doc/dev_guide/av1_encoder.dox

/*!\page encoder_guide AV1 ENCODER GUIDE

\tableofcontents

\section architecture_introduction Introduction

This document provides an architectural overview of the libaom AV1 encoder.

It is intended as a high level starting point for anyone wishing to contribute
to the project, that will help them to more quickly understand the structure
of the encoder and find their way around the codebase.

It stands above and will where necessary link to more detailed function
level documents.

\section  architecture_gencodecs Generic Block Transform Based Codecs

Most modern video encoders including VP8, H.264, VP9, HEVC and AV1
(in increasing order of complexity) share a common basic paradigm. This
comprises separating a stream of raw video frames into a series of discrete
blocks (of one or more sizes), then computing a prediction signal and a
quantized, transform coded, residual error signal. The prediction and residual
error signal, along with any side information needed by the decoder, are then
entropy coded and packed to form the encoded bitstream. See Figure 1: below,
where the blue blocks are, to all intents and purposes, the lossless parts of
the encoder and the red block is the lossy part.

This is of course a gross oversimplification, even in regard to the simplest
of the above codecs.  For example, all of them allow for block based
prediction at multiple different scales (i.e. different block sizes) and may
use previously coded pixels in the current frame for prediction or pixels from
one or more previously encoded frames. Further, they may support multiple
different transforms and transform sizes and quality optimization tools like
loop filtering.

\image html genericcodecflow.png "" width=70%

\section architecture_av1_structure AV1 Structure and Complexity

As previously stated, AV1 adopts the same underlying paradigm as other block
transform based codecs. However, it is much more complicated than previous
generation codecs and supports many more block partitioning, prediction and
transform options.

AV1 supports block partitions of various sizes from 128x128 pixels down to 4x4
pixels using a multi-layer recursive tree structure as illustrated in figure 2
below.

\image html av1partitions.png "" width=70%

AV1 also provides 71 basic intra prediction modes, 56 single frame inter prediction
modes (7 reference frames x 4 modes x 2 for OBMC (overlapped block motion
compensation)), 12768 compound inter prediction modes (that combine inter
predictors from two reference frames) and 36708 compound inter / intra
prediction modes. Furthermore, in addition to simple inter motion estimation,
AV1 also supports warped motion prediction using affine transforms.

In terms of transform coding, it has 16 separable 2-D transform kernels
{ DCT, ADST, fADST, IDTX }2 that can be applied at up to 19 different scales
from 64x64 down to 4x4 pixels.

When combined together, this means that for any one 8x8 pixel block in a
source frame, there are approximately 45,000,000 different ways that it can
be encoded.

Consequently, AV1 requires complex control processes. While not necessarily
a normative part of the bitstream, these are the algorithms that turn a set
of compression tools and a bitstream format specification, into a coherent
and useful codec implementation. These may include but are not limited to
things like :-

- Rate distortion optimization (The process of trying to choose the most
  efficient combination of block size, prediction mode, transform type
  etc.)
- Rate control (regulation of the output bitrate)
- Encoder speed vs quality trade offs.
- Features such as two pass encoding or optimization for low delay
  encoding.

For a more detailed overview of AV1's encoding tools and a discussion of some
of the design considerations and hardware constraints that had to be
accommodated, please refer to *** TODO link to Jingning's AV1 overview paper.

Figure 3 provides a slightly expanded but still simplistic view of the
AV1 encoder architecture with blocks that relate to some of the subsequent
sections of this document. In this diagram, the raw uncompressed frame buffers
are shown in dark green and the reconstructed frame buffers used for
prediction in light green. Red indicates those parts of the codec that are
(or may be) lossy, where fidelity can be traded off against compression
efficiency, whilst light blue shows algorithms or coding tools that are
lossless. The yellow blocks represent non-bitstream normative configuration
and control algorithms.

\image html av1encoderflow.png "" width=70%

\section architecture_command_line The Libaom Command Line Interface

 Add details or links here: TODO ? elliotk@

\section architecture_enc_data_structures Main Encoder Data Structures

The following are the main high level data structures used by the libaom AV1
encoder:

 - \ref AV1_COMP
 - Add details, references or links here: TODO ? urvang@


\section architecture_enc_use_cases Encoder Use Cases

The libaom AV1 encoder is configurable to support a number of different use
cases and rate control strategies.

