Older blog entries for joolean (starting at number 82)

Clutter

Further adventures in game development: I've been working with Clutter to create a primitive client-side game engine to integrate with gzochi. Clutter's a 2-D scene graph library for writing OpenGL applications in C. It's the canvas library behind Mutter and, from what I can tell, a bunch of next-level GTK+ stuff. The documentation says it's for building "visually rich graphical user interfaces," but I don't see why you couldn't also use it to, say, manage the layout of a game screen, which is what I'm trying to use it for: There are a lot of things I don't miss from my brief career as a professional Flash developer, but computing dirty rectagles and figuring out the draw order for actors in a scene isn't something I was looking forward to writing myself. Finding Clutter felt serendipitous.

One thing Clutter doesn't come with, though, is a component that can render frame-based animations, which I'm modeling as sets of raster images that get painted on the stage in sequence at a specified rate. I've built some components that can load image atlases from disk or over the web and carve them up into frames in memory, and I've been trying to get Clutter to turn those frames into a flipbook. It took a lot of meditating over the documentation and some coaching from Clutter developers on IRC for me to get that doing animations this way wasn't going to be a misuse of the library -- but it would be something I'd have to write myself.

Clutter does ship with some data structures you can use for displaying images, so that's what I tried first. There are two: ClutterTexture, which is a GObject subtype of ClutterActor; and ClutterImage, which implements the ClutterContent interface and can thus be used to render ClutterActors that don't know how to render themselves. I saw some indication online that ClutterTexture was probably on its way out, so the first technique I used to implement my animation system was to have a handler for the "new-frame" signal on an animation-specific ClutterTimeline update the internal state of the animated actor's ClutterImage content with the pixel data for a new frame (via clutter_image_set_data). To my surprise, it actually worked (go Clutter!) but it also drove the CPU utilization on my netbook up to about 60%. Here's why: Since Clutter gets its drawing done via OpenGL (actually via Cogl, a GL/GLES compatibility layer), before you can display an image to the framebuffer, Clutter wants to get a handle to a CoglTexture, a block of allocated (and filled-out) GPU texture memory. Pushing textures to the GPU is expensive, so doing it every frame is pretty much a non-starter.

The next thing I tried was maintaining a roster of ClutterImage objects, each with static, pre-uploaded data for a single frame of animation, and updating the content of my sprite actor with the requisite image (via clutter_actor_set_content) on every new frame. This was an improvement, but only got me down to about 40% utilization. I think the problem with this approach might have been that updating the content at the actor level triggers an invalidation that forces some unnecessary invalidations of the scene graph that are expensive to recalculate, but I didn't investigate deeply.

At the suggestion of someone on IRC, the next thing I tried was creating my own implementation of the ClutterContent interface such that the paint_content function would use a pre-uploaded Cogl texture to paint each required frame. The paint_content function has the prototype:

void (*) (ClutterContent *, ClutterActor *, ClutterPaintNode *);

...where ClutterPaintNode argument is a local root of the render graph that corresponds to the actor to which the content is attached (content can be shared across multiple actors). To get your content painted, you need to attach a child paint node to that root. The ClutterPaintNode interface doesn't expose any primitive drawing callbacks, so I don't think they want you to write your own, but there's a provided implementation (ClutterTexturePaintNode) that paints the contents of the texture you give it. I built my ClutterContent implementation around this paint node type -- and it worked, but I was still at around 30% CPU. When I profiled my application, I found that the invalidation signal was being fired much more frequently than the new-frame signal, and the default mechanism for handling invalidation -- which I wasn't sure I wanted to override -- queues a paint operation.

