/* * 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 www.aomedia.org/license/software. 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 www.aomedia.org/license/patent. */ #ifndef AOM_AV1_ENCODER_PICKLPF_H_ #define AOM_AV1_ENCODER_PICKLPF_H_ #ifdef __cplusplus extern "C" { #endif #include "av1/encoder/encoder.h" struct yv12_buffer_config; struct AV1_COMP; /*!\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 * * \remark 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, * "LPF_PICK_FROM_FULL_IMAGE_NON_DUAL" is used. \n * 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 * "LPF_PICK_FROM_FULL_IMAGE", \n * 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" #endif #endif // AOM_AV1_ENCODER_PICKLPF_H_
Codebase Browser
chromium