The principle use cases for which it is optimised are as follows:

 - <b>Video on Demand / Streaming</b>
 - <b>Low Delay or  Live Streaming</b>
 - <b>Video Conferencing / Real Time Coding (RTC)</b>
 - <b>Fixed Quality / Testing</b>

Other examples of use cases for which the encoder could be configured but for
which there is less by way of specific optimizations include:

 - <b>Download and Play</b>
 - <b>Disk Playback</b>>
 - <b>Storage</b>
 - <b>Editing</b>
 - <b>Broadcast video</b>

Specific use cases may have particular requirements or constraints. For example:

<b>Video Conferencing:</b>  In a video conference we need to encode the video
in real time and to avoid any coding tools that could increase latency, such
as frame look ahead.

<b>Live Streams:</b> In cases such as “live streaming of games or events” it
may be possible to allow some limited buffering of the video and use of
lookahead coding tools to improve encoding quality. However,  whilst a lag of
a second or two may be fine given the one way nature of this type of video,
it is clearly not possible to use tools such as two pass coding.

<b>Broadcast:</b> Broadcast video (e.g. digital TV over satellite) may have
specific requirements such as frequent and regular key frames (e.g. once per
second or more) as these are important as entry points to users when switching
channels. There may also be  strict upper limits on bandwidth over a short
window of time.

<b>Download and Play:</b> Download and play applications may have less strict
requirements in terms of local frame by frame rate control but there may be a
requirement to accurately hit a file size target for the video clip as a
whole. Similar considerations may apply to playback from mass storage devices
such as DVD or disk drives.

<b>Editing:</b> In certain special use cases such as offline editing, it may
be desirable to have very high quality and data rate but also very frequent
key frames or indeed to encode the video exclusively as key frames. Lossless
video encoding may also be required in this use case.

<b>VOD / Streaming:</b> One of the most important and common use cases for AV1
is video on demand or streaming, for services such as YouTube and Netflix. In
this use case it is possible to do two or even multi-pass encoding to improve
compression efficiency. Streaming services will often store many encoded
copies of a video at different resolutions and data rates to support users
with different types of playback device and bandwidth limitations.
Furthermore, these services support dynamic switching between multiple
streams, so that they can respond to changing network conditions.

Exact rate control when encoding for a specific format (e.g 360P or 1080P on
YouTube) may not be critical, provided that the video bandwidth remains within
allowed limits. Whilst a format may have a nominal target data rate, this can
be considered more as the desired average egress rate over the video corpus
rather than a strict requirement for any individual clip. Indeed, in order
to maintain optimal quality of experience for the end user, it may be
desirable to encode some easier videos or sections of video at a lower data
rate and harder videos or sections at a higher rate.

VOD / streaming does not usually require very frequent key frames (as in the
broadcast case) but key frames are important in trick play (scanning back and
forth to different points in a video) and for adaptive stream switching. As
such, in a use case like YouTube, there is normally an upper limit on the
maximum time between key frames of a few seconds, but within certain limits
the encoder can try to align key frames with real scene cuts.

Whilst encoder speed may not seem to be as critical in this use case, for
services such as YouTube, where millions of new videos have to be encoded
every day, encoder speed is still important, so libaom allows command line
control of the encode speed vs quality trade off.

<b>Fixed Quality / Testing Mode:</b> Libaom also has a fixed quality encoder
pathway designed for testing under highly constrained conditions.


\section architecture_enc_rate_ctrl Rate Control

 Add details here.

\subsection architecture_enc_vbr Variable Bitrate (VBR) Encoding

 Add details here.

\subsection architecture_enc_1pass_lagged 1 Pass Lagged VBR Encoding

 Add details here.

\subsection architecture_enc_rc_loop The Main Rate Control Loop

 Add details here.

\subsection architecture_enc_fixed_q Fixed Q Mode

 Add details here.

\section architecture_enc_src_proc Source Frame Processing

 Add details here.

