Zero overhead backward compatibility

The Animation Compression Library finally supports backward compatibility (going forward once 2.0 comes out). I’m really happy with how it turned out so I thought I would write a bit about how the ACL decompression is designed.


At runtime, decompressing animation data could not be easier:

acl::decompression_context<custom_decompression_settings> context;

// Seek 1.0 second into our compressed animation
// and don't use rounding to interpolate, acl::sample_rounding_policy::none);

custom_writer writer(output_data);

A small context object is created and bound to our compressed data. Its construction is cheap enough that it can be done on the stack on demand. It can then subsequently be used (and re-used) to seek and decompress.

A key design goal is to have as little overhead as possible: pay only for what you use. This is achieved through templating in two ways:

  • The custom_decompression_settings argument on the context object controls what features are enabled or disabled.
  • The custom_writer wraps whatever container you might be using in your game engine to represent the animation data. This is to make sure no extra copying is required.

The decompression settings are where the magic happens.

Compile time user control

There are many game engines out there each handling animation in their own specific way. In order to be able to integrate as seamlessly as possible, ACL exposes a small struct that can be overriden to control its behavior. By leveraging constexpr and templating, features that aren’t used or needed can be removed entirely at compile time to ensure zero cost at runtime.

Here is how it looks:

struct decompression_settings
  // Whether or not to clamp the sample time when `seek(..)`
  // is called. Defaults to true.
  static constexpr bool clamp_sample_time() { return true; }

  // Whether or not the specified track type is supported.
  // Defaults to true.
  // If a track type is statically known not to be supported,
  // the compiler can strip the associated code.
  static constexpr bool is_track_type_supported(track_type8 /*type*/)
  { return true; }

  // Other stuff ...

  // Which version we should optimize for.
  // If 'any' is specified, the decompression context will
  // support every single version with full backwards
  // compatibility.
  // Using a specific version allows the compiler to
  // statically strip code for all other versions.
  // This allows the creation of context objects specialized
  // for specific versions which yields optimal performance.
  static constexpr compressed_tracks_version16 version_supported()
  { return compressed_tracks_version16::any; }

  // Whether the specified rotation/translation/scale format
  // are supported or not.
  // Use this to strip code related to formats you do not need.
  static constexpr bool is_rotation_format_supported(rotation_format8 /*format*/)
  { return true; }

  // Other stuff ...

  // Whether rotations should be normalized before being
  // output or not. Some animation runtimes will normalize
  // in a separate step and do not need the explicit
  // normalization.
  // Enabled by default for safety.
  static constexpr bool normalize_rotations() { return true; }

Extending this is simple and clean:

struct default_transform_decompression_settings : public decompression_settings
  // Only support transform tracks
  static constexpr bool is_track_type_supported(track_type8 type)
  { return type == track_type8::qvvf; }

  // By default, we only support the variable bit rates as
  // they are generally optimal
  static constexpr bool is_rotation_format_supported(rotation_format8 format)
  { return format == rotation_format8::quatf_drop_w_variable; }

  // Other stuff ...

A new struct is created to inherit from the desired decompression settings and specific functions are defined to hide the base implementations, thus replacing them.

By templating the decompression_context with the settings structure, it can be used to determine everything needed at compile time:

  • Which memory representation is needed depending on whether we are decompressing scalar or transform tracks.
  • Which algorithm version to support and optimize for.
  • Which features to strip when they aren’t needed.

This is much nicer than the common C way to use #define macros. By using a template argument, multiple setting objects can easily be created (with type safety) and used within the same application or file.

Backward compatibility

By using the decompression_settings, we can specify which version we optimize for. If no specific version is provided (the default behavior), we will branch and handle all supported versions. However, if a specific version is provided, we can strip the code for all other versions removing any runtime overhead. This is clean and simple thanks to templates.

template<compressed_tracks_version16 version>
struct decompression_version_selector {};

// Specialize for ACL 2.0's format
template<> struct
  static bool is_version_supported(compressed_tracks_version16 version)
  { return version == compressed_tracks_version16::v02_00_00; }

  template<class decompression_settings_type, class context_type>
  ACL_FORCE_INLINE static bool initialize(context_type& context, const compressed_tracks& tracks)
    return acl_impl::initialize_v0<decompression_settings_type>(context, tracks);

  // Other stuff ...

// Specialize to support all versions
template<> struct
  static bool is_version_supported(compressed_tracks_version16 version)
    return version >= compressed_tracks_version16::first && version <= compressed_tracks_version16::latest;

  template<class decompression_settings_type, class context_type>
  static bool initialize(context_type& context, const compressed_tracks& tracks)
    // TODO: Note that the `any` decompression can be optimized further to avoid a complex switch on every call.
    const compressed_tracks_version16 version = tracks.get_version();
    switch (version)
    case compressed_tracks_version16::v02_00_00:
      return decompression_version_selector<compressed_tracks_version16::v02_00_00>::initialize<decompression_settings_type>(context, tracks);
      ACL_ASSERT(false, "Unsupported version");
      return false;

  // Other stuff ...

This is ideal for many game engines. For example Unreal Engine 4 always compresses locally and caches the result in its Derived Data Cache. This means that the compressed format is always the latest one used by the plugin. As such, UE4 only needs to support a single version and it can do so without any overhead.

Other game engines might choose to support the latest two versions, emiting a warning to recompress old animations while still being able to support them with very little overhead: a single branch to pick which context to use.

More general applications might opt to support every version (e.g. a glTF viewer).

Note that backward compatibility will only be supported for official releases as the develop branch is constantly subject to change.


This C++ customization pattern is clean and simple to use and it allows a compact API with a rich feature set. It was present in a slightly different form ever since ACL 0.1 and the more I use it, the more I love it.

In fact, in my opinion the Animation Compression Library and Realtime Math contain some of the best code (quality wise) that I’ve ever written in my career. Free from time or budget constraints, I can carefully craft each facet to the best of my ability.

ACL 2.0 continues to progress nicely. It is still missing a few features but it is already an amazing step up from 1.3.