#include "libavutil/imgutils.h"
#include "avcodec.h"
#include "internal.h"
#include "vp8.h"
#include "vp8data.h"
#include "rectangle.h"
#include "thread.h"
Go to the source code of this file.
Defines | |
#define | MV_EDGE_CHECK(n) |
#define | XCHG(a, b, xchg) |
#define | MARGIN (16 << 2) |
#define | REBASE(pic) pic ? pic - &s_src->frames[0] + &s->frames[0] : NULL |
Functions | |
static void | free_buffers (VP8Context *s) |
VP8 compatible video decoder. | |
static int | vp8_alloc_frame (VP8Context *s, AVFrame *f) |
static void | vp8_release_frame (VP8Context *s, AVFrame *f, int prefer_delayed_free, int can_direct_free) |
static void | vp8_decode_flush_impl (AVCodecContext *avctx, int prefer_delayed_free, int can_direct_free, int free_mem) |
static void | vp8_decode_flush (AVCodecContext *avctx) |
static int | update_dimensions (VP8Context *s, int width, int height) |
static void | parse_segment_info (VP8Context *s) |
static void | update_lf_deltas (VP8Context *s) |
static int | setup_partitions (VP8Context *s, const uint8_t *buf, int buf_size) |
static void | get_quants (VP8Context *s) |
static VP56Frame | ref_to_update (VP8Context *s, int update, VP56Frame ref) |
Determine which buffers golden and altref should be updated with after this frame. | |
static void | update_refs (VP8Context *s) |
static int | decode_frame_header (VP8Context *s, const uint8_t *buf, int buf_size) |
static av_always_inline void | clamp_mv (VP8Context *s, VP56mv *dst, const VP56mv *src) |
static int | read_mv_component (VP56RangeCoder *c, const uint8_t *p) |
Motion vector coding, 17.1. | |
static av_always_inline const uint8_t * | get_submv_prob (uint32_t left, uint32_t top) |
static av_always_inline int | decode_splitmvs (VP8Context *s, VP56RangeCoder *c, VP8Macroblock *mb) |
Split motion vector prediction, 16.4. | |
static av_always_inline void | decode_mvs (VP8Context *s, VP8Macroblock *mb, int mb_x, int mb_y) |
static av_always_inline void | decode_intra4x4_modes (VP8Context *s, VP56RangeCoder *c, int mb_x, int keyframe) |
static av_always_inline void | decode_mb_mode (VP8Context *s, VP8Macroblock *mb, int mb_x, int mb_y, uint8_t *segment, uint8_t *ref) |
static int | decode_block_coeffs_internal (VP56RangeCoder *c, DCTELEM block[16], uint8_t probs[16][3][NUM_DCT_TOKENS-1], int i, uint8_t *token_prob, int16_t qmul[2]) |
static av_always_inline int | decode_block_coeffs (VP56RangeCoder *c, DCTELEM block[16], uint8_t probs[16][3][NUM_DCT_TOKENS-1], int i, int zero_nhood, int16_t qmul[2]) |
static av_always_inline void | decode_mb_coeffs (VP8Context *s, VP56RangeCoder *c, VP8Macroblock *mb, uint8_t t_nnz[9], uint8_t l_nnz[9]) |
static av_always_inline void | backup_mb_border (uint8_t *top_border, uint8_t *src_y, uint8_t *src_cb, uint8_t *src_cr, int linesize, int uvlinesize, int simple) |
static av_always_inline void | xchg_mb_border (uint8_t *top_border, uint8_t *src_y, uint8_t *src_cb, uint8_t *src_cr, int linesize, int uvlinesize, int mb_x, int mb_y, int mb_width, int simple, int xchg) |
static av_always_inline int | check_dc_pred8x8_mode (int mode, int mb_x, int mb_y) |
static av_always_inline int | check_tm_pred8x8_mode (int mode, int mb_x, int mb_y) |
