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GenThree Overview

April 15, 2012

This post is part of the series “Debris: Opening the box“.

Now that the code is released, I’m going to change things up a bit. Originally, I was going to describe Werkkzeug 3’s operator execution engine in isolation; but now there’s enough material to give you the story in the proper order, with all the false starts and backtracking that went into it, which should explain some of the design quirks of the final result. Because it all starts with GenThree, the tool we wrote for Candytron (Note: I originally wrote this text for the fr_public repository).

Introduction

GenThree was the next tool Chaos started after Werkkzeug1. See diary.txt for the early history (if you can read German). I joined the project some time after the last entry in diary.txt, in early January 2003. This marks the first time that Chaos and me collaborated directly; before then, Chaos had used my packers, but we hadn’t shared much code.

After joining, I initially took over the Mesh generator and started work on the “FX Chain” (postprocessing effects), while Chaos focused on the GUI and Scripting parts of the system. After the first 2 months or so it rapidly becomes very hard to say who did exactly what because there was no clear top-level division of labor; we worked quite well together, to the point where we sometimes weren’t certain who had written what. As an aside, if you can read German, concept.txt contains a bunch of notes by Chaos, again mostly from the early phases of the project. Some of these ideas ended up in GenThree (and later Werkkzeug3), some never got implemented, and some we tried but rejected.

Scripting

Anyway; even when I started (this was in the lead-up to Candytron), we were very clearly aiming at producing kkrieger; Candytron was intended to be a kind of milestone. The thing that differentiates GenThree from our previous systems was that it was intended to replace the Operator Graph that had been at the center of our previous tools with a scripting language. The Operator Stacking UI still exists, but outputs script code instead of a blob describing the graph. The intention was to use this UI to build up components, but do all of the high-level glue (and, ideally, a lot of the game logic for kkrieger) as a script.

We were considering several kinds of approaches, and different levels of abstraction – e.g. the top of concept.txt has a detailed sketch of a quite low-level scripting system top that would compile to regular x86 code, plus the associated back-end. This was never implemented; the idea evolved into another scripting system that was intended to run-time compile to regular x86 code. Then we’d store the (compressed) byte code plus a small runtime engine instead of regular x86 code, with the hope that the byte code would be smaller. The language was designed to facilitate easy compilation; it was very FORTH-like, so an initial interpreter could just use explicit stacks and threaded code. This one never happened either, primarily because we dreaded going all-in on something this experimental with a code base that was intended to last till kkrieger at the very least. However, the underlying idea (store all our code in a form that’s more amenable to compression) never went away, and some years later metamorphosed into dispack/disfilter, the x86 code transform used by kkrunchy.

But back to GenThree. Scripting system approach number three, and what you see here, was CSL – Chaos’ Scripting Language (he did all the work on that one). The initial idea is described in concept.txt under “Scripted Demo System”. The runtime system is still very FORTH-like, but it uses three stacks instead of the customary two:

  • The I-Stack or integer stack, which contains 16.16 fixed point numbers (there’s no support for floating-point types in script code)
  • The R-Stack or return stack, used to implement calls, loops etc.
  • The O-stack or object stack, which holds references to complex composite objects like Bitmaps, Meshes and so forth. These objects are typed.

The operation of the I- and R-stacks is only visible to the bytecode interpreter; the language itself is C-like and exposes familiar control structures to the user. The O-stack is different; it’s explicitly manipulated by the code.

Rather than explaining this to you in boring detail, I just recommend you check out system.txt. This defines the set of operators available to every project, and displays some of the unconventional features of the system:

At the beginning, a bunch of classes are defined. These correspond to classes of values on the O-stack. The fields listed define instance variables; I don’t remember how exactly that particular binding worked. Each class also has a magic ID, written as hexadecimal number. Since we use 16.16 fixed point, they’re written in a somewhat funky way here; they’re regular 32-bit ints on the C++ side.

