlds we display, and yet so far we author and control content with tools that simulate what can be done with real cameras, in real sets. And we have full control over the -physics- of the simulations we do, and yet we are very wary of allowing tweaks that break physical laws. Why?


James Turrell's installations play with real-world lights and spaces
I think that a lot of it is a reaction, a very understandable and very reasonable reaction, against the chaos that we had before physically-based rendering techniques. 

We are just now trying to figure everything we need to know about PBR, and trying to educate our artists to this methodology, and that has been without doubt the single most important visual revolution of the last few years in realtime (and even offline) graphics. 
PBR gives us certainty, gives us ways to understand visual problems in quantitative terms, and gives us a language with less degrees of freedom for artists, with parameters that are clearly separated, orthogonal (lights, materials, surfaces...).

This is great, when everything has a given degree of realism by construction, artists don't have to spend time trying just to achieve realism through textures and models, they can actually focus on higher level goal of focusing on -what- to show, of actual design.

But now, as we learn more of the craft of physically based models, now is the time to learn again how and why to programmatically break it! We have to understand that breaking physics is not a problem per se, the problem is breaking physics for no good reason. 
For example, let's consider adjusting the "intensity" of global illumination, which is something that is not uncommon in rendering engines, it's often a control that artists ask for. The problem is entirely of math, or correctness, but of intent. 


Lighting (softness/bounces) can be a hint of scale
Why are we breaking energy conservation? Is it because we made some mistakes in the math, and artists are correcting? Is it because artists did create worlds with incorrect parameters, for what they are trying to achieve? Or is it because we consciously want to communicate something with that choice, for example, distorting the sense of scale of the scene? The last, is a visual language choice, the former are just errors which should be corrected, finding the root cause instead of adding a needless degree of freedom to our system.

Nothing should be off the table, if we understand why we want certain hacks, the opposite, we should start with physical simulations and find what we can play with, and why. 
Non-linear geometric distortions? Funky perspective projections (iirc there were racing games in the far past that did play with that)? Color shifts? Bending light rays

And even more possibilities are open with better displays, with stereopsis, HDR, VR... What if we did sometimes kill, or even invert the stereo projection, for some objects? Or change their color, or shading, across eyes? 


3D printed dress by Iris Van Herpen
All other artistic disciplines, from fashion to architecture, rush at new technologies, new tools, trying to understand how they can be employed, hacked, bent, for new effects. 

We are still in our infancy, it's understandable, realistic realtime rendering is still young (even if we look at -gameplay- in games, that arguably is much more studied and refined than visuals as an art), but it's time to start being more aware, I'd say, of our limits, start experimenting more.

06 February, 2016

Low-resolution effects with depth-aware upsampling

I have to confess, till recently I was never fond of doing half or quarter res effects via a bilateral upsampling step. It's a very popular technique, but all the times I tried it I found it causing serious edge artifacts... 
On Fight Night Champion I ended up shipping AO and deferred shadows without any depth aware upsampling (just separating the ring and fighters from the background, and using a bias towards over-shadowing); Space Marines ended up shipping with a bilateral upsampling on AO (but no bilateral blurring or noise) but it still had artifacts. In the end it sort-of worked, via some hacks that were good enough to ship, but that I never really understood.

For Call of Duty Black Ops 3 we needed to compute some effects (volumetric lighting) at quarter-res or less, to respect the performance budgets we had, so depth-aware upsampling was definitely a necessity, so I needed to investigate a bit more into it.
A quite extreme example of "god rays" in COD:BO3
I found a solution that is very simple, that I understand quite well, and that works well in practice. I'm sure it's something many other games are doing and many other people discovered (due to its simplicity), but I'm not aware of it being presented publicly, so here it is, my notes on how not to suck at bilateral upsampling:

1) Bilateral weighting doesn't make a lot of sense for upsampling.

The most commonly used bilateral upsampling scheme works by using the same four texels that would be involved in bilinear filtering, but changing their weights by multiplying them by a function of the depth difference between the true surface (high res z-buffer) and their depths (low-res z-buffer).

This method makes little sense, really, because you can have the extreme case where the bilinear weights select only one sample, but that sample is not similar to the surface depth you need at all! Samples that are not detected to be part of the full-res surface should simply be ignored, regardless of how "strongly" biliear wants to access them...