static av_always_inline int | check_intra_pred8x8_mode (int mode, int mb_x, int mb_y) |
static av_always_inline int | check_intra_pred8x8_mode_emuedge (int mode, int mb_x, int mb_y) |
static av_always_inline int | check_tm_pred4x4_mode (int mode, int mb_x, int mb_y) |
static av_always_inline int | check_intra_pred4x4_mode_emuedge (int mode, int mb_x, int mb_y, int *copy_buf) |
static av_always_inline void | intra_predict (VP8Context *s, uint8_t *dst[3], VP8Macroblock *mb, int mb_x, int mb_y) |
static av_always_inline void | vp8_mc_luma (VP8Context *s, uint8_t *dst, AVFrame *ref, const VP56mv *mv, int x_off, int y_off, int block_w, int block_h, int width, int height, int linesize, vp8_mc_func mc_func[3][3]) |
luma MC function | |
static av_always_inline void | vp8_mc_chroma (VP8Context *s, uint8_t *dst1, uint8_t *dst2, AVFrame *ref, const VP56mv *mv, int x_off, int y_off, int block_w, int block_h, int width, int height, int linesize, vp8_mc_func mc_func[3][3]) |
chroma MC function | |
static av_always_inline void | vp8_mc_part (VP8Context *s, uint8_t *dst[3], AVFrame *ref_frame, int x_off, int y_off, int bx_off, int by_off, int block_w, int block_h, int width, int height, VP56mv *mv) |
static av_always_inline void | prefetch_motion (VP8Context *s, VP8Macroblock *mb, int mb_x, int mb_y, int mb_xy, int ref) |
static av_always_inline void | inter_predict (VP8Context *s, uint8_t *dst[3], VP8Macroblock *mb, int mb_x, int mb_y) |
Apply motion vectors to prediction buffer, chapter 18. | |
static av_always_inline void | idct_mb (VP8Context *s, uint8_t *dst[3], VP8Macroblock *mb) |
static av_always_inline void | filter_level_for_mb (VP8Context *s, VP8Macroblock *mb, VP8FilterStrength *f) |
static av_always_inline void | filter_mb (VP8Context *s, uint8_t *dst[3], VP8FilterStrength *f, int mb_x, int mb_y) |
static av_always_inline void | filter_mb_simple (VP8Context *s, uint8_t *dst, VP8FilterStrength *f, int mb_x, int mb_y) |
static void | filter_mb_row (VP8Context *s, AVFrame *curframe, int mb_y) |
static void | filter_mb_row_simple (VP8Context *s, AVFrame *curframe, int mb_y) |
static void | release_queued_segmaps (VP8Context *s, int is_close) |
static int | vp8_decode_frame (AVCodecContext *avctx, void *data, int *data_size, AVPacket *avpkt) |
static av_cold int | vp8_decode_init (AVCodecContext *avctx) |
static av_cold int | vp8_decode_free (AVCodecContext *avctx) |
static av_cold int | vp8_decode_init_thread_copy (AVCodecContext *avctx) |
static int | vp8_decode_update_thread_context (AVCodecContext *dst, const AVCodecContext *src) |
Variables | |
static const uint8_t | subpel_idx [3][8] |
AVCodec | ff_vp8_decoder |
#define MARGIN (16 << 2) |
#define MV_EDGE_CHECK | ( | n | ) |
{\ VP8Macroblock *edge = mb_edge[n];\ int edge_ref = edge->ref_frame;\ if (edge_ref != VP56_FRAME_CURRENT) {\ uint32_t mv = AV_RN32A(&edge->mv);\ if (mv) {\ if (cur_sign_bias != sign_bias[edge_ref]) {\ /* SWAR negate of the values in mv. */\ mv = ~mv;\ mv = ((mv&0x7fff7fff) + 0x00010001) ^ (mv&0x80008000);\ }\ if (!n || mv != AV_RN32A(&near_mv[idx]))\ AV_WN32A(&near_mv[++idx], mv);\ cnt[idx] += 1 + (n != 2);\ } else\ cnt[CNT_ZERO] += 1 + (n != 2);\ }\ }
Referenced by decode_mvs().
