## Saturday, March 29, 2014

### UE4 available to all

The big news from Epic at GDC this year was that Unreal Engine 4 is now available to everyone for only $19/month + 5% royalties. There's been a lot said already about why this is cool, opinions, comparisons to competitors and so forth. From a business model point of view I feel it has been covered better than I could possibly say. If you are interested check out Mark Rein's post or hear it from the man himself Tim Sweeney for very compelling reasons why this is a good idea for us to provide and for developers to subscribe. I wanted to give my own personal perspective as an engineer. What Epic just did is absolutely revolutionary and I am privileged to be part of it. I am super excited! Let me explain why. If you've followed this blog for a while you may remember I gave Epic huge props for releasing UDK back in 2009. That was great for artists and designers, not that great for programmers. This is the next step down that path and its a doozy. Now there have been naysayers that compare the numbers 19/month + 5%, is that better or worse, is it new or been done before, and so on. They are missing the point. It isn't the price, it isn't even the features. The revolution is this: for a 1/3 the cost of a new video game you can get access to the complete source code for our cutting edge game engine. The entire engine source code, every bit of it, the exact same we use in house and the same provided to private licensees, is available to you for only$19 (except for XB1 and PS4 code we aren't legally able to give you due to NDAs). This is the same tech that will be powering many of the top AAA games developed this generation. As Mark put it "Right now 7 of the top 21 (!) all-time highest rated Xbox 360 games (by Metacritic score) were powered by Unreal Engine 3" and you damn well better bet we are going to do the same or better this time around with UE4.

Now you may say John Carmack and id have open sourced their engines many times. That is true and I commend them for it. John has had a lasting impact on how many coders, myself included, write software due to these efforts. Unfortunately, the strict licensing terms have resulted in basically no commercial products using the id engines from that open source license. John has even lamented this recently "It is self-serving at this point, but looking back, I wish I could have released all the Id code under a more permissive license. GPL never did really do anything for game code, and I do wonder whether it was a fundamental cultural incompatibility. GPL was probably the best that could have flown politically for the code releases -- posterity without copy-paste into competition." And my own personal twist with this situation is after Zenimax bought id, their engines were no longer commercially licensable meaning anything built on top of that code base had to be scraped. GPL is simply not an option for commercial games.

There are other examples of open sourced game engines or cheaply priced engine source code but they all have something in common which is they were never up to date. The id engines were only open sourced years after the game that used them shipped. Id had already moved onto their next technology. For example the engine that powered Doom 3 wasn't opened sourced until 6 years after the game shipped. By that point Rage had even shipped. The reason for this is obvious. The point where releasing the code is non-threatening to execs is the point where a competitor can't use the code to their advantage. This means every single engine code base available to indie devs or students was either not commercially developed or purposely not competitive in its capabilities.

With the UE4 subscription, you can today get complete source that is totally up to date and will continue to be. This isn't any old engine, its the same cutting edge technology that will be powering many of the top AAA games of this generation. And as soon as we add new features you'll get them. In fact you'll even see the work in progress before they're done. How crazy is that?

Look, its easy to brush this off as the guy who works for Epic telling me I should give him money. That isn't why I'm writing this. What I am most excited about by far is in what I can share and give back the the gamedev community. That is the reason I have this blog, its the reason I have presented at conferences and will again in the future.

If you are a teacher, check out the license terms, they are ridiculously nice for schools. If you are a student, tell your school about it or grab a personal subscription.

For all those wanting to get into the game industry, the same response you will hear from everyone is make something. No one lands a game job because of their great grades or fancy degrees. They get it by making something cool. I wrote my own engine. That's how I landed my first job. Although I learned a ton if I were in that place today I would modify UE4. There is literally nothing else more applicable to a engine programming position than proving you can do the work. Hell, make something cool enough we'll hire you!

I can't wait to see what people make with this. It's a great time to be a programmer!

## Sunday, December 15, 2013

### Tone mapping

When working with HDR values, two troublesome situations often arise.

The first happens when one tries to encode an HDR color using an encoding that has a limited range, for instance RGBM. Values outside the range still need to be handled gracefully, ie not clipped.

The second happens when an HDR signal is under sampled. One very bright sample can completely dominate the result. In path tracing these are commonly called fireflies.

