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* Copyright (c) 2016, Alliance for Open Media. All rights reserved
* This source code is subject to the terms of the BSD 2 Clause License and
* the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
* was not distributed with this source code in the LICENSE file, you can
* obtain it at If the Alliance for Open
* Media Patent License 1.0 was not distributed with this source code in the
* PATENTS file, you can obtain it at
#ifdef __cplusplus
extern "C" {
#include "av1/encoder/encoder.h"
struct yv12_buffer_config;
struct AV1_COMP;
int av1_get_max_filter_level(const AV1_COMP *cpi);
/*!\brief Algorithm for AV1 loop filter level selection.
* \ingroup in_loop_filter
* This function determines proper filter levels used for in-loop filter
* (deblock filter).
* \param[in] sd The pointer of frame buffer
* \param[in] cpi Top-level encoder structure
* \param[in] method The method used to select filter levels
* \par
* method includes:
* \arg \c LPF_PICK_FROM_FULL_IMAGE: Try the full image with different values.
* \arg \c LPF_PICK_FROM_FULL_IMAGE_NON_DUAL: Try the full image filter search
* with non-dual filter only.
* \arg \c LPF_PICK_FROM_SUBIMAGE: Try a small portion of the image with
* different values.
* \arg \c LPF_PICK_FROM_Q: Estimate the level based on quantizer and frame type
* \arg \c LPF_PICK_MINIMAL_LPF: Pick 0 to disable LPF if LPF was enabled last
* frame
* \return Nothing is returned. Instead, filter levels below are stored in the
* "loopfilter" structure inside "cpi":
* \arg \c filter_level[0]: the vertical filter level for Y plane
* \arg \c filter_level[1]: the horizontal filter level for Y plane
* \arg \c filter_level_u: the filter level for U plane
* \arg \c filter_level_v: the filter level for V plane
* \n
* \b Overview
* \par
* The workflow of deblock filter is shown in Fig.1. \n
* Boundary pixels pass through a non-flatness check, followed by a step that
* determines smoothness and selects proper types of filters
* (4-, 6-, 8-, 14-tap filter). \n
* If non-flatness criteria is not satisfied, the encoder will not apply
* deblock filtering on these boundary pixels.
* \image html filter_flow.png "Fig.1. The workflow of deblock filter" width=70%
* \par
* The non-flatness is determined by the boundary pixels and thresholds as shown
* in Fig.2. \n
* Filtering is applied when \n
* \f$|p_0-p_1|<thr_1\f$ and \f$|q_0-q_1|<thr_1\f$ and
* \f$2*|p_0-q_0|+|p_1-q_1|/2<thr_2\f$ \n
* \image html filter_thr.png "Fig.2. Non-flatness of pixel boundary" height=40%
* \par
* Thresholds ("thr_1" and "thr_2") are determined by the filter level. \n
* In AV1, for each frame, we employ the four filter levels, based on these
* observations: \n
* Luma and chroma planes have different characteristics, including subsampling
* (different plane size), coding quality (chroma planes are better coded). \n
* Therefore chroma planes need less deblocking filtering than luma plane. \n
* In addition, content texture has different spatial characteristics: vertical
* and horizontal direction may need different level of filtering. \n
* The selection of these filter levels is described in the following section.
* \par
* \b Algorithm
* \par
* The encoder selects filter levels given the current frame buffer, and the
* method. \n
* By default, "LPF_PICK_FROM_FULL_IMAGE" is used, which should provide
* the most appropriate filter levels. \n
* For video on demand (VOD) mode, if speed setting is larger than 5,
* For real-time mode, if speed setting is larger than 5, "LPF_PICK_FROM_Q" is
* used.
* \par
* "LPF_PICK_FROM_FULL_IMAGE" method: determine filter levels sequentially
* by a filter level search procedure (function "search_filter_level"). \n
* The order is: \n
* First search and determine the filter level for Y plane.
* Let vertical filter level (filter_level[0]) and the horizontal filter level
* (filter_level[1]) be equal to it. \n
* Keep the horizontal filter level the same and search and determine the
* vertical filter level. \n
* Search and determine the horizontal filter level. \n
* Search and determine filter level for U plane. \n
* Search and determine filter level for V plane.
* \par
* Search and determine filter level is fulfilled by function
* "search_filter_level". \n
* It starts with a base filter level ("filt_mid") initialized by the
* corresponding last frame's filter level. \n
* A filter step ("filter_step") is determined as:
* filter_step = filt_mid < 16 ? 4 : filt_mid / 4. \n
* Then a modified binary search strategy is employed to find a proper
* filter level. \n
* In each iteration, set filt_low = filt_mid - filter_step,
* filt_high = filt_mid + filter_step. \n
* We now have three candidate levels, "filt_mid", "filt_low" and "filt_high".
* \n
* Deblock filtering is applied on the current frame with candidate filter
* levels and the sum of squared error (SSE) between source and filtered frame
* is computed. \n
* Set "filt_best" to the filter level of the smallest SSE. If "filter_best"
* equals to "filt_mid", halve the filter_step. Otherwise, set filt_mid =
* filt_best. \n
* Go to the next iteration until "filter_step" is 0. \n
* Note that in the comparison of SSEs between SSE[filt_low] and SSE[filt_mid],
* a "bias" is introduced to slightly raise the filter level. \n
* It is based on the observation that low filter levels tend to yield a smaller
* SSE and produce a higher PSNR for the current frame, \n
* while oversmoothing it and degradating the quality for prediction for future
* frames and leanding to a suboptimal performance overall. \n
* Function "try_filter_frame" is the referrence for applying deblock filtering
* with a given filter level and computatition of SSE.
* \par
* "LPF_PICK_FROM_FULL_IMAGE_NON_DUAL" method: almost the same as
* just without separately searching for appropriate filter levels for vertical
* and horizontal filters.
* \par
* "LPF_PICK_FROM_Q" method: filter levels are determined by the
* quantization factor (q). \n
* For 8 bit: \n
* Keyframes: filt_guess = q * 0.06699 - 1.60817 \n
* Other frames: filt_guess = q * inter_frame_multiplier + 2.48225 \n
* inter_frame_multiplier = q > 700 ? 0.04590 : 0.02295 \n
* For 10 bit and 12 bit: \n
* filt_guess = q * 0.316206 + 3.87252 \n
* Then filter_level[0] = filter_level[1] = filter_level_u = filter_level_v =
* clamp(filt_guess, min_filter_level, max_filter_level) \n
* Where min_filter_level = 0, max_filter_level = 64 \n
* The equations were determined by linear fitting using filter levels
* generated by "LPF_PICK_FROM_FULL_IMAGE" method.
void av1_pick_filter_level(const struct yv12_buffer_config *sd,
struct AV1_COMP *cpi, LPF_PICK_METHOD method);
#ifdef __cplusplus
} // extern "C"