#define REBASE | ( | pic | ) | pic ? pic - &s_src->frames[0] + &s->frames[0] : NULL |
Definition at line 1808 of file vp8.c.
Referenced by vp8_decode_update_thread_context().
#define XCHG | ( | a, | ||
b, | ||||
xchg | ||||
) |
static av_always_inline void backup_mb_border | ( | uint8_t * | top_border, | |
uint8_t * | src_y, | |||
uint8_t * | src_cb, | |||
uint8_t * | src_cr, | |||
int | linesize, | |||
int | uvlinesize, | |||
int | simple | |||
) | [static] |
Definition at line 836 of file vp8.c.
Referenced by filter_mb_row(), and filter_mb_row_simple().
static av_always_inline int check_dc_pred8x8_mode | ( | int | mode, | |
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 878 of file vp8.c.
Referenced by check_intra_pred8x8_mode(), and check_intra_pred8x8_mode_emuedge().
static av_always_inline int check_intra_pred4x4_mode_emuedge | ( | int | mode, | |
int | mb_x, | |||
int | mb_y, | |||
int * | copy_buf | |||
) | [static] |
Definition at line 934 of file vp8.c.
Referenced by intra_predict().
static av_always_inline int check_intra_pred8x8_mode | ( | int | mode, | |
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 898 of file vp8.c.
Referenced by intra_predict().
static av_always_inline int check_intra_pred8x8_mode_emuedge | ( | int | mode, | |
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 908 of file vp8.c.
Referenced by intra_predict().
static av_always_inline int check_tm_pred4x4_mode | ( | int | mode, | |
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 924 of file vp8.c.
Referenced by check_intra_pred4x4_mode_emuedge().
static av_always_inline int check_tm_pred8x8_mode | ( | int | mode, | |
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 888 of file vp8.c.
Referenced by check_intra_pred8x8_mode_emuedge().
static av_always_inline void clamp_mv | ( | VP8Context * | s, | |
VP56mv * | dst, | |||
const VP56mv * | src | |||
) | [static] |
Definition at line 407 of file vp8.c.
Referenced by decode_mvs().
static av_always_inline int decode_block_coeffs | ( | VP56RangeCoder * | c, | |
DCTELEM | block[16], | |||
uint8_t | probs[16][3][NUM_DCT_TOKENS-1], | |||
int | i, | |||
int | zero_nhood, | |||
int16_t | qmul[2] | |||
) | [static] |
c | arithmetic bitstream reader context | |
block | destination for block coefficients | |
probs | probabilities to use when reading trees from the bitstream | |
i | initial coeff index, 0 unless a separate DC block is coded | |
zero_nhood | the initial prediction context for number of surrounding all-zero blocks (only left/top, so 0-2) | |
qmul | array holding the dc/ac dequant factor at position 0/1 |
Definition at line 764 of file vp8.c.
Referenced by decode_mb_coeffs().
static int decode_block_coeffs_internal | ( | VP56RangeCoder * | c, | |
DCTELEM | block[16], | |||
uint8_t | probs[16][3][NUM_DCT_TOKENS-1], | |||
int | i, | |||
uint8_t * | token_prob, | |||
int16_t | qmul[2] | |||
) | [static] |
c | arithmetic bitstream reader context | |
block | destination for block coefficients | |
probs | probabilities to use when reading trees from the bitstream | |
i | initial coeff index, 0 unless a separate DC block is coded | |
qmul | array holding the dc/ac dequant factor at position 0/1 |
Definition at line 698 of file vp8.c.
Referenced by decode_block_coeffs().
static int decode_frame_header | ( | VP8Context * | s, | |
const uint8_t * | buf, | |||
int | buf_size | |||
) | [static] |
Definition at line 270 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void decode_intra4x4_modes | ( | VP8Context * | s, | |
VP56RangeCoder * | c, | |||
int | mb_x, | |||
int | keyframe | |||
) | [static] |
Definition at line 613 of file vp8.c.