In both cases the obvious solution is to reduce the range. This sounds exactly like tonemapping so break out those tone mapping operators, right? Well yes and no. Common tone mapping operators work on color channels individually. This has the downside of desaturating the colors which can look really bad if later operations attenuate the values, for instance reflections, glare, or DOF.

Instead I use a function that modifies only the luminance of the color. The simplest of which is this:
$$T(color) = \frac{color}{ 1 + \frac{luma}{range} }$$
Where $T$ is the tone mapping function, $color$ is the color to be tone mapped, $luma$ is the luminance of $color$, and $range$ is the range that I wish to tone map into. If the encoding must fit RGB individually in range then $luma$ is the max RGB component.

Inverting this operation is just as easy. $$T_{inverse}(color) = \frac{color}{ 1 - \frac{luma}{range} }$$
This operation, when used to reduce fireflies, can also be thought of as a weighting function for each sample: $$weight = \frac{1}{ 1 + luma }$$
For a weighted average, sum all samples and divide by the summed weights. The result will be the same as if the samples were tone mapped using $T$ with $range$ of 1, averaged, then inverse tone mapped using $T_{inverse}$.

If a more expensive function is acceptable then keeping more of the color range linear is best. To do this use the functions below where 0 to $a$ is linear and $a$ to $b$ is tone mapped. $$T(color) = \left\{ \begin{array}{l l} color & \quad \text{if luma \leq a}\\ \frac{color}{luma} \left( \frac{ a^2 - b*luma }{ 2a - b - luma } \right) & \quad \text{if luma \gt a} \end{array} \right.$$ $$T_{inverse}(color) = \left\{ \begin{array}{l l} color & \quad \text{if luma \leq a}\\ \frac{color}{luma} \left( \frac{ a^2 - ( 2a - b )luma }{ b - luma } \right) & \quad \text{if luma \gt a} \end{array} \right.$$
These are same as the first two functions if $a=0$ and $b=range$.

I have used these methods for lightmap encoding, environment map encoding, fixed point bloom, screen space reflections, path tracing, and more.

## Saturday, August 3, 2013

### Specular BRDF Reference


First let me define alpha that will be used for all following equations using UE4's roughness: $$\alpha = roughness^2$$

## Normal Distribution Function (NDF)

The NDF, also known as the specular distribution, describes the distribution of microfacets for the surface. It is normalized [12] such that: $$\int_\Omega D(\mv) (\ndotm) d\omega_i = 1$$ It is interesting to notice all models have $\frac{1}{\pi \alpha^2}$ for the normalization factor in the isotropic case.

Blinn-Phong [2]: $$D_{Blinn}(\mv) = \frac{1}{ \pi \alpha^2 } (\ndotm)^{ \left( \frac{2}{ \alpha^2 } - 2 \right) }$$ This is not the common form but follows when $power = \frac{2}{ \alpha^2 } - 2$.

Beckmann [3]: $$D_{Beckmann}(\mv) = \frac{1}{ \pi \alpha^2 (\ndotm)^4 } \exp{ \left( \frac{(\ndotm)^2 - 1}{\alpha^2 (\ndotm)^2} \right) }$$

GGX (Trowbridge-Reitz) [4]: $$D_{GGX}(\mv) = \frac{\alpha^2}{\pi((\ndotm)^2 (\alpha^2 - 1) + 1)^2}$$

GGX Anisotropic [5]: $$D_{GGXaniso}(\mv) = \frac{1}{\pi \alpha_x \alpha_y} \frac{1}{ \left( \frac{(\mathbf{x} \cdot \mv)^2}{\alpha_x^2} + \frac{(\mathbf{y} \cdot \mv)^2}{\alpha_y^2} + (\ndotm)^2 \right)^2 }$$

The geometric shadowing term describes the shadowing from the microfacets. This means ideally it should depend on roughness and the microfacet distribution.

Implicit [1]: $$G_{Implicit}(\lv,\vv,\hv) = (\ndotl)(\ndotv)$$

Neumann [6]: $$G_{Neumann}(\lv,\vv,\hv) = \frac{ (\ndotl)(\ndotv) }{ \mathrm{max}( \ndotl, \ndotv ) }$$

Cook-Torrance [11]: $$G_{Cook-Torrance}(\lv,\vv,\hv) = \mathrm{min}\left( 1, \frac{ 2(\ndoth)(\ndotv) }{\vdoth}, \frac{ 2(\ndoth)(\ndotl) }{\vdoth} \right)$$

Kelemen [7]: $$G_{Kelemen}(\lv,\vv,\hv) = \frac{ (\ndotl)(\ndotv) }{ (\vdoth)^2 }$$

### Smith

The following geometric shadowing models use Smith's method[8] for their respective NDF. Smith breaks $G$ into two components: light and view, and uses the same equation for both: $$G(\lv, \vv, \hv) = G_{1}(\lv) G_{1}(\vv)$$ I will define $G_1$ below for each model and skip duplicating the above equation.

