Just spent half a day debugging this, so here it is for the future reference of the internets.

In a deferred rendering setup (see Game Angst for a good discussion of deferred shading & lighting), lights are applied using data from screen-space buffers. Position, normal and other things are reconstructed from buffers and lighting is computed “in screen space”.

Because each light is applied to a portion of the screen, the pixels it computes can belong to different objects. If in any place of lighting computation you use textures with mipmaps, be careful. Most common use for mipmapped light textures is light “cookies” (aka “gobo”).

Let’s say we have a very simple scene with a spot light:

Light’s angular attenuation comes from a texture like this:

If the texture has mipmaps and you sample it using the “obvious” way (e.g. tex2Dproj), you can get something like this:

Black stuff around the sphere is no good! It’s not the infamous half-texel offset in D3D9, not a driver bug, not a shader compiler bug and not the nature trying to prevent you from writing a deferred renderer.

It’s the mipmapping.

Mipmaps of your cookie texture look like this (128x128, 16x16, 8x8, 4x4 shown):

Now, take two adjacent pixels, where one belongs to the edge of the sphere, and the other belongs to the background object (technically you take a 2x2 block of pixels, but just two are enough to illustrate the point). When the light is applied, cookie texture coordinates for those pixels are computed. It can happen that the coordinates are very different, especially when pixels “belong” to entirely different surfaces that are quite far away from each other.

What the GPU does when texture coordinates of adjacent pixels are very different? Chooses a lower mipmap level so that texel to pixel density roughly matches 1:1. On the edges of this “wrong” screenshot, it happens that very small mipmap level is sampled, which is either black or white color (see 4x4 mip level).

What to do here? You could disable mip-mapping (which is not good for performance and not good for image quality). You could drop some smallest mip levels which might be enough and not that bad for performance. Another option is to manually supply LOD level or derivatives to sampling instructions, using something else than cookie texture coordinates. For example, derivative in view space position, or something like that. This might not be possible on lower shader models though.

…I took a plane to Copenhagen. Oh, this sounds familiar…

Well ok, it all started a bit before:

I exchanged some emails with David and Joachim and they invited me for a gamejam in their office. Then one thing led to another, I was young and needed money (oops! wrong topic) and on January 2006 I started working on this thing called “Unity”.

Unity was at version 1.2.1 then. Since then we’ve released about a dozen new versions, added hundreds (or thousands?) of new features, a handful of new platforms and have grown a lot.

Also, we stopped saying “Sales are INSANE!!!!11” whenever they exceeded a whopping ten thousand euros per week. Seriously, that much money in 2006 was a big thing. Our Windows build machine was a single core Celeron with 512MB RAM because that’s what we could afford! Well ok, we’re still saying “sales are insane!” from time to time, just the threshold has gone way up.

Occasionally we’d get excited about strangest things. I think this email is about some car model from ATI that was on front page of our website in 2006. It’s beyond me why we’d put a car on Unity website, but somehow it seemed to make sense at the time.

It would take too much space to list all the awesome things that happened in those four years. I got to work on some things too, like Windows Web Player, Direct3D renderer, shadows, editor for Windows and whatnot. But I mostly concentrate on creating trouble, which does not seem to hinder Unity that much. I need to get more efficient!

Seriously though, it has been an amazing ride so far, and I hope it will only become better. Thanks to everyone at Unity Technologies and the community!

Rock on!

I’m catching up on various GPU hacks that exist for Direct3D 9 (things like native shadow mapping, render to vertex buffer, etc.). Turns out there’s a lot of them, but all the information is scattered around the intertubes.

So here are the D3D9 hacks known to me in one place.

Let me know if I missed something or got something wrong. I also want to figure out if Intel GPUs/drivers implement any of them.

A tweet by Michael Hutchinson on C#/Mono usage in games caused me to do a couple of short replies (one, two). But then I started thinking a bit more, and here’s a longer post on what is needed for C# (and more specifically Mono) to be used in games more.

In Unity we use Mono to do game code (well, Unity users are doing that, not us). Overall it’s great; it has tons of advantages, loads of awesome and a flying ninja here and there. But no technology is perfect, right?

Edit: Miguel rightly points out in the comments that Mono team is solving or has already solved some of these issues already. In some areas they are moving so fast that we at Unity can’t keep up!

#1: Garbage Collector

Most game developers do not like Garbage Collection (GC) very much. Typically, the more limited/hardcore their target platform is, the more they dislike GC. The reason? Most GC implementations cause rather unpredictable spikes.

Here’s a run of something recorded in the (awesome) Unity 2.6 profiler. Horizontal axis is time, vertical is CPU time spent in that frame:

At the bottom you see dark red thingies appearing once in a while. This is garbage collector kicking in, because some script code is allocating some memory at runtime.