Referenced by decode_mb_mode().
static av_always_inline void decode_mb_coeffs | ( | VP8Context * | s, | |
VP56RangeCoder * | c, | |||
VP8Macroblock * | mb, | |||
uint8_t | t_nnz[9], | |||
uint8_t | l_nnz[9] | |||
) | [static] |
Definition at line 775 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void decode_mb_mode | ( | VP8Context * | s, | |
VP8Macroblock * | mb, | |||
int | mb_x, | |||
int | mb_y, | |||
uint8_t * | segment, | |||
uint8_t * | ref | |||
) | [static] |
Definition at line 638 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void decode_mvs | ( | VP8Context * | s, | |
VP8Macroblock * | mb, | |||
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 523 of file vp8.c.
Referenced by decode_mb_mode().
static av_always_inline int decode_splitmvs | ( | VP8Context * | s, | |
VP56RangeCoder * | c, | |||
VP8Macroblock * | mb | |||
) | [static] |
Split motion vector prediction, 16.4.
Definition at line 459 of file vp8.c.
Referenced by decode_mvs().
static av_always_inline void filter_level_for_mb | ( | VP8Context * | s, | |
VP8Macroblock * | mb, | |||
VP8FilterStrength * | f | |||
) | [static] |
Definition at line 1392 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void filter_mb | ( | VP8Context * | s, | |
uint8_t * | dst[3], | |||
VP8FilterStrength * | f, | |||
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 1422 of file vp8.c.
Referenced by filter_mb_row().
static void filter_mb_row | ( | VP8Context * | s, | |
AVFrame * | curframe, | |||
int | mb_y | |||
) | [static] |
Definition at line 1523 of file vp8.c.
Referenced by vp8_decode_frame().
static void filter_mb_row_simple | ( | VP8Context * | s, | |
AVFrame * | curframe, | |||
int | mb_y | |||
) | [static] |
Definition at line 1542 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void filter_mb_simple | ( | VP8Context * | s, | |
uint8_t * | dst, | |||
VP8FilterStrength * | f, | |||
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 1492 of file vp8.c.
Referenced by filter_mb_row_simple().
static void free_buffers | ( | VP8Context * | s | ) | [static] |
VP8 compatible video decoder.
Copyright (C) 2010 David Conrad Copyright (C) 2010 Ronald S. Bultje Copyright (C) 2010 Jason Garrett-Glaser
This file is part of Libav.
Libav is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.
Libav is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with Libav; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
Definition at line 37 of file vp8.c.
Referenced by vp8_decode_flush_impl(), and vp8_decode_update_thread_context().
static void get_quants | ( | VP8Context * | s | ) | [static] |
Definition at line 198 of file vp8.c.
Referenced by decode_frame_header().
static av_always_inline const uint8_t* get_submv_prob | ( | uint32_t | left, | |
uint32_t | top | |||
) | [static] |
Definition at line 445 of file vp8.c.
Referenced by decode_splitmvs().
static av_always_inline void idct_mb | ( | VP8Context * | s, | |
uint8_t * | dst[3], | |||
VP8Macroblock * | mb | |||
) | [static] |
Definition at line 1340 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void inter_predict | ( | VP8Context * | s, | |
uint8_t * | dst[3], | |||
VP8Macroblock * | mb, | |||
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Apply motion vectors to prediction buffer, chapter 18.
Definition at line 1260 of file vp8.c.
Referenced by vp8_decode_frame().
static av_always_inline void intra_predict | ( | VP8Context * | s, | |
uint8_t * | dst[3], | |||
VP8Macroblock * | mb, | |||
int | mb_x, | |||
int | mb_y | |||
) | [static] |
Definition at line 968 of file vp8.c.
Referenced by vp8_decode_frame().
static void parse_segment_info | ( | VP8Context * | s | ) | [static] |
Definition at line 139 of file vp8.c.
Referenced by decode_frame_header().
static av_always_inline void prefetch_motion | ( | VP8Context * | s, | |
VP8Macroblock * | mb, | |||
int | mb_x, | |||
int | mb_y, | |||
int | mb_xy, | |||
int | ref | |||
) | [static] |
Definition at line 1238 of file vp8.c.