Beckmann [4]: $$c = \frac{\ndotv}{ \alpha \sqrt{1 - (\ndotv)^2} }$$ $$G_{Beckmann}(\vv) = \left\{ \begin{array}{l l} \frac{ 3.535 c + 2.181 c^2 }{ 1 + 2.276 c + 2.577 c^2 } & \quad \text{if c < 1.6}\\ 1 & \quad \text{if c \geq 1.6} \end{array} \right.$$

Blinn-Phong:
The Smith integral has no closed form solution for Blinn-Phong. Walter [4] suggests using the same equation as Beckmann.

GGX [4]: $$G_{GGX}(\vv) = \frac{ 2 (\ndotv) }{ (\ndotv) + \sqrt{ \alpha^2 + (1 - \alpha^2)(\ndotv)^2 } }$$ This is not the common form but is a simple refactor by multiplying by $\frac{\ndotv}{\ndotv}$.

Schlick-Beckmann:
Schlick [9] approximated the Smith equation for Beckmann. Naty [1] warns that Schlick approximated the wrong version of Smith, so be sure to compare to the Smith version before using. $$k = \alpha \sqrt{ \frac{2}{\pi} }$$ $$G_{Schlick}(\vv) = \frac{\ndotv}{(\ndotv)(1 - k) + k }$$

Schlick-GGX:
For UE4, I used the Schlick approximation and matched it to the GGX Smith formulation by remapping $k$ [10]: $$k = \frac{\alpha}{2}$$

## Fresnel

The Fresnel function describes the amount of light that reflects from a mirror surface given its index of refraction. Instead of using IOR we instead use the parameter or $F_0$ which is the reflectance at normal incidence.

None: $$F_{None}(\mathbf{v}, \mathbf{h}) = F_0$$

Schlick [9]: $$F_{Schlick}(\mathbf{v}, \mathbf{h}) = F_0 + (1 - F_0) ( 1 - (\vdoth) )^5$$

Cook-Torrance [11]: $$\eta = \frac{ 1 + \sqrt{F_0} }{ 1 - \sqrt{F_0} }$$ $$c = \vdoth$$ $$g = \sqrt{ \eta^2 + c^2 - 1 }$$ $$F_{Cook-Torrance}(\mathbf{v}, \mathbf{h}) = \frac{1}{2} \left( \frac{g - c}{g + c} \right)^2 \left( 1 + \left( \frac{ (g + c)c - 1 }{ (g - c)c+ 1 } \right)^2 \right)$$

## Optimize

Be sure to optimize the BRDF shader code as a whole. I choose these forms of the equations to either match the literature or to demonstrate some property. They are not in the optimal form to compute in a pixel shader. For example, grouping Smith GGX with the BRDF denominator we have this: $$\frac{ G_{GGX}(\lv) G_{GGX}(\vv) }{4(\ndotl)(\ndotv)}$$ In optimized HLSL it looks like this:

float a2 = a*a;
float G_V = NoV + sqrt( (NoV - NoV * a2) * NoV + a2 );
float G_L = NoL + sqrt( (NoL - NoL * a2) * NoL + a2 );
return rcp( G_V * G_L );

If you are using this on an older non-scalar GPU you could vectorize it as well.

### References

[1] Hoffman 2013, "Background: Physics and Math of Shading"
[2] Blinn 1977, "Models of light reflection for computer synthesized pictures"
[3] Beckmann 1963, "The scattering of electromagnetic waves from rough surfaces"
[4] Walter et al. 2007, "Microfacet models for refraction through rough surfaces"
[5] Burley 2012, "Physically-Based Shading at Disney"
[6] Neumann et al. 1999, "Compact metallic reflectance models"
[7] Kelemen 2001, "A microfacet based coupled specular-matte brdf model with importance sampling"
[8] Smith 1967, "Geometrical shadowing of a random rough surface"
[9] Schlick 1994, "An Inexpensive BRDF Model for Physically-Based Rendering"
[10] Karis 2013, "Real Shading in Unreal Engine 4"
[11] Cook and Torrance 1982, "A Reflectance Model for Computer Graphics"
[12] Reed 2013, "How Is the NDF Really Defined?"

