[In an in-depth technical article, Neversoft co-founder Mick West discusses performance concerns when optimizing asset processing for games, including the basic nature of the common problems and in-depth solutions for keeping the pipeline efficient.]
The fundamental building block of any game asset pipeline is the asset processing tool. An asset processing tool is a program or piece of code that takes data in one format and performs some operations on it, such as converting it into a target specific format, or performing some calculation, such as lighting or compression. This article discusses the performance issues with these tools and gives some ideas for optimization with a focus on minimizing I/O.
The Ugly Stepsister
Asset conversion tools are too often neglected during development. Since they are usually well specified and discrete pieces of code, they are often tasked to junior programmers. Generally, any programmer can easily create a tool that works to a simple specification, and at the start of a project the performance of the tool is not so important because the size of the data involved is generally small and the focus is simply on getting things up and running.
However, toward the end of the project, the production department often realizes that a large amount of time is being wasted waiting for these tools to complete their tasks. The accumulation of near-final game data and the more rapid iterations in the debugging and tweaking phase of the project make the speed of these tools of paramount importance.
Further, time may be wasted trying to optimize the tools at this late stage, and there’s a significant risk that bugs will be introduced into the asset pipeline (and the game) when making significant changes to processes and code during the testing phase.
Hence, it’s highly advisable to devote sufficient time to optimizing your asset pipeline early in development. It’s also advisable to use the people who are highly experienced in doing the types of optimizations needed. This early application of optimization is another example of what I call mature optimization (see “Mature Optimization,” Game Developer, January 2006).
There’s a limited number of man hours available in the development of a game. If you wait until the need for optimization becomes apparent, you will have already wasted hundred of hours.
The Nature of the Data
Asset processing tools come in three flavors: converters, calculators, and packers. Converters take data that are arranged in a particular set of data structures and rearrange them into another set of data structures, which are often machine- or engine-specific. A good example here is a texture converter, which might take textures in .PNG format and convert it to a form that can be directly loaded into the graphic memory of the target hardware.
Asset calculators take an asset or group of assets and perform some set of calculations on them such as calculating lighting and shadows or creating normal maps. Since these operations involve a lot of calculations and several passes over the data, they typically take a lot longer than the asset conversion tools. Sometimes they take large assets, such as high-resolution meshes, and produce smaller assets, such as displacement maps.
The third processing tool type, asset packers, take the individual assets and package them into data sets for use in particular instances in the game, generally without changing them much.
Using an asset packer might involve simply gathering all the files used by one level of the game and arranging them into a .WAD file. Or it might involve grouping files in such a way that streaming can be effectively performed when moving from one area of the game to another. Since the amount of data can be very large, the packing process might take a lot of time and be very resource intensive, requiring lots of memory and disk space, especially for final builds.
You may be surprised how often the simplest method of optimization is overlooked. Are you letting the content creators use the debug version of a tool? It’s a common mistake for junior programmers, but even the most experienced among us sometimes overlook this simple step.
So before you do anything, try turning the optimization settings on and off to make sure there’s a noticeable speed difference. Then, in release mode, try tweaking some settings, such as “optimize for speed” and “optimize for size.” Depending on the nature of the data (and the hardware your tools are running on), you might actually get faster code if you use “optimize for size.” The optimal optimization setting can vary from tool to tool.
Be careful when tweaking the optimization settings to test the speed of your code. In a multitasking operating system like Windows XP, a lot is going on, so your timings might vary dramatically from one run to the next. Taking the average is not always a useful measure either, as it can be greatly skewed by random events. A more accurate way is to compare the lowest times of multiple runs of two different settings, as that will be closest to the “pure” run.
Parallelize Your Code
Most PCs now have some kind of multicore and/or hyper-threading. If your tools are written in the traditional mindset of a single processing thread, you’re wasting a significant amount of the silicon you paid for, as well as the time of the artists and level designers as they wait for their assets to be converted.
Since the nature of asset data is generally to be large chunks of homogeneous data, such as lists of vertices and polygons, it’s generally very amenable to data level parallelization with worker threads, where the same code is run on multiple chunks of similar data concurrently, taking advantage of the cache. For details on this approach see “Particle Tuning” (Game Developer, April 2006).