| /* |
| * 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_ |