\section architecture_enc_hierachical Hierarchical Coding

 Add details here.

\section architecture_enc_tpl Temporal Dependency Modelling

 Add details here.

\section architecture_enc_partitions Block Partition Search

 Add details here.

\section architecture_enc_inter_modes Inter Prediction Mode Search

 Add details here.

\section architecture_enc_intra_modes Intra Mode Search

 Add details here.

\section architecture_enc_tx_search Transform Search

 Add details here.

\section architecture_loop_filt Loop Filtering

 Add details here.

\section architecture_loop_rest Loop Restoration Filtering

 Add details here.

\section architecture_cdef CDEF

 Add details here.

\section architecture_entropy Entropy Coding

 Add details here.

*/

/*!\defgroup encoder_algo Encoder Algorithm
 *
 * The encoder algorithm describes how a sequence is encoded, including high
 * level decision as well as algorithm used at every encoding stage.
 */

/*!\defgroup high_level_algo High-level Algorithm
 * \ingroup encoder_algo
 * This module describes sequence level/frame level algorithm in AV1.
 * More details will be added.
 * @{
 */

 /*!\defgroup frame_coding_pipeline Frame Coding Pipeline
    \ingroup high_level_algo

 To encode a frame, first call \ref av1_receive_raw_frame() to obtain the raw
 frame data. Then call \ref av1_get_compressed_data() to encode raw frame data
 into compressed frame data. The main body of \ref av1_get_compressed_data()
 is \ref av1_encode_strategy(), which determines high-level encode strategy
 (frame type, frame placement, etc.) and then encodes the frame by calling
 \ref av1_encode(). In \ref av1_encode(), \ref av1_first_pass() will execute
 the first_pass of two-pass encoding, while \ref encode_frame_to_data_rate()
 will perform the final pass for either one-pass or two-pass encoding.

 The main body of \ref encode_frame_to_data_rate() is
 \ref encode_with_recode_loop_and_filter(), which handles encoding before
 in-loop filters (with recode loops encode_with_recode_loop(), or without
 any recode loop \ref encode_without_recode()), followed by in-loop filters
 (deblocking filters \ref loopfilter_frame(), CDEF filters and restoration
 filters \ref cdef_restoration_frame()).

 Except for rate/quality control, both encode_with_recode_loop() and
 \ref encode_without_recode() call \ref av1_encode_frame() to manage reference
 frame buffers and to perform the rest of encoding that does not require
 operating external frames by \ref encode_frame_internal(), which is the
 starting point of \ref partition_search.
 */

 /*!\defgroup two_pass_algo Two Pass Mode
    \ingroup high_level_algo

 In two pass mode, the input file is passed into the encoder for a quick
 first pass, where statistics are gathered. These statistics and the input
 file are then passed back into the encoder for a second pass. The statistics
 help the encoder reach the desired bitrate without as much overshooting or
 undershooting.

 During the first pass, the codec will return "stats" packets that contain
 information useful for the second pass. The caller should concatenate these
 packets as they are received. In the second pass, the concatenated packets
 are passed in, along with the frames to encode. During the second pass,
 "frame" packets are returned that represent the compressed video.

 A complete example can be found in `examples/twopass_encoder.c`. Pseudocode
 is provided below to illustrate the core parts.

