Halo's high dynamic range technique is achieved using a method very much like some of HDR photography out there. Two shots of the scene at two different exposure levels are taken, which are then recombined via a pixel shader. This dual framebuffer is the reason that the game runs at a sub-HD resolution with no AA - that's down to the 10MB limit of the 360's eDRAM, the onboard memory attached to the Xenos GPU, the secret weapon that often gives the Microsoft hardware the advantage in our face-off features.
Aside from impressive lighting, another strength of the Halo engine is the sheer scale of the scenes it's capable of drawing. You'll see more of the vast, expansive warzones in our second video, but put simply, the size of some of the levels is absolutely staggering. The Storm and Floodgate are the two most dramatic examples in the game, with some Halo 3 users estimating a 20km to 30km (!) length to the Forerunner structure.
Moving on, another aspect of the Halo tech we expect to be utilised fully in ODST concerns what you might call the warzone simulation. You can see this in full effect at the tail-end of the ODST trailer and we show its implementation in Halo 3 in the video below. Bungie's code is able to simultaneously process a bewilderingly large array of in-area opponents at any given moment, something it excels at in comparison to pretty much all other console-based first-person shooters. It's also the key to the new Firefight mode, which pits the ODST troops against wave after wave of enemy forces.
Now, of course, there are the considerable rendering considerations to be taken into account here, but it's also worth remembering another key strength of Bungie's engine - the AI.
There's a fully realised, artificial intelligence behind every creature in the Halo world and seeing that utilised in a massive, expansive environment operating independently to the presence of Master Chief, is quite something. Depending on other factors that affect CPU load, the game is capable of giving intelligent actions to between 20 and 30 enemies at any given moment. There around 290 different AI behaviours in Halo 3, each with up to ten different permutations. That the game manages to sustain 30FPS as well as it does is another very impressive element of the core technology.
Speaking of frame-rate, in our playthrough we were very surprised to note the appearance of v-sync tearing. It's minimal, barely noticeable, accounting for just 0.4 per cent of our captured footage (there's more of an impact in split-screen mode), but what has us scratching our heads is why it is there at all. Some scenes with heavy alpha effects and overdraw can cause lost frames with no sign of tearing. Other areas (such as the first Scarab attack) tear fairly easily, but for reasons we can't quite gather.
Part of the Halo equation that is often ignored is its split-screen multiplayer component. Few first-person shooters incorporate this feature these days, but Bungie's code has a strong multiplayer focus that integrates both online and offline play. Two-player split-screen sees the action scaled down into a 4:3 window with each player getting 960x360's worth of resolution. Black borders fill the sides on a widescreen display with the action perfectly proportioned on an older display. Performance remains remarkably solid for the most part at 30FPS, although you will see more screen tear in this mode than you will in the single-player mode. What is quite remarkable is how untouched the Halo experience is - no apparent reduction in enemy numbers, no dialling back on the graphical bling. It's all there.
The four-player split-screen must have been crafted from some kind of arcane Bungie coding voodoo. Once again, it's detail-complete - the only thing we noticed missing were the shadowmaps. What is highly intriguing is the fact that each quarter screen (640x360) is being individually rendered, and features individual screen tear in each mini-window, as opposed to a line across the entire screen appearing when the engine gets stressed. Our guess? Four mini-buffers are drawn in the 360 GPU's onboard eDRAM, with a single pass of each buffer resolved to memory and then combined at 33ms intervals giving 30FPS. This discrete rendering approach doesn't seem to affect drops in frame-rate though - when the engine lags in one window, it slows down the experience for everyone.
ODST looks set to replicate Halo 3's repertoire of split-screen modes, with two-player co-operative on a local console and the ability to hook up with another 360 in split-screen mode for four-player campaign action. Four-player split-screen is limited to the Halo 3 multiplayer competitive compilation located on the second DVD in the ODST pack.
So, with an engine with so many strengths, what were the elements that perhaps held the developers back and can be addressed in ODST? In many senses, Bungie was tied to the past with its work on the first Xbox. A lot of prototyping for Halo 3 was actually carried out on the older hardware that the devs had mastered so well. For example, the Cortana level started out life on the older Microsoft hardware, where the biological look was first pioneered. Other issues Bungie encountered were rather more bizarre. The original Xbox 360 devkits had exactly the same amount of memory as the retail unit, meaning that the game only had around 335MB of memory for content due to SDK overheads - a deficit of around 50MB. This has been resolved in the new Microsoft kits, and should give Bungie a very useful boost for ODST.
In part though, this did mean that Bungie's streaming tech had to be phenomenally good. Their own figures show that the 360's DVD drive is capable of 6-18MB/s throughput (100-240ms seek time) while the hard disk stats came in at 17-30MB (10-30ms seek). The streaming code was so efficient, and so geared towards the hardware, that a New Xbox Experience hard-disk install actually slows the game down considerably. Expect to see that situation reversed for ODST.
Look forward to a rich audio experience with ODST too. Bungie went to town with audio on Halo 3, with over 34,000 lines of dialogue in the game and a cast of 34 voice actors. There's even level of detail (LOD) iterations of each line, with the game able to cross-fade between 'near' and 'far' recordings dynamically. Audio is so important to Halo 3 that decompressing and playing the game's sound accounts for almost the entirety of the processing power of one of Xenos's CPU cores (using both hardware threads), and audio takes up almost 50 per cent of any given level's data. Moving the lion's share of the multiplayer modes onto a second disc means that Bungie can have a zero-compromise approach to ODST, making full use of the 6.8GB available on the DVD.
Our conclusion? ODST's going to be one to watch. Maybe not a graphical showcase to humble Killzone 2, but still a good-looking game with gameplay scenarios tailored to the unique properties of Bungie's proprietary engine. We've picked apart the Halo tech to showcase the best the engine has to offer, but who knows what other enhancements, tweaks and tuning improvements are under the bonnet and yet to be revealed? As soon as we find out, you'll be the first to know.
Do you want to know more? Keep up to date with all matters technical and performance-related in the Digital Foundry channel.
Halo 3: ODST is out for Xbox 360 on 22nd September. Check out Eurogamer's E3 Halo 3: ODST hands-on preview for more.