Add TPL section to encoder overview document
Change-Id: I4d42d85a1041a9c94c57717290d22d4c64d1fc22
diff --git a/doc/dev_guide/av1_encoder.dox b/doc/dev_guide/av1_encoder.dox
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--- a/doc/dev_guide/av1_encoder.dox
+++ b/doc/dev_guide/av1_encoder.dox
@@ -1,4 +1,4 @@
-/*!\page encoder_guide AV1 ENCODER GUIDE
+/*!\page encoder_guide AV1 ENCODER GUIDE
\tableofcontents
@@ -481,8 +481,100 @@
Add details here.
\section architecture_enc_tpl Temporal Dependency Modelling
+The temporal dependency model runs at the beginning of each GOP. It builds the
+motion trajectory within the GOP in units of 16x16 blocks. The temporal
+dependency of a 16x16 block is evaluated as the predictive coding gains it
+contributes to its trailing motion trajectory. This temporal dependency model
+reflects how important a coding block is for the coding efficiency of the
+overall GOP. It is hence used to scale the Lagrangian multiplier used in the
+rate-distortion optimization framework.
- Add details here.
+\subsection architecture_enc_tpl_config Configurations
+
+The temporal dependency model and its applications are by default turned on in
+libaom encoder for the VoD use case. To disable it, use --tpl-model=0 in the
+aomenc configuration.
+
+
+\subsection architecture_enc_tpl_algoritms Algorithms
+
+The scheme works in the reverse frame processing order over the source frames,
+propagating information from future frames back to the current frame. For each
+frame, a propagation step is run for each MB. it operates as follows:
+
+<ul>
+ <li> Estimate the intra prediction cost in terms of sum of absolute Hadamard
+ transform difference (SATD) noted as intra_cost. It also loads the motion
+ information available from the first-pass encode and estimates the inter
+ prediction cost as inter_cost. Due to the use of hybrid inter/intra
+ prediction mode, the inter_cost value is further upper bounded by
+ intra_cost. A propagation cost variable is used to collect all the
+ information flowed back from future processing frames. It is initialized as
+ 0 for all the blocks in the last processing frame in a group of pictures
+ (GOP).</li>
+
+ <li> The fraction of information from a current block to be propagated towards
+ its reference block is estimated as:
+\f[
+ propagation\_fraction = (1 − inter\_cost/intra\_cost)
+\f]
+ It reflects how much the motion compensated reference would reduce the
+ prediction error in percentage.</li>
+
+ <li> The total amount of information the current block contributes to the GOP
+ is estimated as intra_cost + propagation_cost. The information that it
+ propagates towards its reference block is captured by:
+
+\f[
+ propagation\_amount =
+ (intra\_cost + propagation\_cost) ∗ propagation\_fraction
+\f]</li>
+
+ <li> Note that the reference block may not necessarily sit on the grid of
+ 16x16 blocks. The propagation amount is hence dispensed to all the blocks
+ that overlap with the reference block. The corresponding block in the
+ reference frame accumulates its own propagation cost as it receives back
+ propagation.
+
+\f[
+ propagation\_cost = propagation\_cost +
+ (\frac{overlap\_area}{(16*16)} ∗ propagation\_amount)
+\f]</li>
+
+ <li> In the final encoding stage, the distortion propagation factor of a block
+ is evaluated as \f$(1 + \frac{propagation\_cost}{intra\_cost})\f$, where the second term
+ captures its impact on later frames in a GOP.</li>
+
+ <li> The Lagrangian multiplier is adapted at the 64x64 block level. For every
+ 64x64 block in a frame, we have a distortion propagation factor:
+
+\f[
+ dist\_prop[i] = 1 + \frac{propogation\_cost[i]}{intra\_cost[i]}
+\f]
+
+ where i denotes the block index in the frame. We also have the frame level
+ distortion propagation factor:
+
+\f[
+ dist\_prop = 1 +
+ \frac{\sum_{i}propogation\_cost[i]}{\sum_{i}intra\_cost[i]}
+\f]
+
+ which is used to normalize the propagation factor at the 64x64 block level. The
+ Lagrangian multiplier is hence adapted as:
+
+\f[
+ λ[i] = λ[0] * \frac{dist\_prop}{dist\_prop[i]}
+\f]
+
+ where λ0 is the multiplier associated with the frame level QP. The
+ 64x64 block level QP is scaled according to the Lagrangian multiplier.
+</ul>
+
+\subsection architecture_enc_tpl_keyfun Key Functions
+
+- The TPL model is built in (TODO REF) av1_tpl_setup_stats().
+- Its application to the QP offset is triggered in (TODO REF) setup_delta_q().
\section architecture_enc_partitions Block Partition Search