## Sunday, July 28, 2013

### Epic, SIGGRAPH, etc

I'm resurrecting this blog from the dead. I'm sorry it's been neglected for a year but I've been busy. If you follow me on twitter (@BrianKaris) then this probably isn't news, but for those that don't here's an update:

A year ago I left Human Head and accepted a position on the rendering team at Epic Games. Since then we made the UE4 Infiltrator demo. I've worked on temporal AA, reflections, shading, materials, and other misc cool stuff for UE4 and games being developed here at Epic. I'm surrounded by a bunch of really smart, talented people, with whom it has been a pleasure to work.

Just this last week I presented in the SIGGRAPH 2013 course: Physically Based Shading in Theory and Practice. If you saw my talk and are interested in the subject but haven't looked at the course notes I highly suggest you follow that link and check them out as well as the other presenter's materials. Like previous years, the talks are only a taste of the content that the course notes cover in detail.

Now with that out of the way, hopefully I can start making some good posts again.

## Monday, May 14, 2012

The system I described last time allowed specular highlights to reach large distances but only requires calculating them on the tiles where they will show up. This is great but it means now we must calculate shadows for these very large distances. Growing the shadow maps to include geometry at a much greater distance is hugely wasteful. Fortunately there is a solution.

Before I get to that though I want to talk about a concept I think is going to be very important for next gen renderers and that is having more than one representation for scene geometry. Matt Swoboda talked about this in his GDC presentation this year [1] and I am in complete agreement with him. We will need geometry in similar formats as we've had in the past for efficient rasterization (vertex buffers, index buffers, displacement maps). This will be used whenever the rays are coherent simply because HW rasterization is much faster than any other algorithm currently for coherent rays. Examples of use are primary rays and shadow rays in the form of shadow maps.

Incoherent rays will be very important for next gen renderers but we need a different representation to efficiently trace rays. Any that support tracing cones will likely be more useful than ones which can only trace rays. Possible representations are signed distance fields [2][1][9], SVOs [3], surfel trees [4], and billboard cloud trees [5][9]. I'll also include screen space representations although these don't store the whole scene. These include mip map chains of min/max depth maps [6], variance depth maps [7] and adaptive transparency screen buffers [8]. Examples of use for these trace friendly data structures are indirect diffuse (radiosity), indirect specular (reflections) and sparse shadowing of direct specular. The last one is what helps with our current issue.

The Samaritan demo[9] from Epic had a very similar issue that they solved in the same way I am suggesting. They had many point lights which generated specular highlights at any distance. To shadow them they did a cone trace in the direction of the reflection vector against a signed distance field that was stored in a volume texture. This was already being done for other reflections so using that data to shadow the point lights doesn’t come at much cost. The signed distance field data structure could be swapped with any of the others I listed. What is important is that the shadowing is calculated with a cone trace.

What I propose as the solution to our problem is to use traditional shadow maps only within the diffuse radius. Do a cone trace down the reflection vector. The cone trace will return a visibility function that any specular outside the range of a shadow map can cheaply use to shadow.

Actually, having shadowing data independent from the lights means it can be used for culling as well. The max unoccluded ray distance can be accumulated per tile which puts a cap on the culling cone for light sources. I anticipate this form of occlusion culling will actually be a very significant optimization.

This shadowing piece of the puzzle means the changes I suggested in my last post, in theory, come at a fairly low cost assuming you already do cone tracing for indirect specular. That may seem like a large assumption but to demonstrate how practical cone tracing is, a very simple, approximate form of cone tracing can be done purely against the depth buffer. This is what I do with screen space reflections on current gen hardware. I don’t do cone tracing exactly but instead reduce the trace distance with low glossiness and fade out the samples at the end of the trace. This acts like occlusion coverage fades by the radius of the cone at the point of impact which is a visually acceptable approximation. In other words the crudest form of cone tracing can already be done in current gen. It is fairly straightforward to extend this to true cone tracing on faster hardware using one of the screen space methods I listed. Replacing screen space with global is much more complex but doable.