Referenced by vp8_decode_frame().
static int read_mv_component | ( | VP56RangeCoder * | c, | |
const uint8_t * | p | |||
) | [static] |
Motion vector coding, 17.1.
Definition at line 416 of file vp8.c.
Referenced by decode_mvs(), and decode_splitmvs().
static VP56Frame ref_to_update | ( | VP8Context * | s, | |
int | update, | |||
VP56Frame | ref | |||
) | [static] |
Determine which buffers golden and altref should be updated with after this frame.
The spec isn't clear here, so I'm going by my understanding of what libvpx does
Intra frames update all 3 references Inter frames update VP56_FRAME_PREVIOUS if the update_last flag is set If the update (golden|altref) flag is set, it's updated with the current frame if update_last is set, and VP56_FRAME_PREVIOUS otherwise. If the flag is not set, the number read means: 0: no update 1: VP56_FRAME_PREVIOUS 2: update golden with altref, or update altref with golden
Definition at line 243 of file vp8.c.
Referenced by update_refs().
static void release_queued_segmaps | ( | VP8Context * | s, | |
int | is_close | |||
) | [static] |
Definition at line 1555 of file vp8.c.
Referenced by vp8_decode_frame(), and vp8_decode_free().
static int setup_partitions | ( | VP8Context * | s, | |
const uint8_t * | buf, | |||
int | buf_size | |||
) | [static] |
Definition at line 172 of file vp8.c.
Referenced by decode_frame_header().
static int update_dimensions | ( | VP8Context * | s, | |
int | width, | |||
int | height | |||
) | [static] |
Definition at line 109 of file vp8.c.
Referenced by decode_frame_header().
static void update_lf_deltas | ( | VP8Context * | s | ) | [static] |
Definition at line 160 of file vp8.c.
Referenced by decode_frame_header().
static void update_refs | ( | VP8Context * | s | ) | [static] |
Definition at line 259 of file vp8.c.
Referenced by decode_frame_header().
static int vp8_alloc_frame | ( | VP8Context * | s, | |
AVFrame * | f | |||
) | [static] |
Definition at line 49 of file vp8.c.
Referenced by vp8_decode_frame().
static void vp8_decode_flush | ( | AVCodecContext * | avctx | ) | [static] |
static void vp8_decode_flush_impl | ( | AVCodecContext * | avctx, | |
int | prefer_delayed_free, | |||
int | can_direct_free, | |||
int | free_mem | |||
) | [static] |
Definition at line 85 of file vp8.c.
Referenced by update_dimensions(), vp8_decode_flush(), and vp8_decode_free().
static int vp8_decode_frame | ( | AVCodecContext * | avctx, | |
void * | data, | |||
int * | data_size, | |||
AVPacket * | avpkt | |||
) | [static] |
static av_cold int vp8_decode_free | ( | AVCodecContext * | avctx | ) | [static] |
static av_cold int vp8_decode_init | ( | AVCodecContext * | avctx | ) | [static] |
static av_cold int vp8_decode_init_thread_copy | ( | AVCodecContext * | avctx | ) | [static] |
static int vp8_decode_update_thread_context | ( | AVCodecContext * | dst, | |
const AVCodecContext * | src | |||
) | [static] |
static av_always_inline void vp8_mc_chroma | ( | VP8Context * | s, | |
uint8_t * | dst1, | |||
uint8_t * | dst2, | |||
AVFrame * | ref, | |||
const VP56mv * | mv, | |||
int | x_off, | |||
int | y_off, | |||
int | block_w, | |||
int | block_h, | |||
int | width, | |||
int | height, | |||
int | linesize, | |||
vp8_mc_func | mc_func[3][3] | |||
) | [static] |
chroma MC function
s | VP8 decoding context | |
dst1 | target buffer for block data at block position (U plane) | |
dst2 | target buffer for block data at block position (V plane) | |
ref | reference picture buffer at origin (0, 0) | |
mv | motion vector (relative to block position) to get pixel data from | |
x_off | horizontal position of block from origin (0, 0) | |
y_off | vertical position of block from origin (0, 0) | |
block_w | width of block (16, 8 or 4) | |
block_h | height of block (always same as block_w) | |
width | width of src/dst plane data | |
height | height of src/dst plane data | |
linesize | size of a single line of plane data, including padding | |
mc_func | motion compensation function pointers (bilinear or sixtap MC) |
Definition at line 1163 of file vp8.c.
