Saturday, January 17, 2015

Good lossless codec API design

Lossless codec design and implementation seems to be somewhat of a black art. I've seen many potentially good lossless codecs come out with almost useless interfaces (or none at all). Here are some attributes I've seen of good codecs:

- If you want others to use your codec in their apps, don't just provide a single command line executable with a awkward as hell command line interface. Support static libraries and SO's/DLL's, otherwise it's not useful to a large number of potential customers no matter how cool your codec is.

- Minimum number of source files, preferably in all-C.
If you use C++, don't rely on a ton of 3rd party crap like boost, etc. It just needs to compile out of the box.

Related: Programmers are generally a lazy bunch and hate mucking around with build systems, make files, etc. Make it easy for others to build your stuff, or just copy & paste into your project. Programmers will gladly sacrifice some things (such as raw perf, features, format compatibility, etc. - see stb_image) if it's fast and trivial to plop your code into their project. Don't rely on a ton of weird macros that must be configured by a custom build system.

- Even if the codec is C++, provide the interface in pure C so the codec can be trivially interfaced to other languages.

- You must support Linux, because some customers only run Linux and they will refuse to use Wine to run your codec (even if it works fine).

- Must provide heap alloc callbacks, so the caller can redirect all allocations to their own system.

- Support a "compile as ANSI C" mode, so it's easy to get your codec minimally working on new platforms. The user can fill in the platform specific stuff (atomics, threading, etc.) later, if needed.

Related: If you use threads, support basic pthreads and don't use funky stuff like pthread spinlocks (because platforms like OSX don't support them). Basic pthreads is portable across many platforms (even Win32 with a library like pthreads-win32, but just natively support Win32 too because it's trivial).

- Don't assume you can go allocate a single huge 256MB+ block on the heap. On mobile platforms this isn't a hot idea. Allocate smaller blocks, or ideally just 1 block and manage the heap yourself, or don't use heaps.

- Streaming support, to minimize memory consumption on small devices. Very important in the mobile world.

- Expose a brutally simple API for memory to memory compression.

- Support a zlib-compatible API. It's a standard, everybody knows it, and it just works. If you support this API, it becomes almost trivial to plop your codec into other existing programs. This allows you to also leverage the existing body of zlib docs/knowledge.

- Support in-place memory to memory decompression, if you can, for use in very memory constrained environments.

- Single threaded performance is still important: Codecs which depend on using tons of cores (for either comp or decomp) to be practical aren't useful on many mobile devices.

- In many practical use cases, the user doesn't give a hoot about compression performance at all. They are compressing once and distributing the resulting compressed data many times, and only decompressing in their app. So expose optional parameters to allow the user to tune your codec's internal models to their data, like LZMA does. Don't worry about the extra time needed to compress, we have the cloud and 40+ core boxes.

- Provide a "reinit()" API for your codec, so the user can reuse all those expensive heap allocations you've made on the first init on subsequent blocks.

- Deal gracefully with already compressed, or incompressible data. Don't expand it, except by a tiny amount, and don't slow to a crawl. Related: don't fall over on very compressible data, or data containing repeated strings, etc.

- Communicate the intended use cases and assumptions up front:
Is it a super fast but low ratio codec that massively trades off ratio for speed?
Is it a symmetrical codec, i.e. is compression throughput roughly equal to decompression?
Is it a asymmetric codec, where (typically) compression time is longer than decompression time?
Is the codec useful on tiny or small blocks, or is it intended to be used on large solid blocks of data?
Does your codec require a zillion cores or massive amounts of RAM to be practical at all?

- Test and tune your codec on mobile and console devices. You'll be surprised at the dismally low performance available vs. even mid-range x86 devices. These are the platforms that benefit greatly from data compression systems, so by ignoring this you're locking out a lot of potential customers of your codec. The world is not just x86.

One some CPU's, stuff like int divides, variable length int shifts, and L2 cache misses are surprisingly expensive. On some platforms, CPU load hit stores can crush performance on seemingly decent looking code.