The result is hopefully point light specularity “just works”. The problem is then shifted to determining which lights in the world to attempt to draw. Considering we have >10000 in one map in Prey 2 this may not be easy :). Honestly I haven’t thought about how to solve this yet.

I, like everyone else who has talked about tiled light culling, am leaving out an important part which is how to efficiently meld shadow maps and tiled culling for the diffuse portion. I will be covering ideas on how to handle that next time.

Finally, I want to reach out to all that have read these posts that if you have an idea on how the cone based culling can be adapted to a blinn distribution please let me know.

[1] http://directtovideo.wordpress.com/2012/03/15/get-my-slides-from-gdc2012/
[2] http://iquilezles.org/www/material/nvscene2008/rwwtt.pdf
[3] http://maverick.inria.fr/Publications/2011/CNSGE11b/GIVoxels-pg2011-authors.pdf
[4] http://www.mpi-inf.mpg.de/~ritschel/Papers/Microrendering.pdf
[5] http://graphics.cs.yale.edu/julie/pubs/bc03.pdf
[7] http://www.punkuser.net/vsm/vsm_paper.pdf
[9] http://www.nvidia.com/content/PDF/GDC2011/GDC2011EpicNVIDIAComposite.pdf

## Sunday, April 29, 2012

### Tiled Light Culling

First off I'm sorry that I haven't updated this blog in so long. Much of what I have wanted to talk about on this blog, but couldn't, was going to be covered in my GDC talk but that was cancelled due to forces outside my control. If you follow me on twitter (@BrianKaris) you probably heard all about it. My comments were picked up by the press and quoted in every story about Prey 2 since. That was not my intention but oh, well. So, I will go back to what I was doing which is to talk here about things I am not directly working on.

### Tiled lighting

There has been a lot of talk and excitement recently concerning tiled deferred [1][2] and tiled forward [3] rendering.

I’d like to talk about an idea I’ve had on how to do tile culled lighting a little differently.

The core behind either tiled forward or tiled deferred is to cull lights per tile. In other words for each tile, calculate which of the lights on screen affect it. The base level of culling is done by calculating a min and max depth for the tile and using this to construct a frustum. This frustum is intersected with a sphere from the light to determine which lights hit solid geometry in that tile. More complex culling can be done in addition to this such as back faced culling using a normal cone.

This very basic level of culling, sphere vs frustum, only works with the addition of an artificial construct which is the radius of the light. Physically correct light falloff is inverse squared.

### Light falloff

Small tangent I've been meaning to talk about for a while. To calculate the correct falloff from a sphere or disk light you should use these two equations [4]:

Falloff:
$$Sphere = \frac{r^2}{d^2}$$
$$Disk = \frac{r^2}{r^2+d^2}$$

If you are dealing with light values in lumens you can replace the r^2 factor with 1. For a sphere light this gives you 1/d^2 which is what you expected. The reason I bring this up is I found it very helpful in understanding why the radiance appears to approach infinity when the distance to the light approaches zero. Put a light bulb on the ground and this obviously isn’t true. The truth from the above equation is the falloff approaches 1 when the distance to the sphere approaches zero. This gets hidden when the units change from lux to lumens and the surface area gets factored out. The moral of the story is don’t allow surfaces to penetrate the shape of a light because the math will not be correct anymore.

### Culling inverse squared falloff

Back to tiled culling. Inverse squared falloff means there is no distance in which the light contributes zero illumination. This is very inconvenient for a game world filled with lights. Two possibilities, first is to subtract a constant term from the falloff but max with 0. The second is windowing the falloff with something like (1-d^2/a^2)^2. The first loses energy over the entire influence of the light. The second loses energy only away from the source. I should note the tolerance should be proportional to the lights intensity. For simplicity I will use the following for this post:
$$Falloff = max( 0, \frac{1}{d^2}-tolerance)$$

The distance cutoff can be thought of as an error tolerance per light. Unfortunately glossy specular doesn’t work well in this framework at all. The intensity of a glossy, energy conserving specular highlight, even for a dielectric, will be WAY higher than the lambert diffuse. This spoils that idea of the distance falloff working as an error tolerance for both diffuse and specular because they are at completely different scales. In other words, for glossy specular, the distance will have to be very large for even a moderate tolerance, compared to diffuse.