 During the first pass, the uncompressed frames are passed in and stats
 information is appended to a byte array.

~~~~~~~~~~~~~~~{.c}
// For simplicity, assume that there is enough memory in the stats buffer.
// Actual code will want to use a resizable array. stats_len represents
// the length of data already present in the buffer.
void get_stats_data(aom_codec_ctx_t *encoder, char *stats,
                    size_t *stats_len, bool *got_data) {
  const aom_codec_cx_pkt_t *pkt;
  aom_codec_iter_t iter = NULL;
  while ((pkt = aom_codec_get_cx_data(encoder, &iter))) {
    *got_data = true;
    if (pkt->kind != AOM_CODEC_STATS_PKT) continue;
    memcpy(stats + *stats_len, pkt->data.twopass_stats.buf,
           pkt->data.twopass_stats.sz);
    *stats_len += pkt->data.twopass_stats.sz;
  }
}

void first_pass(char *stats, size_t *stats_len) {
  struct aom_codec_enc_cfg first_pass_cfg;
  ... // Initialize the config as needed.
  first_pass_cfg.g_pass = AOM_RC_FIRST_PASS;
  aom_codec_ctx_t first_pass_encoder;
  ... // Initialize the encoder.

  while (frame_available) {
    // Read in the uncompressed frame, update frame_available
    aom_image_t *frame_to_encode = ...;
    aom_codec_encode(&first_pass_encoder, img, pts, duration, flags);
    get_stats_data(&first_pass_encoder, stats, stats_len);
  }
  // After all frames have been processed, call aom_codec_encode with
  // a NULL ptr repeatedly, until no more data is returned. The NULL
  // ptr tells the encoder that no more frames are available.
  bool got_data;
  do {
    got_data = false;
    aom_codec_encode(&first_pass_encoder, NULL, pts, duration, flags);
    get_stats_data(&first_pass_encoder, stats, stats_len, &got_data);
  } while (got_data);

  aom_codec_destroy(&first_pass_encoder);
}
~~~~~~~~~~~~~~~

 During the second pass, the uncompressed frames and the stats are
 passed into the encoder.

~~~~~~~~~~~~~~~{.c}
// Write out each encoded frame to the file.
void get_cx_data(aom_codec_ctx_t *encoder, FILE *file,
                 bool *got_data) {
  const aom_codec_cx_pkt_t *pkt;
  aom_codec_iter_t iter = NULL;
  while ((pkt = aom_codec_get_cx_data(encoder, &iter))) {
   *got_data = true;
   if (pkt->kind != AOM_CODEC_CX_FRAME_PKT) continue;
   fwrite(pkt->data.frame.buf, 1, pkt->data.frame.sz, file);
  }
}

void second_pass(char *stats, size_t stats_len) {
  struct aom_codec_enc_cfg second_pass_cfg;
  ... // Initialize the config file as needed.
  second_pass_cfg.g_pass = AOM_RC_LAST_PASS;
  cfg.rc_twopass_stats_in.buf = stats;
  cfg.rc_twopass_stats_in.sz = stats_len;
  aom_codec_ctx_t second_pass_encoder;
  ... // Initialize the encoder from the config.

  FILE *output = fopen("output.obu", "wb");
  while (frame_available) {
    // Read in the uncompressed frame, update frame_available
    aom_image_t *frame_to_encode = ...;
    aom_codec_encode(&second_pass_encoder, img, pts, duration, flags);
    get_cx_data(&second_pass_encoder, output);
  }
  // Pass in NULL to flush the encoder.
  bool got_data;
  do {
    got_data = false;
    aom_codec_encode(&second_pass_encoder, NULL, pts, duration, flags);
    get_cx_data(&second_pass_encoder, output, &got_data);
  } while (got_data);

  aom_codec_destroy(&second_pass_encoder);
}
~~~~~~~~~~~~~~~
 */

 /*!\defgroup look_ahead_buffer The Look-Ahead Buffer
    \ingroup high_level_algo

 A program should call \ref aom_codec_encode() for each frame that needs
 processing. These frames are internally copied and stored in a fixed-size
 circular buffer, known as the look-ahead buffer. Other parts of the code
 will use future frame information to inform current frame decisions;
 examples include the first-pass algorithm, TPL model, and temporal filter.
 Note that this buffer also keeps a reference to the last source frame.

 The look-ahead buffer is defined in \ref av1/encoder/lookahead.h. It acts as an
 opaque structure, with an interface to create and free memory associated with
 it. It supports pushing and popping frames onto the structure in a FIFO
 fashion. It also allows look-ahead when using the \ref av1_lookahead_peek()
 function with a non-negative number, and look-behind when -1 is passed in (for
 the last source frame; e.g., firstpass will use this for motion estimation).
 The \ref av1_lookahead_depth() function returns the current number of frames
 stored in it. Note that \ref av1_lookahead_pop() is a bit of a misnomer - it
 only pops if either the "flush" variable is set, or the buffer is at maximum
 capacity.