After that, there are some global functions and variables. Note that functions can be assigned IDs, just like classes. This is part of a light-weight binding scheme. By convention, positive IDs are used for script functions that are called by C++ code (so the OnInit, OnFrame and OnSound functions are forward declarations for code that’s supposed to be written in CSL), while negative IDs are script declarations for functions implemented in native C++ code. There’s a table in the C++ code mapping those magic numbers to function pointers – and nothing else. Note that each function has type information on the parameters; this is detailed enough to manually build a stack frame and call into the given C++ function, without needing any layer of glue code or marshalling. This is one of the more unconventional design decisions in CSL; it was done both for convenience and to reduce code size, since mechanical wrapper/glue code is very repetitive and adds zero value. Also note that each function comes with a description of what types of objects are expected to be at the top of the O-stack before execution, and what ends up on the O-stack afterwards. To pick a declaration at random:

void MeshMaterial(
  int id = 1 [ 1 .. 255 ],
  int mask = 0 [ "mask8:f" ],
) ( mesh material -- mesh ) = -0x0b,"mod link2",'m';

the ( mesh material -- mesh ) line here denotes that this operator expects a mesh and a material object on the O-stack, and returns a new mesh object. It also means that the op has binding number -0xb (-11), the "mod link2" is a list of annotation tags that can influence GUI, code generation and memory allocation (“mod” here means that the operator can modify an object in-place, for example), and the ‘m’ assigns the hotkey ‘m’ to this operator in the GUI. You get the idea.

There’s even some operators completely written in CSL – cf. the Crashzoom and FXWideBlur filters, both of which make use of the language and call to a single underlying function Blend4x written in C++ that is used to implement various render-to-texture effects.

Going back up a level, I mentioned that there’s still a GUI behind all this. Well, the op-stacking UI is alive and kicking in GenThree, so how does that part work internally? Just look at data/candytron_final_064.csl, the final generated source code for Candytron. The first part of this is just system.txt, which I have just described. After that comes a bunch of generated code corresponding to whatever the user built in the Op-Stacking GUI (this is OnGenerate and friends) and the animation/timeline timeline_OnInit and timeline_OnFrame). All of this is code with a very regular structure that’s intended to compress well. Finally, at the very end there’s the bit starting with // new text – this is actually code entered in the tool itself by the user. The idea was that you could code in there too, but it never saw serious use.

So all this looks pretty nice on paper, right? C++ code providing low-level functions and runtime services, a script engine to tie it all together, and a very decoupled UI that’s loosely coupled: anything that can compile to CSL goes.

The only problem was that (language quirks like the fixed-point centric world view aside) it didn’t work well for what we needed it for. It had reasonable density for code, but as a data representation (and all the Ops are really more data than code) it sucked. Our previous systems had a very compact representation for ops, and lots of tricks to quantize and pack parameter values into a small space. In the byte code-based system, it all boiled down into general “push value” and “call function” op codes, and while the type information was there for execution purposes (to convert the 16.16 ints into floats when necessary, for example), none of that structure was evident or easily exploitable in the byte code. As a result, compression ratios of the byte code sucked – compared to what we were used to, anyway. Candytron had significantly less procedurally generated content than our average intro, yet still spent about 8k (after compression!) on describing it, whereas most of our intros around that time needed maybe 6k for the ops.

On the other side, the script runtime system was fairly big too – much bigger than the more specialized operator execution engines we’d used before (or after). And it just wasn’t pulling its weight. So the summer after we released Candytron, Chaos threw away the scripting parts and most of the existing GUI, but kept the Operators – Werkkzeug3 was born. Still, the base system and most of the content generation parts (and their interface) survived. As one of several side effects, that means that Wz3 used (and still uses) the same “explicit description of stack layout” method for parameters. This has far less benefits in a non-script environment (and we probably wouldn’t have done it if we had been shooting for ops from the beginning), but it’s really just the only part of the original scripting engine that survived.