I was feeling a bit down in the mouth about the prospects of animating my sprite at 20% CPU or less. I started Googling various combinations of "clutter" and "animation," and somehow arrived at the Gitorious page for Rob Staudinger's clutter-sprite project, which promises obliquely to provide "A sprite actor for clutter." It didn't look like much, but I started rifling the source files to see if I could learn something. Sure enough, I found that Rob cleverly overrides ClutterActor's paint function and uses cogl_rectangle to paint the frame of animation directly from an image stored as a CoglMaterial, skirting the Clutter render tree entirely. Using a variation on his technique, I was able to get my utilization down to about 25%, which seems like it might be as low as I'm going to get with my netbook's GPU and this version of Clutter.
gzochi

Having at least temporarily put that security fuss to bed, I decided to implement a few of the remaining missing features of the gzochi server container; specifically, periodic task rescheduling and some light application metrics reporting. The result is a fresh release, 0.4. It's been sync'd to Savannah's trusty network of mirrors by now -- so go get it! Software's never, you know, done, but I think with this release the framework might be feature-complete, at least in terms of its v1 functionality. (A "distributed mode" might be in the works for v2, if I ever get around to reading how RedDwarf Server went about that.) So what's left to work on? Bugs, for sure. In particular, there are several worrisome memory leaks. Unit test coverage. Performance, because there's no reason the container shouldn't really scream, especially with recent GNU Guile builds.

But wow: the rare joy of getting to be a user.

Like a lot of programmers, I think, I developed a mode of thinking about and designing a software system as a set of mostly independent components, each with a limited, discrete function, working in concert to produce a complex epiphenomenal behavior. Until relatively recently, though, I didn't think of these systems as potentially spanning multiple processes or machines. It may seem like a trivial observation, but I've come to find it useful to think of complex systems as appliances that use some set of computing hardware to host one or more processes whose combined behavior forms the behavior of the whole system. The benefit of this kind of thinking is that you no longer need to figure out a reasonable way to wedge a web server into, say, your spreadsheet application process code. Instead, you've got your web server, and you've got your spreadsheet. The difficulty is that you may need to launch and coordinate several processes -- or machines -- to get the complete appliance into the right state, such that its different parts are relaying data back and forth and responding to requests properly.

krb5

...Which brings me to my plodding, ongoing experiments in writing an online game. I'd invested quite a bit of time attempting to model the concept of downloadable assets of different types from within my gzochi application code before ultimately deciding that the game server had no business manipulating asset data. That kind of thing, I figure, is the rightful purview of some kind of independent asset management system that's aware of user authorization but not necessarily game state. So I set about figuring out how to manage authorization across processes, and, naturally, Kerberos came to mind. Everything you read about Kerberos steers you in the direction of using it via GSS, the Generic Security Services API. A lot of what you read about GSS suggests that perhaps you ought to consider using SASL, the Simple Authentication & Security Layer. So I did. On first glance, SASL looked like a bad fit -- your SASL-ized applications get to enter into negotiations over which of a set of mutually-supported authentication mechanisms will be used to initiate a session. I guess the idea is that you want secure authentication and you don't care how it happens. But I did care how it happened. So I dropped down to GSS, and found that at first it sort of made sense: Everything is a principal and has credentials, and two principals can create a security context with each other through which they can securely exchange information. But the GSS API designers seemed desperate to avoid explicit representation of anything that might remotely suggest that it's a wrapper around any particular security implementation, much less Kerberos -- no ticket-granting tickets, no service tickets, no distinction between user and service principals. I spent weeks trying to figure out how to model the authentication and authorization flow I had in mind: A client application would obtain a TGT for user with a password, and then use it to obtain tickets to authenticate with the asset server and game server.

When, out of frustration, I dug into the verboten krb5 API, I found it easy to understand -- in the course of trying to get GSS to work I'd figured out the details of key tables and credential caches -- and had something approximating my desired architecture working in an evening. And it's, like, ten lines of code. So I'm kind of on board with what Simon Josefsson says in the appendix of the GNU GSS manual:

...GSS may not be the simplest solution available to solve actual problems, since otherwise more projects would have chosen to take advantage of the work that went into GSS instead of using another framework (or designing their own solution).
I'm with him right up until he says the only circumstance under which you should use GSS is when you're sure you want a Kerberos 5 implementation. Bzzzht!
gzochi

gzochi 0.3 is out -- go get it! The big news in this release is that there's much more scalable and robust support for transaction execution: transactions can time out, get rolled back, and then get retried automatically. This was the functionality that I was most eager / most scared to add to the server, and the fact that it's there and works predictably and quickly is a major confidence boost. The only thing that's missing at this point from, say, a minimum viable product point of view is support for preiodic task scheduling. And I'll be working on that shortly.