A better option is to simply -choose- between bilinear filtering or nearest depth point sampling, based on if the low-res samples are part of the high-res surface or not. This can be done in a variety of ways, for example:

- lerp(bilinear_weights, depth_weights, f(depth_discontinuity)) * four_samples
- lerp(biliear_sample, best_depth_sample, f(depth_discontinuity))
- bilinear_fetch(lerp(bilinear_texcoords, best_depth_texcoords, f(depth_discontinuity)))

Where the weighting function f() is quite "sharp" or even just a step function. The latter scheme is similar to nVidia's "nearest depth sampling", it's the fastest alternative but in Black Ops 3 I ended up sharply going from bilateral to "depth only" weights if a too big discontinuity is detected in the four bilinear texels.

2) Choose the low-res samples to maximise the chances of finding a representative.

It's widely known that a depth buffer can't be downsampled averaging values, that would result in depths that do not exist in the original buffer, and that are not representative of any surface, but "floating" in between surfaces at edge discontinuities. So either min or max filtering is used, commonly preferring nearest-to-camera samples, with the reasoning that closest surfaces are more important, and thus should be sampled more (McGuire tested various strategies in the context of SSAO, see Table 1 here).

But if we think in terms of the reconstruction filter and its failure cases, it's clear that preferring a single set of depths doesn't make a lot of sense. We want to maximize the chance of finding, among the texels we consider for upsamping, some that represent well the surfaces in the full resolution scene. Effectively in the downsampling step we're selecting on points we want to compute the low-res effect, clearly we want to do that so we distribute samples evenly across surfaces.

A good way of doing this is to chose per each sample in the downsampled z-buffer, a surface that is different from the ones of its neighbors. There are many ways this could be done, but the simplest is to just alternate min and max downsampling in a checkerboard patter, making sure that for each 2x2 quad, if we are in a region that has multiple surfaces, at least two of them will be represented in the low-res buffer. 

In theory it's possible to push even more surfaces in a quad, for example we could record the second smallest or second biggest, or the median or any other scheme (even a quasi-random choice) to select a depth (we shouldn't use averages though, as these will generate samples that belong to no surface), but in practice this didn't seem to work great with my upsampling, I guess because it reduces spatial resolution in favour of depth resolution, but your mileage may vary depending on the effect, the upsampling filter and the downsampling ratio.

Some residual issues can be seen sometimes (upper right),
when there is no good point sample in the 2x2 neighborhood.

Further notes.

The nearest-depth upsampling with a min/max checkerboard pattern downsampling worked well enough for Black Ops 3 that no further research was done, but there are still things that could be clearly improved:

- Clustering for depth selection.
A compute shader could do actual depth clustering to try to understand how many surfaces there are in an area, and chose what depths to store and the tradeoffs between depth resolution and screenspace resolution.

- Gradients.
Depth discontinuity in the upsampling step is a very simplistic metric, more information can be used to understand if samples belong to the same surface, like normals, g-buffer attributes and so on.

- Wider filters.
Using a 2x2 quad of samples for the upsampling filter is convenient as it allows to naturally fall back to bilinear if we think the samples are representative of the high-res surface, but there is no reason to limit the search to such neighborhood, wider filters could be used, both for higher-order filtering and to have better chances of finding representative samples.

- Better filtering of the representative depth samples.
There is no reason to revert to point-sampling in presence of discontinuities (or purely depth-weighted sampling), it's still possible to reject samples that are not representative of the surface while weighting the useful ones with a filter that depends on the subtexel  position.
Special cases could be considered for horizontal and vertical edges, where we could do 1d linear interpolation on the axis of the surface. Bart Wronski has something along these lines here (and the idea of baking an UV offset to be reused by different effects also allows in general to use more complex logic, and amortize it among effects).

- "Separable" bilateral filters.
Often when depth-aware upsampling is employed we also use depth-aware (bilateral) filters, typically blurs. These are often done in separate horizontal/vertical passes, even if technically such filters are not separable at all. 
This is particularly a problem with depth-aware filters because the second pass will use values that are not anymore relative to the depths in the low-res depth buffer, but result from a combination of samples from the first pass, done at different depths.