Code organization

Let’s start at the beginning: _start.cpp (and _startdx.cpp). These two files (plus associated header files) form the OS/rendering abstraction. Yes that’s right, no other file in the project includes any of the Windows or D3D header files, it’s all abstracted away. Fundamentally this is not hard because a demo or intro really doesn’t care that much about what OS it’s running on; what it wants is a window, a nice way to switch state and render triangles, a pipe to output sound to and a way to get current timing information (and maybe keypresses). The tools need a bit more on the input side (proper mouse and keyboard input at least) and some file IO, but it’s still a very limited set of functionality.

The tools use the aforementioned two files; intros use _startintro.cpp, which is a size-optimized mash-up of both _start and _startdx. There’s also _startgdi (for GDI-based GUI-only rendering) and _startgl (GL-based), both of which were never properly completed and don’t work, so there’s not much to talk about.

_start performs initialization and then calls sAppHandler. sAppHandler is implemented in the actual main program and is basically an event handler. To give an example of what constitutes events:

  • sAPPCODE_CONFIG – display a configuration dialog. (Called before the 3D API is initialized)
  • sAPPCODE_INIT – main initialization phase (after 3D API is initialized).
  • sAPPCODE_EXIT – similarly, shutdown phase.
  • sAPPCODE_KEY – keyboard input.
  • sAPPCODE_PAINT – paint a new frame.

There’s a few more, but you get the idea. There’s two main versions of this: the “tool” runtime in main.cpp and the “player” runtime in mainplayer.cpp. The tool version sets up a window and then runs the GUI event loop; the player version does whatever initialization is necessary and then plays back the demo.

The GUI is a regular retained-mode event loop-based affair. Everything is rendered using the 3D API (D3D in our case) though, and re-rendered every frame, which greatly cuts down on updating bugs :). There’s a lot of UI and UI-related code in GenThree, Werkkzeug3 etc., but that kind of code doesn’t make for very exciting exposition, so I’ll just gloss over it here.

The scripting engine that I’ve talked about is split into two files: cslce.cpp, which implements the scanner, parser and bytecode generator, and cslrt.cpp, the bytecode interpreter and runtime system.

CSL is a language suitable for one-pass compilation with semantic processing and code generation interleaved with code generation. It can be processed with a straightforward Recursive Descent parser. This class of languages has a long tradition and leads to simple, fast (but not very smart) compilers without depending on any special parser generator tools. CSL in particular was greatly inspired by LCC and its code as described in the book “A Retargetable C Compiler: Design and Implementation” – a very worthwhile read if you would like to expand your horizon on a type of Software Engineering that’s underappreciated: Simple, straightforward, very well thought-out no-nonsense C code. (As you might be able to tell, Chaos and me really like that book). Anyway.

The second part, cslrt, implements the runtime. Since the bytecode is stack-based with a small set of built-in operations, this is really quite straightforward.

On the player side, the rest of the source, including all the generators, binds loosely to the script runtime, which calls the shots. The table of script functions, together with some glue code, is in genplayer.cpp. The rest is just a bag of script-callable functions, which I’ll describe one functional group at a time.

Mesh generation

This is something we tried to push hard for this intro – much more so than in our previous releases. The mesh generator contains a bunch of ideas, some
of which worked out well, and lots that didn’t. The core mesh data structures are half-edge based, and the implementation is contained in genmesh.cpp and genmesh.hpp. I’ve written about this in detail before – just look at the mesh-processing articles linked from the parent post.

One part I haven’t talked about before, and that only appears in GenThree, is the Recorder – the Rec* group of functions in the GenMesh class. This was an experimental approach for mesh animation that we used heavily in Candytron. The basic idea was to separate the topological processing (which involves tricky and often slow manipulation of complex data structures) from the vertex processing (which, for a lot of operators, simply computs new vertices as linear combinations of existing ones, which is simple and relatively fast).