Now of course, it is possible to write your script code so that it does no allocations (or almost no allocations). Preallocate your objects into pools, manually invoke GC when there’s a game situation when a small hickup won’t affect gameplay, etc. In fact, a lot of iPhone games made with Unity do that.

But that kind of side steps the whole advantage of “garbage collector almost frees you from doing memory management”. If you’re not allocating anything anyway, GC could just as well not be there!

A little side story. Me and Unity’s iPhone tech lead ReJ tried to explain what GC is to a non-programmer. Here’s what we came up with:

Garbage Collection is this cleaning service for lazy people. They can just leave any garbage on the floor in their house, and once in a while a garbage guy comes, collects all the garbage and takes it outside. Now, there are some intricacies in the service. First, you never know when the garbage guy will come. You might be taking a shower, doing a meditation or having some “sexy time” - and it’s in the service agreement that when a garbage guy comes, you have to let him in to do his work.

Second thing is, the garbage guy is usually some homeless drunkard. He smells so bad that when he comes, you have to stop whatever you were doing, go outside and wait until he’s done with the garbage collection. Even your neighbors, who might be doing something entirely else in parallel, actually have to stop and idle while garbage is being collected in your house!

There are variations of this GC service. One variation is called “moving GC”, where the garbage guy also rearranges your furniture while collecting the garbage - he moves it all into one side of your house. This is so that you can buy a bigger piece of furniture, or throw a huge piece of garbage - and there will be enough unused space for you to do that! Of course this way GC process takes somewhat longer, but hey, you get all your stuff nicely packed into one corner.

Can’t you see that this service is the greatest idea of all time?

This is quite a harsh attitude towards GC, and of course it’s exaggerated. But there is some truth to it. So how could GC be fixed?

GC fix #1: more control

More explicit control on when & how long GC runs. I want to say to the garbage guy, “come everyday at 4PM and do your work for 20 minutes”. In the game, I’d want to call GC with an upper time limit, say 1 millisecond for each call, and I would be calling that 30 times per second.

GC fix #2: sometimes I want to clean garbage myself

Inefficiencies and unpredictability of GC cause people to do even more work than a normal, oldskool memory allocation. Why not provide an option to deal with deallocations manually? I.e. a keyword reallynew could allocate an object that is not part of garbage collected world. It would function as a regular .NET object, just it would be user’s responsibility to reallydelete it.

Mono is already extending .NET (see SIMD and continuations). Maybe it makes sense to add some way to bypass garbage collector?

#2: Distribution Size

Using C#/.NET in a game requires having .NET runtime. None of the interesting platforms are guaranteed to have it, and even on Windows you can’t count on it being present. Mono is great here in a sense that it can be used on many more platforms than Microsoft’s own .NET. It’s also great on distribution size, but only if you compare it to Microsoft’s .NET.

In Unity Web Player, we package Mono DLL + mscorlib assembly into something like 1.5 megabytes (after LZMA compression). Which is great compared to 20+ megabytes of .NET runtime, but not that great it you compare it so, say, Lua runtime (which is less than 100 kilobytes).

On some platforms (iPhone, Xbox 360, PS3, …) it’s not possible to generate code at runtime, so Mono’s JIT does not work. All code that’s written in C# has to be precompiled to machine code ahead of time (AOT compilation). This is not a problem per se, but because .NET framework was never designed with small size and few dependencies in mind, doing anything will ultimately pull in a lot of code.

We joke that doing anything in C# will result in an XML parser being included somewhere. This is not that far from the truth; e.g. calling float.ToString() will pull in whole internationalization system, which probably somewhere needs to read some global XML configuration file to figure out whether daylight savings time is active when Eastern European Brazilian Chinese calendar is used.

Size fix #1: custom core .NET libraries?

For game uses, most of “fat” stuff in .NET runtime is not really needed. float.ToString() could just always use period as a decimal separator. Core libraries could consist just of essential collections (list, array, hash table) and maybe a String class, with just essential methods. Maybe it’s worth sacrificing some of the generality of .NET if that could shave off a couple of megabytes from your iPhone game size?

Of course this is very much doable; “all that is needed” (tm) is writing custom mscorlib+friends, and telling C# compiler to not ever reference any of the “real” libraries.

Size fix #2: make Mono runtime smaller

Uncompressed Mono DLL in our Windows build is 1.5 megabytes. We have turned off all the easy stuff (profiler, debugger, logging, COM, AOT etc.). But probably some more could be stripped away. Do our games really need multiple AppDomains? Some fancy marshalling? I don’t know, it just feels that 1.5MB is a lot.