The filter can still look right if we can always correctly reject samples not belonging to the surface at center texel of a filter, because anyway the filtered value is from the surface of the center texel, so doing the second pass using a rejection logic that uses attributes (depth...) at the center of the filtered value sort-of works (it's still a depth of the right surface). 
In practice though that's not always the case, especially if the rejection is done with depth distances only, and it causes visible bleeds in the direction of the second filter pass. A better alternative in these cases (if the surface sample rejection can't be fixed...) is to do separate passes not in an horizontal/vertical fashion but in a staggered grid (e.g. first considering a NxN box filter pass then doing a second pass by sampling every N pixels in horizontal and vertical directions).

24 January, 2016

Color grading and excuses

I started jotting down some notes for this post a month ago maybe, after watching bridge of spies on a plane to New York. An ok movie if you ask me, with very noticeable, heavy-handed color chocies and for some reasons a heavy barrel distortion in certain scenes. 

Heavy barrel distortion, from the Bridge of Spies trailer. Anamorphic lenses?
I'm quite curious to understand the reasoning behind said distortion, what it's meant to convey, but this is not going to be a post criticizing the overuse of grading, I think that's already something many people are beginning to notice and hopefully avoid. Also I'm not even entirely sure it's really a "problem", it might be even just fatigue

For decades we didn't have the technology to reproduce colors accurately, so realistic color depiction was the goal to achieve. With digital technology perfect colors are "easy", so we started experimenting with ways to do more, to tweak them and push them to express certain atmospheres/emotions/intentions, but nowadays we get certain schemes that are repeated over and over so mechanically it becomes stale (at least in my opinion). We'll need something different, break the rules, find another evolutionary step to keep pushing the envelope.

Next-NEXT gen? Kinemacolor
What's more interesting to me is of course the perspective of videogame rendering. 

We've been shaping our grading pretty much after the movie pipelines, we like the word "filmic", we strive to reproduce the characteristics and defects of real cameras, lenses, film stocks and so on. 
A surprising large number of games, of the most different genres, all run practically identical post-effect pipelines (at least in the broad sense, good implementations are still rare). You'll have your bloom, a "filmic" tone mapping, your color-cube grading, depth of field and motion blur, and maybe vignette and color aberration. Oh, and lens flares, of course... THE question is: why? Why we do what we do? 

Dying light shows one of the heavier-handed CA in games
One argument that I hear sometimes is that we adopt these devices because they are good, they have so much history and research behind them that we can't ignore. I'm not... too happy with this line of reasoning. 
Sure, I don't doubt that the characteristic curves of modern film emulsions were painstakingly engineered, but still we should't just copy and paste, right? We should know the reasoning that led to these choices, the assumptions made, check if these apply to us. 
And I can't believe that these chemical processes fully achieved even the ideal goals their engineers had, real-world cameras have to operate under constraints we don't have.
In fact digital cameras are already quite different than film, and yet if you look at the work of great contemporary photographers, not everybody is rushing to apply film simulation on top of them...

Furthermore, did photography try to emulate paintings? Cross-pollination is -great-, but every media has its own language, its own technical discoveries. We're really the only ones trying so hard to be emulators; Even if you look at CGI animated movies, they seldom employ many effects borrowed from real-world cameras, it's mostly videogames that are obsessed with such techniques.

Notice how little "in your face" post-fx are in a typical Pixar movie...
A better reason someone gave me was the following: games are hard enough, artists are comfortable with a given set of tools, the audience is used to a given visual language, so by not reinventing it we get good enough results, good productivity and scenes that are "readable" from the user perspective.

There is some truth behind this, and lots of honesty, it's a reasoning can lead to good results if followed carefully. But it turns out that in a lot of cases, in our industry, we don't even apply this line of thinking. And the truth is that more often than not we just copy "ourselves", we copy what someone else did in the industry without too much regard with the details, ending up in a bastard pipeline that doesn't really resemble film or cameras.