 The buffer is stored in the \ref AV1_COMP::lookahead field.
 It is initialized in the first call to \ref aom_codec_encode(), in the
 \ref av1_receive_raw_frame() sub-routine. The buffer size is defined by
 the g_lag_in_frames parameter set in the
 \ref aom_codec_enc_cfg_t::g_lag_in_frames struct.
 This can be modified manually but should only be set once. On the command
 line, the flag "--lag-in-frames" controls it. The default size is 19 for
 non-realtime usage and 1 for realtime. Note that a maximum value of 35 is
 enforced.

 A frame will stay in the buffer as long as possible. As mentioned above,
 the \ref av1_lookahead_pop() only removes a frame when either flush is set,
 or the buffer is full. Note that each call to \ref aom_codec_encode() inserts
 another frame into the buffer, and pop is called by the sub-function
 \ref av1_encode_strategy(). The buffer is told to flush when
 \ref aom_codec_encode() is passed a NULL image pointer. Note that the caller
 must repeatedly call \ref aom_codec_encode() with a NULL image pointer, until
 no more packets are available, in order to fully flush the buffer.

 */

/*! @} - end defgroup high_level_algo */

/*!\defgroup partition_search Partition Search
 * \ingroup encoder_algo
 A frame is first split into tiles in \ref encode_tiles(), with each tile
 compressed by av1_encode_tile(). Then a tile is processed in superblock rows
 via \ref av1_encode_sb_row() and then \ref encode_sb_row().

 Partition search starts by superblocks that are sequentially processed in
 \ref encode_sb_row(). For a superblock, two search modes are supported
 corresponding to the encoding configurations, \ref encode_nonrd_sb() is for
 1-pass and real-time modes, while \ref encode_rd_sb() performs more
 exhaustive searches.

 Partition search over the recursive quad-tree space is implemented by
 recursively calling \ref nonrd_use_partition(), \ref rd_use_partition(), or
 rd_pick_partition() and returning best options for sub-trees to their
 parent partitions.

 In libaom, partition search lays on top of mode search (predictor, transform,
 etc.) instead of being a separate module, the interface of mode search is
 \ref pick_sb_modes(), which connects \ref partition_search with
 \ref inter_mode_search and \ref intra_mode_search. To make good decisions,
 reconstruction is also required in order to build references and contexts, it
 is implemented by \ref encode_sb() at sub-tree level and \ref encode_b() at
 coding block level.
 * @{
 */
/*! @} - end defgroup partition_search */

/*!\defgroup intra_mode_search Intra Mode Search
 * \ingroup encoder_algo
 * This module describes intra mode search algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup intra_mode_search */

/*!\defgroup inter_mode_search Inter Mode Search
 * \ingroup encoder_algo
 * This module describes inter mode search algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup inter_mode_search */

/*!\defgroup palette_mode_search Palette Mode Search
 * \ingroup intra_mode_search
 * This module describes palette mode search algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup palette_mode_search */

/*!\defgroup transform_search Transform Search
 * \ingroup encoder_algo
 * This module describes transform search algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup transform_search */

/*!\defgroup coefficient_coding Transform Coefficient Coding and Optimization
 * \ingroup encoder_algo
 * This module describes the algorithms of transform coefficient coding and optimization in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup coefficient_coding */

/*!\defgroup in_loop_filter In-loop Filter
 * \ingroup encoder_algo
 * This module describes in-loop filter algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup in_loop_filter */

/*!\defgroup in_loop_cdef CDEF
 * \ingroup encoder_algo
 * This module describes the CDEF parameter search algorithm
 * in AV1. More details will be added.
 * @{
 */
/*! @} - end defgroup in_loop_restoration */

/*!\defgroup in_loop_restoration Loop Restoration
 * \ingroup encoder_algo
 * This module describes the loop restoration search
 * and estimation algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup in_loop_restoration */

/*!\defgroup rate_control Rate Control
 * \ingroup encoder_algo
 * This module describes rate control algorithm in AV1.
 * More details will be added.
 * @{
 */
/*! @} - end defgroup rate_control */