#3: Porting to New Platforms

You know this classic: “There’s no portable code. There’s only code that’s been ported.”

Most existing gaming platforms are quite weird. Most upcoming smartphone platforms also are quite weird, each in their own interesting way. Porting a large project like Mono is not easy, especially since parts of it (JIT or AOT engine) highly depend on the platform.

For Unity iPhone, unexpected discovery that it’s not possible to JIT on iPhone made the initial release be delayed by something like 4 months. It did not help that in early iPhone SDK builds JIT was actually possible, and Apple decided to disable runtime generated code later. Making Mono actually work there required significant work both from Mono team and from Unity. We still have one guy working almost exclusively on Mono+iPhone issues!

Of course, maybe all the Mono iPhone work made porting to new platforms easier as a byproduct. But so far we don’t have Mono ported to any other platform, up to production quality. So judging from experience, we now always assume Mono port will be a pain, just because “some nasty surprises will come up” (and they always do).

#4: Small Stuff

There is a ton of small bits where extending .NET would benefit gaming scenarios. For example:

Suppose there is some array on the native engine side; for example vertex positions in a mesh (3xFloat for each vertex). Is it possible to make that piece of memory be represented as a native struct array for .NET side? So that it would not involve any extra memory copies, but N vertices somewhere in memory would look just like Vector3[N] for C#?

On a similar note, having “strided arrays” would be useful. For example, mesh data is often interleaved, so for each vertex there is a position, normal, UVs and so on. It would be cool if in C# position array would still look like Vector3[N], but internally the distance between each element would be larger than 12 bytes required for Vector3.

Where do we go from here?

The above are just random ideas, and I’m not complaining about Mono. It is great! It’s just not perfect. Mono being open source is a very good thing, which means pretty much any interested party can improve it as needed. So rock on.

Reading “Rendering Technology at Black Rock Studios” made me realize that cascaded shadow maps I did 2+ years ago in Unity 2.0 are probably called “deferred shadowing”. Since I never wrote how they are done… here:

The process is roughly this (all of this is DX9 level tech on PCs; later tech or consoles could and should use more optimizations):

1. Render shadow map cascades. All of them packed into one shadow map via viewports.

2. Collect shadows into screen sized render target. This is the shadow term.

3. Blur the shadow term.

4. In regular forward rendering, use shadow term in screen space.

More detail:

Render Shadow Cascades

Nothing fancy here. All cascades packed into a single shadow map. For example two 512x512 cascades would be packed into 1024x512 shadow map side by side.

Screen-space Shadow Term

Render all shadow receivers with a shader that “collects” shadow map term. In effect, shadows from all cascades are collected into a screen-sized texture. After this step, original cascaded shadowmaps are not needed anymore.

Unity supports up to 4 shadow map cascades, which neatly fit into a float4 register in the pixel shader. Correct cascade is sampled just once, without using static or dynamic branching. Pixel shader pseudocode:

 float4 near = float4 (z >= _LightSplitsNear);
 float4 far = float4 (z < _LightSplitsFar);
 float4 weights = near * far;
 float2 coord =
     i._ShadowCoord[0] * weights.x +
     i._ShadowCoord[1] * weights.y +
     i._ShadowCoord[2] * weights.z +
     i._ShadowCoord[3] * weights.w;
 float sm = tex2D (_ShadowMapTexture, coord.xy).r;

Additionally, shadow fadeout is applied here (shadows in Unity can be cast up to specified distance from the camera, and they fade out when approaching that distance).

After this I end up having shadow term in screen space. Note that here I do not do any shadow map filtering; that is done in screen space later.

On PCs in DX9 there is (or there was?) no easy/sane way to read depth buffer in the pixel shader, so while collecting shadows the shader also outputs depth packed into two channels of the render target.

Screen-space Shadow Blur

Previous step results in screen space shadow term and depth. Shadow term is blurred into another render target, using a spatially varying Poisson disc-like filter.

Filter size depends on depth (shadow boundaries closer to the camera are blurred more). Filter also discards samples if difference in depth is larger than something, to avoid blurring over object boundaries. It’s not totally robust, but seems to work quite well.

Using shadow term in forward rendering

In forward rendering, this blurred shadow term texture is used. Here shadow term already has filtering & fadeout applied, and the shaders do not need to know anything about shadow cascades. Just read pixel from the texture and use it in lighting computation. Done!

Fin

Back then I didn’t know this would be called “deferred” (that would probably have scared me away!). I don’t know if this approach is any good, but so far it works quite well for Unity needs. Also, reduces shader permutation count a lot, which I like.