When was the last time you saw a movie and you noticed chromatic aberrations? Heavy handed flares and "bloom" (ok, other than in the infamous J.J.Abrams  Star Trek, but hey, he apologized...)? Is the motion blur noticeable? Even film grain is hardly so "in your face", in fact I bet after watching a movie, most of the times, you can't discern if it was shot on film or digitally.
Lots of the defects we simulate are not considered pleasing or artistic, they are aberrations that camera manufacturers try to get rid of, and they became quite versed at it! Hexagonal-shaped bokeh? Maybe on very cheap lenses...


http://www.cs.ubc.ca/labs/imager/tr/2012/PolynomialOptics/
On the other hand lots of other features that -do- matter are completely ignored. Lots of a lens "character" comes from its point spread function, a lens can have a lower contrast but high resolution or the opposite, field curvature can be interesting, out of focus areas don't have a fixed, uniform shape across the image plane (in general all lens aberrations change across it) and so on. We often even leave the choice of antialiasing filters to the user...

Even on the grading side we are sloppy. Are we really sure that our artists would love to work with a movie grading workflow? And how are movies graded anyways? With a constant, uniform color correction applied over the entire image? Or with the same correction applied per environment? Of course not! The grading is done shot by shot, second by second. It's done with masks and rotoscoping, gradients, non-global filters...

A colorist applying a mask
Lots of these tools are not even hard to replicate, if we wanted to; We could for example use stencils to replicate masks, to grade differently skin from sky from other parts of the scene. 
Other things are harder because we don't have shots (well, other than in cinematic sequences), but we could understand how a colorist would work, what an artist could want to express, and try to invent tools that allow a better range of adjustment. Working in worldspace or clipspace maybe, or looking at material attributes, at lighting, and so on.

Ironically people (including myself sometimes) are sometimes instinctively "against" more creative techniques that would be simple in games on the grounds that they are too "unnatural", too different from what we think it's justified by the real camera argument, that we pass on opportunities to recreate certain effects that would be quite normal in movies instead, just because they are not exactly in the same workflow.

Katana, a look development tool.

Scene color control vs post-effect grading.

I think the endgame though is to find our own ways. Why do we grade and push so much on post effects to begin with? I believe the main reason is because it's so easy, it empowers artists with global control over a scene, and allows to do large changes with minimal effort. 

If that's the case though, could we think of different ways to make the life of our artists easier? Why can't we allow the same workflows, the same speed, to operations on source assets? With the added benefit of not needlessly breaking physical laws, thus achieving control in a a more believable way....


Neutral image in the middle. On the right: warm/cold via grading, on the left a similar effect done editing lights. 
Unlike in movies and photography for us it's trivial to change the colors of all the lights (or even of all the materials). We can manipulate subsets of these, hierarchically, by semantic, locally in specific areas, by painting over the world, interpolating between different variants and so on...
Why did we push everything to the last stage of our graphics pipeline? I believe if in photography or movies there was the possibility of changing the world so cheaply, if they had the opportunities we do have, they would exploit them immediately.

Gregory Crewdson

Many of these changes are "easy" as they won't impact the runtime code, just smarter ways to organize properties. Many pipelines are even pushing towards parametric material libraries and composting for texture authoring, which would make even bulk material edits possible without breaking physical models.

We need to think and experiment more. 



P.S. 
A possible concern when thinking of light manipulation is that as the results are more realistic, it might be less appropriate for dynamic changes in game (e.g. transitions between areas). Where grading changes are not perceived as changes in the scene, lighting changes might be, thus potentially creating a more jarring effect.

It might seem I'm very critical of our industry, but there are reasons why we are "behind" other medias, I think. Our surface area is huge, engineers and artists have to care about developing their own tools -while using them-, making sure everything works for the player, make sure everything fits in a console... We're great at these things, there's no surprise then that we don't have the same amount of time to spend thinking about game photography. Our core skills are different, the game comes first.

15 November, 2015

UVic lecture slides


An introduction to what is like to be a rendering engineer or researcher in a AAA gamedev production, and why you might like it. Written for a guest lecture to computer graphics undergrads at University of Victoria.

As most people don't love when I put things on scribd, for now this is hosted from my dropbox.

https://dl.dropboxusercontent.com/u/6809780/BLOG_HOSTING/AP%20Guest%20Lecture.pdf

15 August, 2015

Siggraph 2015 course

As some will have noticed, Michal Iwanicki and I were speakers (well I was much more of a listener, actually) in the physically based shading course this year (thanks again to both Stephens for organizing and invitingus) presenting a talk on approximate models for rendering, while our Activision colleague and rendering lead of Sledgehammer showed some of his studio's work on real world measurements used in Advanced Warfare.