- Beware relying on floating point math in a lossless codec. Different compilers can do different things when optimizing FP math expressions, possibly resulting in compressed outputs which are compiler dependent.

- Test your codec to death on a wide variety of data, then test it again. Random failures are the kiss of death. If your codec is designed for game data then download a bunch of games on Steam, unpack the game data (using typically user provided unpack/modding tools) then add the data to your test corpus.

- Carefully communicate up front about forwards/backwards compatibility.

- Make sure your codec can be built using Emscripten for Javascript compatibility. Or just provide a native Javascript decoder.

- Make sure your compressor supports a 100% deterministic mode, so with the same source data and compressor settings you always get the exact same compressed data every time. This allows integrating your codec into build systems that intelligently check for file modifications.

- "Fuzz" test your codec by randomly flipping bits of compressed data, inserting/removing bits, etc. and make sure your decompressor will never crash or overwrite memory. If your decompressor can crash, make sure you document this so the caller can check the integrity of the compressed data before decompression. Consider providing two decompressors, one that is optimized for raw performance (but can crash), and another hardened decompressor that can't possibly crash on corrupted inputs.

Related: Try to design your bitstream format so the decompressor can detect corruption as quickly as possible. I've seen codecs fall into the "let's output 4GB+ of all-0's" mode on trivially corrupted inputs.

- Your decompressor shouldn't try to read beyond the end of valid input streams, even by a single byte. (In other words, when your decompressor in streaming mode says it needs more bytes to make forward progress, it better mean it.) Otherwise 100% zlib compatibility is out, and trying to read beyond the end makes it harder to use your decompressor on non-seekable input streams. (This can be a tricky requirement to implement in some decoder designs, which is why it's here.)

- Gracefully handle error conditions, especially out of memory conditions. Return a real error code, don't just terminate. Have enough error status codes so developers can easily debug why something went wrong in the field.

- Don't just focus on textual data. The Large Text Compression Benchmark is cool and all, but many customers don't have much text (if any) to distribute or store.

Off topic, but related: The pure Context Mixing based codecs are technically interesting, but every time I try them on real-life game data they are just insanely slow, use massive amounts of RAM, and aren't competitive at all ratio-wise against good old LZ based codecs like LZMA. I'm not claiming CM algorithms can't be improved, but I think the CM devs focus way too much on text (or bog standard formats like JPEG) and not enough on handling arbitrary "wild" binary data.

- Allow the user to optionally disable adler32/crc32/etc. checks during decompression, so they can do it themselves (or not). Computing checksums can be surprisingly expensive.

Also, think about your codec's strengths and weaknesses, and how it will be used in practice. It's doubtful that one codec will be good for all real-world use cases. Some example use cases I've seen from the video game world:

- If a game is displaying a static loading screen, the codec probably has access to almost the entire machine's CPU(s) and possibly a good chunk of temporary memory. The decompressor must be able to keep up with the data provider's (DVD/BlueRay/network) rate, otherwise it'll be the bottleneck. As long as the codec's consumption rate is greater or equal to the provider's data rate, it can use up a ton of CPU (because it won't be the pipeline's bottleneck). A high ratio, heavy CPU, potentially threaded codec is excellent in this case.

- If a game is streaming assets in the background during gameplay, the codec probably doesn't have a lot of CPU available. The decompressor should be optimized for low memory consumption, high performance, low CPU cache overhead, etc. It's fine if the ratio is lower than the best achievable, because streaming systems are tolerant of high latencies.

- Network packet compression: Typically requires a symmetrical, low startup overhead codec that can do a good enough job on small to tiny binary packets. Codecs that support static data models tuned specifically to a particular game's data packets are especially useful here.

- Content updates: Codecs which support delta compression (patching) are especially useful here. New assets generally resemble old ones.

Finally, in many games I've worked on or seen, the vast majority of distributed data falls into a few big buckets: Audio, textures, meshes, animations, executable, compiled shaders, video. The rest of the game's data (scripts, protodata, misc serialized objects) forms a long tail (lots of tiny files and a small percent of the total). It can pay off to support optimizations for these specific data types.

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