Referenced by inter_predict(), and vp8_mc_part().
static av_always_inline void vp8_mc_luma | ( | VP8Context * | s, | |
uint8_t * | dst, | |||
AVFrame * | ref, | |||
const VP56mv * | mv, | |||
int | x_off, | |||
int | y_off, | |||
int | block_w, | |||
int | block_h, | |||
int | width, | |||
int | height, | |||
int | linesize, | |||
vp8_mc_func | mc_func[3][3] | |||
) | [static] |
luma MC function
s | VP8 decoding context | |
dst | target buffer for block data at block position | |
ref | reference picture buffer at origin (0, 0) | |
mv | motion vector (relative to block position) to get pixel data from | |
x_off | horizontal position of block from origin (0, 0) | |
y_off | vertical position of block from origin (0, 0) | |
block_w | width of block (16, 8 or 4) | |
block_h | height of block (always same as block_w) | |
width | width of src/dst plane data | |
height | height of src/dst plane data | |
linesize | size of a single line of plane data, including padding | |
mc_func | motion compensation function pointers (bilinear or sixtap MC) |
Definition at line 1113 of file vp8.c.
Referenced by inter_predict(), and vp8_mc_part().
static av_always_inline void vp8_mc_part | ( | VP8Context * | s, | |
uint8_t * | dst[3], | |||
AVFrame * | ref_frame, | |||
int | x_off, | |||
int | y_off, | |||
int | bx_off, | |||
int | by_off, | |||
int | block_w, | |||
int | block_h, | |||
int | width, | |||
int | height, | |||
VP56mv * | mv | |||
) | [static] |
Definition at line 1206 of file vp8.c.
Referenced by inter_predict().
static void vp8_release_frame | ( | VP8Context * | s, | |
AVFrame * | f, | |||
int | prefer_delayed_free, | |||
int | can_direct_free | |||
) | [static] |
Definition at line 63 of file vp8.c.
Referenced by vp8_decode_flush_impl(), and vp8_decode_frame().
static av_always_inline void xchg_mb_border | ( | uint8_t * | top_border, | |
uint8_t * | src_y, | |||
uint8_t * | src_cb, | |||
uint8_t * | src_cr, | |||
int | linesize, | |||
int | uvlinesize, | |||
int | mb_x, | |||
int | mb_y, | |||
int | mb_width, | |||
int | simple, | |||
int | xchg | |||
) | [static] |
Definition at line 847 of file vp8.c.
Referenced by intra_predict().
{ .name = "vp8", .type = AVMEDIA_TYPE_VIDEO, .id = CODEC_ID_VP8, .priv_data_size = sizeof(VP8Context), .init = vp8_decode_init, .close = vp8_decode_free, .decode = vp8_decode_frame, .capabilities = CODEC_CAP_DR1 | CODEC_CAP_FRAME_THREADS, .flush = vp8_decode_flush, .long_name = NULL_IF_CONFIG_SMALL("On2 VP8"), .init_thread_copy = ONLY_IF_THREADS_ENABLED(vp8_decode_init_thread_copy), .update_thread_context = ONLY_IF_THREADS_ENABLED(vp8_decode_update_thread_context), }
const uint8_t subpel_idx[3][8] [static] |
{ { 0, 1, 2, 1, 2, 1, 2, 1 }, { 0, 3, 5, 3, 5, 3, 5, 3 }, { 0, 2, 3, 2, 3, 2, 3, 2 }, }
Definition at line 1089 of file vp8.c.
Referenced by vp8_mc_chroma(), and vp8_mc_luma().