Before and after :)
If you weren't at Siggraph this year or you missed the course, fear not, we'll publish the course notes soon (I need to do some proof reading and adding bibliography) and the course notes are "the real deal", as in twenty minutes we couldn't do much more than a teaser trailer on stage.

I wanted to write though about the reasons that motivated me to present in the course that material, give some background. Creating approximations might be time consuming sometimes, but it's often not that tricky, per-se I don't think it's the most exciting topic to talk about. 
But it is important, and it is important because we are still too often too wrong. Too many times we use models that we don't completely understand, that are exact under assumptions we didn't investigate and for which we don't know what perceptual error they cause.

You can nowadays point your finger at any random real time rendering technique, really look at it closely, compare with ground truth, and you're more likely than not to find fundamental flaws and simple improvements through approximation.

This is a very painful process, but necessary. PBR is like VR, it's an all or nothing technique. You can't just use GGX and call it a day. Your art has to be (perceptually) precise, your shadows, your GI, your post effect, there is a point where everything "snaps" and things just look real, but it's very easy to be just a bit off and ruin the illusion. 
Worse, errors propagate non-locally as artists try to compensate for our mistakes in the rendering pipeline by skewing the assets to try to reach as best as they can a local minimum.

Moreover, we are also... not helped I fear by the fact that some of these errors are only ours, we commit them in application, but the theory in many cases is clear. We often got research from the seventies and the eighties that we should just read more carefully. 
For decades in computer graphics we rendered images in gamma space, but there isn't anything for a researcher to publish about linear spaces, and even today we largely ignore what colorspaces really are and what we should use, for example.

We don't challenge the assumptions we work with.

A second issue I think is sometimes it's just neater to work with assumptions that it is to work on approximations. And it is preferable to derive our math exactly via algebraic simplifications, the problem is that when we simplify by imposing an assumption, its effects should be measured precisely.

If we consider constant illumination, and no bounces, we can define ambient occlusion, and it might be an interesting tool. But in reality it doesn't exist, so when is it a reasonable approximation? Then things don't exactly work great, so we tweak the concept and create ambient obscurance, which is better, but to a degree even more arbitrary. Of course this is just an example, but note: we always knew that AO is something odd and arbitrary, it's not a secret, but even in this simple case we don't really know how wrong it is, when it's more wrong, and what could be done to make it measurably better.

You might say that even just finding the errors we are making today and what is needed to bridge the gap, make that final step that separates nice images from actual photorealism, is a non-trivial open problem (*).
It's actually much easier to implement a many exciting new rendering features in an engine than to make sure that we got even a very simple and basic renderer is (again perceptually) right. And on the other hand if your goal is photorealism it's surely better to have a very constrained renderer in very constrained environments that is more accurate than a much fancier one used with less care.

I was particularly happy at this Siggraph to see that more and more we are aware of the importance of acquired data and ground truth simulations, the importance of being "correct", and there are many researchers working to tackle these problems that might seem even to a degree less sexy than others, but are really important.

In particular right after our presentations Brent Burley showed, yet again, a perfect mix of empirical observations, data modelling and analytic approximations in his new version of Disney's BRDF, and Luca Fascione did a better job I could ever do explaining the importance of knowing your domain, knowing your errors, and the continuum of PBR evolution in the industry.

P.S. If you want to start your dive into Siggraph content right, start with Alex "Statix" Evans amazingly presentation in the Advances course: cutting edge technology presented through a journey of different prototypes and ideas. 
Incredibly inspiring, I think also because the technical details were sometimes blurred just enough to let your own imagination run wild (read: I'm not smart enough to understand all the stuff... -sadface-). 
Really happy also to see many teams share insights "early" this year, before their games ship, we really are a great community.

P.S. after the course notes you might get a better sense of why I wrote some of my past posts like:

(*) I loved the open problems course, I think we need it each year and we need to think more at what we really need to solve. This is can be a great communication channel between the industry, the hardware manufactures and the academia. Chose wisely...