Another thing that I think is really significant in this release (even though it's not much code) is the addition of the GLib-compatible reference client, which is something I've wanted to add since starting work on the first gzochi example game. Being able to hook callbacks into a select loop (or something similar) is just so much neater, more predictable, and easier to debug than launching a new independent thread to govern, say, your communication with a server, and having to worry about its interactions with other threads in your application. Weirdly enough, I think this is something that I started to appreciate more fully after writing (and re-writing) multi-step client-server interactions in JavaScript.

As I mentioned in an earlier post, I have indeed begun to start building some actual personal projects on top of gzochi. I don't have anything to show for it yet, except that I've been exposed to a fascinating array of problems that belong to the domain of rich client development: Rendering pipelines, dirty rectangles.
r6rs-protobuf

I've started working on a project that depends on some of the code generation features of r6rs-protobuf (by way of a build-aux helper script) and I realized it's been handling library generation all wrong, at least for multi-.proto builds. The source of the trouble was my decision to map a Protocol Buffers package directly to an R6RS library. That's a fine choice if you can be sure that you know about all declarations of that package, but in a lot of cases, you don't -- for example, when you're applying the compiler to each .proto file in a list. Each file might re-declare that package for its own set of definitions, and since R6RS libraries are effectively immutable -- unlike Java packages or C++ namespaces -- you'll wind up generating a bunch of mutually conflicting libraries that share the same name.

So I've made a new release that changes that behavior. Going forward, the library generator will create a library per top-level definition in each package and name that library accordingly. There'll be a lot more libraries generated, but they'll actually be usable. Get it here.
gzochi

I've just released a new version of gzochi, my online game development framework. You should go get it! This release features resolves the most disruptive bugs from the first release and adds a bunch of cool new things like an interactive remote debugger, support for Berkeley DB, and new concurrent vector and hash table implementations. I think this is the point at which I'm going to start more actively dog-fooding with it, especially given that the Liberated Pixel Cup and the OpenGameArt folks have done such a great job of eliciting content from talented pixel artists.

One thing I'd known but hadn't really internalized: Example code takes forever to write. I added a new example game in gzochi 0.2, a scaled-down, Scheme-based port of the original AberMUD with an Ncurses client in C, and it took me weeks and weeks to get the structure of the code into a comprehensible, reasonably factored state and write all the code comments. Obviousy, there's tremendous benefit in having clear, well-annotated example code -- it's often the first thing people look at when they download your project -- so I think it's probably okay that it took me so long. But considering how quickly I did the rest of the work for the release, I think this might be the last example I add for a while.

A few notes on some older projects:

r6rs-protobuf

In the course of writing the lexer and the corresponding tests for r6rs-thrift I realized that r6rs-protobuf just flat out didn't support "//"-style comments. That's embarrassing -- not least of all because it made the library more or less unusable for real work, and thus nobody must have been using it successfully. But I've fixed that and assembled a new release. Get it here.

SCSS

Antono Vasiljev correctly pointed out that the API method scss:scss->css in SCSS doesn't seem to work with the same arguments as the function in the same name in Chicken's version of SCSS (which I only just found out about). In fact, it didn't work at all, following a redesign of SCSS's stylesheet data structure several versions and years ago. I've brought it up to date -- made it almost robust -- and after I make a few more fixes I'll put together a release of SCSS as well.

r6rs-thrift

I've just thrown together a release of a little project I started during this year's LibrePlanet conference, an implementation of the Apache Thrift framework in R6RS Scheme: r6rs-thrift. I thought it would be a fun, quick hack -- my assumption was that I could lift a lot of the type system and code generation stuff from r6rs-protobuf, my Scheme Protocol Buffers implementation from last year -- but it turned out that Thrift's a bit more complicated. Which is not to say that it wasn't fun to work on. But Thrift includes pluggable wire protocols, parameterized types, and some interesting variations on Protocol Buffers' vanilla structs (unions and exceptions). And there's not a whole lot (read: any) documentation, so I had to pretty much reverse engineer the serialization and codegen semantics by picking through the thriftc source code.