This points to there being two different tolerances, one for diffuse the other for specular. If these both just affect the radius of influence we might as well just set the radius of both as the maximum because diffuse doesn’t take anything more to calculate than specular. Fortunately, maximum intensity of the specular inversely scales with the size of the highlight. This of course is the entire point of energy conservation but energy conservation helps us in culling. The higher the gloss, the larger the radius of influence the tighter the cone of influencing normals.

If it isn’t clear what I mean, think of a chrome ball. With a mirror finish, a light source, even as dim as a candle, is visible at really large distances. The important area on the ball is very small, just the size of the candle flame’s reflection. The less glossy the ball, the less distance the light source is visible but the more area on the ball the specular highlight covers.

Before we can cull using this information we need specular to go to zero past a tolerance just like distance falloff. The easiest is to subtract the tolerance from the specular distribution and max it with zero. For simplicity I will use phong for this post:
$$Phong = max( 0, \frac{n+2}{2}dot(L,R)^n-tolerance)$$

### Specular cone culling

This nicely maps to a cone of L vectors per pixel that will give a non-zero specular highlight.

Cone axis:
$$R = 2 N dot( N, V ) - V$$

Cone angle:
$$Angle = acos \left( \sqrt[n]{\frac{2 tolerance}{n+2}} \right)$$

Just like how a normal cone can be generated for the means of back face culling, these specular cones can be unioned for the tile and used to cull. We can now cull specular on a per tile basis which is what is exciting about tiled light culling.

I should mention the two culling factors need to actually be combined for specular. The sphere for falloff culling needs to expand based on gloss. The (n+2)/2 should be rolled into the distance falloff which leaves angle as just acos(tolerance^(1/n)). I’ve leave these details as an exercise for the reader. Now, to be clear I'm not advocating having diffuse and specular light lists. I'm suggesting culling the light if diffuse is below tolerance AND spec is below tolerance.

This leaves us with a scheme much like biased importance sampling. I haven’t tried this so I can’t comment on how practical it is but it has the potential to produce much more lively reflective surfaces due to having more specular highlights for minimal increase in cost. It also is nice to know your image is off by a known error tolerance from ground truth (per light in respect to shading).

The way I handle this light falloff business for current gen in P2 is by having all lighting beyond the artist set bounds of the deferred light get precalculated. For diffuse falloff I take what was truncated from the deferred light and add it to the lightmap (and SH probes). For specular I add it to the environment map. This means I can maintain the inverse squared light falloff and not lose any energy. I just split it into runtime and precalculated portions. Probably most important, light sources that are distant still show up in glossy reflections. This new culling idea may get that without the slop that comes from baking it into fixed representations.

I intended to also talk about how to add shadows but this is getting long. I'll save it for the next post.

References:
[1] http://visual-computing.intel-research.net/art/publications/deferred_rendering/
[4] http://www.iquilezles.org/www/articles/sphereao/sphereao.htm

## Wednesday, August 10, 2011

### New Prey 2 screenshot

It has been a long time since I updated this blog with substantial content but I wanted to point out that Bethesda just released this new screenshot of Prey 2. It's a great shot but it's also a fantastic demonstration of some new graphical features I added to our latest build of the game.

First, there's the depth of field in the background which is HDR circular bokeh DOF.

Secondly, in the puddles on the ground, you will see screen space reflections. They aren't planer reflections, they work on every surface and run on every platform. SSR really adds a ton of dimension and accuracy to our wet, metal filled, alien noir city. I can't talk yet about how it works unfortunately.

So, check it out and tell me what you think. Hopefully in not too long I can start talking about how some of the tech works but for now you just get a glimpse.

-Brian

## Sunday, January 30, 2011

### Virtualized volume textures

First off it's been a very long time since I made a post. Sorry about that. I've found it difficult to come up with subjects to discuss that I both know enough about and am allowed to publicly talk about. For many, personal hobby projects can be the source of subjects to write about but all the at home pet project stuff I do, I do in the HH codebase and check in if it is a success. Personally I find this more rewarding than the alternative because it can go into a commercial product that hopefully will be seen by millions as well as it can get artist love which is very hard to get with hobby projects. The two biggest downsides are that I no longer own work I do in my free time and I can't easily talk about it. Now on to a technique that fits the bill as I have no particular commercial use for at the moment.