So Chaos had the idea to separate the two. The topological processing just runs once, at init time. All the topological modifications are done at that time, but vertex manipulations get recorded to a log. Most of the time, this log is played back immediately and then thrown away – but if the user wants to animate something, he can turn on “proper” recording for the mesh, which means that we actually keep the log for runtime evaluation. Then, every frame, a bunch of input parameters can be changed and the log is played back. Since we only modify vertices not indices or connectivity, we just need to stream the new data into a vertex buffer. This system is far more flexible than regular skinning; several scenes in Candytron first skin the girl mesh (unsubdivided), then subdivide it once, extrude parts of it, and subdivide again, for example. The extrusion and subdivision operators are relatively heavyweight, since they try to deal correctly with hard edges, discontinuities and so forth, but the part that runs at runtime only only does very simple operations on vectors, so it’s quite fast.

While a neat idea, we ultimately killed this one off too. It worked just fine, but in practice just phrasing all our dynamic mesh animation in terms of skinning made things simpler and more orthogonal at the back end, allowed us to do more work in the mesh consolidation/vertex buffer generator phase, and simplified the mesh code a bit (since there was no longer a need to separate topology and geometry processing into two parts and explicitly record every parameter that went into vertex generation). And finally, a single static skinning setup can be baked into a vertex shader for performance; the more complex recorder system, with its variable input-output relationships and different operations done in different sequence, not so much. Note that we still ended up using SW skinning in kkrieger/debris for other reasons (shadow volumes), but the decision to remove the recorder was made long before we commited to shadow volumes.

Finally, there’s Mesh_Babe, which was used to get the girl mesh (her name is “Josie”, by the way) into the intro. On the Editor side, this just loads an exported mesh (.XSI file in this case, since giZMo – the artist on this project – was using XSI). However, the XSI file is much too big to use in a 64k, so we implemented some (then) cutting-edge mesh compression; the papers had only been published a few months prior! The algorithm we ended up using was described in “Near-Optimal Connectivity Encoding of 2-Manifold Polygon Meshes” by Khodakovsky et al.; there was a separate paper “Compressing Polygon Mesh Connectivity with Degree Duality Prediction” by Isenburg that described the same basic idea in a slightly different framework that probably would’ve been easier to implement in GenMesh, but I realized that too late. I’ll spare you the details – suffice to say this was my first (but by no means last) contact with the fact that being able to understand a paper and being able to implement it correctly are two different things, and one of them is much harder than the other :).

Anyway, the idea was that we’d generate rough basic geometry for kkrieger using a conventional modeler and export it. That was in fact the main reason to be implementing a polygon mesh compression scheme that was this general. But in the end we went down a very different road for level building, so this code too ended up being unused anywhere else. (Starting to see a pattern here?)

Texture generation

This code has the distinction of being the least experimental of all the things we tried in Candytron. It also is the only piece of the whole thing that survived into the Debris era without being substantially rewritten or outright replaced. Part of this was the Second System Effect (e.g. GenMesh was clearly overengineered for what we needed), but mostly it was just the result of us going out of our comfort zone established in previous tools and trying to approach things differently. Most of the time it didn’t work out that well, but it wasn’t at all obvious from the outset that this would happen, and it was certainly a learning experience.

Anyway, on to the actual texture generator. Like the RG2 texture generator (but unlike the original fr-08 Generator or Werkkzeug1), GenThree uses an integer format with 16 bits per color channel, to make sure there’s enough precision headroom even after several stages of color adjustment and layer composition. Though we store 16 bits per channel, we actually only use 15 bits – a compromise to navigate the odd set of MMX instructions available at the time: note that PMULHUW was only added with the Pentium 3, and there’s no unsigned version of PMADDWD – dealing with unsigned 16-bit quantities was simply awkward. We also allow ourselves to be slightly sloppy with regards to rounding and such, since we have enough extra bits not to care too deeply about what goes on in the least-significant bits. This makes the code somewhat simpler, though in the grand scheme of things, it probably didn’t matter. Finally, this representation means that a single ARGB pixel fits inside a MMX register, and there’s no need to do any unpacking or packing to do multiplies (MMX only provides 16-bit multiplies).