One interesting thing that came out Thrift's type system is that because you can effectively declare a type at the same time you're declaring a field of that type -- via, say, a container type as a field in a struct -- I had to add a layer of indirection to the type resolution system such that a descriptor for a user-defined type is declared (and registered) once and then used via an explicit "type reference" record everywhere else. Type references themselves can be parameterized to support Thrift's "generic" containers. I was initially worried about introducing that construct, but it ended up being reasonably discrete and quite powerful. I'm considering porting it back to r6rs-protobuf, just for, you know, funsies.

I was able to port the builder pattern that Protocol Buffers uses over to this implementation, which I thought was a neat little coup. Thrift is, you know, fine, but the mutability of the data types generated by the official compiler has always irked me, in part because it means there's no validation applied until you're about to marshal something over the wire. I suppose you could mount a philosophical defense to the effect that you don't really need an object to be well-formed until that point, but that presumes an awful lot about how you're planning to use your generated data structures.

At any rate: I made it; go get it.
gzochi

That was a quick four months! I've mostly had my head down developing this thing. I spent several weeks cranking out documentation, and then close to a month just working on the example game I wanted to ship with it, both on code clarity and on restructuring art assets. It's the first time I've ever shipped really comprehensive example code, and I'm very proud of the result, even if it has a decidedly retro flavor.

...Because I now have an 0.1 release that you can go download! It's actually very cool and hopefully easy to get started with (I also spent a week just futzing with Autotools scripts.)

As I pointed out on the GNU Guile mailing list, this was a project I started right after graduating from college, and it's what got me into Guile development in the first place: I had a much worse design in mind for how I was going to handle computing resource management back in 2003 (thread cancellation), which Guile didn't support at the time, so I committed to adding it. I've since learned almost ten years' worth of lessons about software architecture, and so this system doesn't capitalize on many features I was sure I needed way back when and thus set out to build from scratch -- dynamic user interface mark-up, e.g. -- but every detour was worth it. I think.

Now, if I could just remember what I was planning to do with a game development framework back in 2003...
gzochi

After releasing libRUIN a couple of months ago, I had some bandwidth to take on some new stuff, and I decided to revisit the project that had initially gotten me interested in user interface markup and GNU Guile in the first place: gzochi, my Free MMO game platform, ambitiously named after Xochipilli, the Aztec god of games and gambling.

My initial work on gzochi, while it took me down some interesting and fruitful paths (i.e., practically every Free Software project I've been working on for the past eight years), was probably all wrong. The system I'd envisioned involved a direct, first-class server-side model of the UI state of all connected clients, with practically nothing between that and the networking layer. I still want to use gzochi to explore things like dynamic UI deployment, but in the intervening years I've also learned that there's a layer for everything, and I wannt to keep the scope of each layer manageable.

So this time around I decided to decompose the problem a bit more aggressively, and I'm taking my architectural cues from a software project I'd worked with in a professional capacity, the Sun Game Server, aka Project Darkstar, aka RedDwarf Server, which takes the approach I had in mind insofar as providing a server container for games, but which makes no assumptions about the details of the games it hosts beyond providing some general "services" to them. To wit: There are message delivery services, a data retrieval service, and a task scheduling service; and "userland" game code is executed transactionally with respect to these services in order to hide the nasty details of concurrency.

The implementation of gzochi I'm currently pursuing steals the best parts of this architecture while replacing the server container bits with C code (with help from GNU Serveez) and replacing the game bits with Guile Scheme. I've got a working prototype of most of the system. Think this sounds interesting? Come aboard!

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