Irradiance volumes using volume textures is a technique that has been getting some use lately. Check out the following for some places I've noticed it.
Split/Second
Cryengine 3 (Cascaded Light Propagation Volumes)
FEAR 2
Rust

Probably the biggest downside to volumes vs more traditional lightmaps is the resolution. Volume textures take up quite a bit of memory so they need to be fairly low resolution. Unfortunately much of this data is covering empty space. It's convenient for empty space to have some data coverage so that the same solution can be used for dynamic objects but the same resolution is certainly not needed. How would you store a different resolution of volume data on world geometry than in empty space?

The most straight forward solution to me is an indirection texture. Interestingly what this turns into is virtualizing the volume texture just like you would a 2d texture. That indirection volume texture acts like the page table to your physical texture. Each page table texel translates the XYZ coordinates into a page or brick in the physical volume texture and subsequently the UVW coordinates in the physical volume texture. If you need a refresher on virtual texturing check out Sean Barrett's and id's presentations on the topic. All the same limitations apply in 3d as they do in 2d. Pages will need borders to have proper filtering. The smaller the page size the more waste due to borders and the larger the page table gets.

Another way of thinking of this is as a sparse voxel octree. Instead of the page table being managed like a quadtree it would work like an octree. Typically this data structure is thought of only for ray casting but there's nothing inherent about it that requires that. SVO's have also only been stored as trees, requiring traversal to get to the leaves. So long as you have bricks and aren't working with the granularity of individual voxels the traversal can be changed into a single texture look up just like the quadtree traversal in a 2d virtual texture.

Thinking about it as a SVO is helpful because volume data usually has more sparseness than 2d textures. In this case we don't really care about bricks where there is no geometry. If you use a screen read back to determine which pages to load this will happen automatically. Better yet you don't need to even store this data on disk. Better than that you don't need to generate that data in the first place during the offline baking process. Don't worry about dynamic objects. There is still data covering the empty space, it is just at the resolution of one page covering the world. If you need it at a higher resolution than that you can force a minimum depth to the octree.

In the end it still likely can't compete with the resolution a lightmap can get you if high res is what you are looking for. If lower res is the target it probably will be quite a bit more memory efficient because of not having any waste due to 2d parameterization. As for what a good page size would be I'm not sure as I haven't implemented any of this. If I did I probably couldn't talk about it yet. If anyone does implement this I'd love to hear about it.

## Sunday, January 3, 2010

### RGBD

This entire post turned out to be hogwash. I'm wiping it out to prevent the spread of misinformation. If you are interested in why it's all nonsense see my comment below. Thanks to Sean Barrett for pointing it out. The result of all of this has been positive ignoring the part of me looking extremely foolish. RGBM is more useful than I originally claimed because a larger scale factor can be used or if a fairly small range is required the gamma correction is likely not needed.

## Friday, November 6, 2009

### UDK

Epic released UDK today. It is the next step in their plan to completely dominate the game engine licensing market place. They've been doing a pretty good job of that so far. To all the other companies trying to compete I'd suggest following in their footsteps. The reason I believe they are so successful is because they do everything they can to get the engine out there and in peoples hands. You can't be scared of people looking at or stealing your stuff. If devs can't look at it they certainly won't plunk down major money to buy it. Epic has been powering their freight train of engine licensing with visibility of their product. With UDK now they have a brand new group of students and indie developers who are going to be familiar with unreal engine as well as make the barrier to evaluate the engine for commercial purposes practically nothing. You get to see all the tools and a major portion of documentation without even having to talk to someone at Epic. You can see engine updates whenever they release a new build. If you want to know more they will send you more than enough stuff to make up your mind in the form of an evaluation version. Compare that to most of the other engine providers. Many will never let you see any code and require all sorts of paper work to see private viewings of just an engine demonstration. As expected they are also not getting very many licensees. Stop being so paranoid and let people see your stuff.

On to the tech stuff. I haven't spent a ton of time looking at it yet but I've noticed some new things already. Of coarse there's the obvious stuff like Lightmass that they've talked all about. There's docs on all that which is great. I'm going to talk about what they aren't talking about. First they changed the way they encode their lightmaps. Previously it was 3 DXT1's that stored the 3 colors of incoming light in the 3 HL2 basis directions. They still use the same concept now but instead only have 2 DXT1's. The first stores the incoming light intensity in each of the 3 directions as RGB. The second is the average color of all 3 directions. The loss in quality may be small as the error is mainly bumps picking up different colors of light. The memory is 2/3's of what it was so it seems like a smart optimization.