As you might have noticed by now, all of these decisions were made with an eye towards reasonably fast and simple implementations of the basic operations using MMX. It was all designed long before using GPUs for texture generation was a serious option: when GenThree was written, we were already using D3D9 (brand new at the time!), but we were using the fixed-function pipe – cards with shader support were still quite new and not very wide-spread. And when RG2 (which made many of these original design decisions) was written, there wasn’t any programmable HW in the PC market, period.

So you would expect that there’s lots of MMX code in the texture generator. And you would be completely right. At the time, code generation when using intrinsics was simply dreadful, so it’s mostly inline assembly too. The code itself is not terribly interesting – it does what you’d expect, and this was from before we did any tricky optimizations. It’s probably worth looking at the Blur filter, however; it uses the old (as in, OLD) but still awesome rendering trick of factoring triangle filters and gaussian blurs as iterated box filters: 2x box gives you triangle, and 3x box gives you a uniform B-Spline that is within a few percent of a true Gaussian but much cheaper to compute. The nice bit about this is that box filters are really simple to do fast – for every pixel, you scroll the “window” by one pixel, meaning one pixel on the left “drops out” and one pixel on the right “comes in”, while the rest of the sum stays the same. This is very cheap to perform incrementally, and Bitmap_Blur has a straightforward implementation of the idea. Look closely – by the time you’ll see it next in this repository, namely in Werkkzeug3, it will support non-integer blur kernel widths and be all MMX’ed up and quite hard to follow :) (It’s the subject of its own upcoming blog post; go figure).

Lights, Camera, Action!

Lighting, material, camera and scene description all happen in the same file, genmaterial.cpp. All of these are objects, so they have a description as a data structure, but in GenThree these are quite basic. A scene is a list of meshes with associated transform matrices. A camera is just a matrix with a few extra parameters describing FOV and such. A light is also just a bag of values that gets passed along to D3D. A material both contains D3D “material” parameters (which influence lighting) and the textures and render states used at the time. States are generally collected once at initialization time and compiled into a list (for faster state caching), but other than that it’s pretty simple too. The most important part here is that an explicit material representation that describes *all* the state sent to the API is even there. In a heavily data-driven environment like our tools, that is just the natural way to handle things; that it also happens to be efficient is a nice bonus.

The most interesting part of genmaterial has in fact nothing to do with materials at all; for reasons that I don’t remember, part of the GenMesh implementation is in here, namely the part that actually converts GenMeshes to Vertex/Index buffers. Sometime in the middle of kkrieger development, all of the mesh preparation, lighting/material and multipass management stuff got pulled out into a separate module, engine.cpp, where it’s resided ever since. Of course, when we wrote Candytron, all of this code was much simpler, so there you go.

There’s two interesting bits worth noting here: first, note how we handle vertex/index buffers. GenThree introduced GeoBuffers, which bundle a Vertex Buffer with an associated Index Buffer into a neat little package. This model (in one form or another) has been around with us ever since – you really want to treat them as a unit most of the time. The system level code (in _startdx.cpp) handles all the memory management part of it – pretty sweet. There’s also a special GeoBuffer that has a static index buffer (describing a list of quads) that’s used to render, well, quads. For GUI, particles and such. It makes sense to have this as part of your rendering abstraction; quads are common, and having a simple way to render them just makes sense. Also, if you make them a thing at the system level, it’s trivial to adapt to targets that natively support non-indexed quad lists.

The second part is that the mesh preparation code supports several different modes (or “programs”, as they’re called in the code): There’s MPP_STATIC and MPP_DYNAMIC, which are fairly obvious (allocate to dynamic/static vertex buffer please); more interestingly, there are also the “sprites”, “trees”, “thick lines”, “outlines”, and “finns” modes, which are also extensively used in Candytron. All of these represent different ways to turn a given mesh into a vertex and index buffer. The same mechanism was used in kkrieger to generate input data for shadow volume extrusion (note that while there is a MPP_SHADOW there, that’s a different thing than what we did in kkrieger).

That’s all, folks!

There’s more stuff in there, but this covers what I think are the most interesting bits. If there’s questions or something is unclear, don’t hesitate, just ask!

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