XVID，X264等是MPEG4、H264標準的開源編碼器，其中X264只有編碼部分，解碼部分需要FFMPEG完成；XVID有編解碼部分，其中解碼亦可以利用FFMPEG中的MPEG4完成解碼。視頻壓縮算法的計算復雜度，都是比較高的。其中具有最大計算復雜度有三部分：宏塊搜素運動補償部分、碼率控制部分、濾波算法部分；這三部分占據了算法復雜度的絕大部分資源。
XVID參數解釋：
參數的設置集中在FFMPEG中的libxvidff.C中的 ff_xvid_encode_init（）函數里面，主要是對編碼上下文進行初始化賦值操作，此函數中的宏定義在XVID.H之中：
ff_xvid_encode_init(AVCodecContext *avctx)? {
??? int xerr, i;
??? int xvid_flags = avctx->flags;
??? xvid_context_t *x = avctx->priv_data;
??? uint16_t *intra, *inter;
??? int fd;

??? xvid_plugin_single_t single;
??? xvid_ff_pass1_t rc2pass1;
??? xvid_plugin_2pass2_t rc2pass2;
??? xvid_gbl_init_t xvid_gbl_init;
??? xvid_enc_create_t xvid_enc_create;
??? xvid_enc_plugin_t plugins[7];

??? /* Bring in VOP flags from ffmpeg command-line */
??? x->vop_flags = XVID_VOP_HALFPEL; /* Bare minimum quality */ 半像素運動插值
??? if( xvid_flags & CODEC_FLAG_4MV )
??????? x->vop_flags |= XVID_VOP_INTER4V; /* Level 3 */每個宏塊分配四個運動矢量
??? if( xvid_flags & CODEC_FLAG_TRELLIS_QUANT)
??????? x->vop_flags |= XVID_VOP_TRELLISQUANT; /* Level 5 */
??? if( xvid_flags & CODEC_FLAG_AC_PRED )
??????? x->vop_flags |= XVID_VOP_HQACPRED; /* Level 6 * /高質量的AC預測
??? if( xvid_flags & CODEC_FLAG_GRAY )
??????? x->vop_flags |= XVID_VOP_GREYSCALE;

??? /* Decide which ME quality setting to use */
??? x->me_flags = 0;
??? switch( avctx->me_method ) { //宏塊搜索區域，搜索算法
?????? case ME_FULL:?? /* Quality 6 */
?????????? x->me_flags |=? XVID_ME_EXTSEARCH16/* extend PMV by more searches */
?????????????????????? |?? XVID_ME_EXTSEARCH8;/* use diamond/square for extended 8x8 search */

?????? case ME_EPZS:?? /* Quality 4 */
?????????? x->me_flags |=? XVID_ME_ADVANCEDDIAMOND8/* use advdiamond for XVID_ME_EXTSEARCH8 */
?????????????????????? |?? XVID_ME_HALFPELREFINE8
?????????????????????? |?? XVID_ME_CHROMA_PVOP/* also use chroma for P_VOP/S_VOP ME */
?????????????????????? |?? XVID_ME_CHROMA_BVOP;/* also use chroma for B_VOP ME */

?????? case ME_LOG:??? /* Quality 2 */
?????? case ME_PHODS:
?????? case ME_X1:
?????????? x->me_flags |=? XVID_ME_ADVANCEDDIAMOND16/* use advdiamonds instead of diamonds as search pattern */
?????????????????????? |?? XVID_ME_HALFPELREFINE16;

?????? case ME_ZERO:?? /* Quality 0 */
?????? default:
?????????? break;
??? }

??? /* Decide how we should decide blocks */
??? switch( avctx->mb_decision ) { //是否選擇碼率控制方式 avctx->mb_decision=1時，編碼一幀時間迅速增加
?????? case 2:
?????????? x->vop_flags |= XVID_VOP_MODEDECISION_RD;
?????????? x->me_flags |=? XVID_ME_HALFPELREFINE8_RD
?????????????????????? |?? XVID_ME_QUARTERPELREFINE8_RD
?????????????????????? |?? XVID_ME_EXTSEARCH_RD
?????????????????????? |?? XVID_ME_CHECKPREDICTION_RD;
?????? case 1:
?????????? if( !(x->vop_flags & XVID_VOP_MODEDECISION_RD) )
?????????????? x->vop_flags |= XVID_VOP_FAST_MODEDECISION_RD;
?????????? x->me_flags |=? XVID_ME_HALFPELREFINE16_RD
?????????????????????? |?? XVID_ME_QUARTERPELREFINE16_RD;

?????? default:
?????????? break;
??? }

??? /* Bring in VOL flags from ffmpeg command-line */
??? x->vol_flags = 0;
??? if( xvid_flags & CODEC_FLAG_GMC ) {
??????? x->vol_flags |= XVID_VOL_GMC;
??????? x->me_flags |= XVID_ME_GME_REFINE;
??? }
??? if( xvid_flags & CODEC_FLAG_QPEL ) { //是否允許1/4像素
??????? x->vol_flags |= XVID_VOL_QUARTERPEL;
??????? x->me_flags |= XVID_ME_QUARTERPELREFINE16;
??????? if( x->vop_flags & XVID_VOP_INTER4V )
??????????? x->me_flags |= XVID_ME_QUARTERPELREFINE8;
??? }

??? memset(&xvid_gbl_init, 0, sizeof(xvid_gbl_init));
??? xvid_gbl_init.version = XVID_VERSION;
??? xvid_gbl_init.debug = 0;

#ifdef ARCH_POWERPC
??? /* XviD's PPC support is borked, use libavcodec to detect */
#if HAVE_ALTIVEC==1
??? if( has_altivec() ) {
??????? xvid_gbl_init.cpu_flags = XVID_CPU_FORCE | XVID_CPU_ALTIVEC;
??? } else
#endif
??????? xvid_gbl_init.cpu_flags = XVID_CPU_FORCE;
#else
??? /* XviD can detect on x86 */
??? xvid_gbl_init.cpu_flags = 0;
#endif

??? /* Initialize */
??? xvid_global(NULL, XVID_GBL_INIT, &xvid_gbl_init, NULL);

??? /* Create the encoder reference */
??? memset(&xvid_enc_create, 0, sizeof(xvid_enc_create));
??? xvid_enc_create.version = XVID_VERSION;

??? /* Store the desired frame size */
??? xvid_enc_create.width = x->xsize = avctx->width; //圖像寬度
??? xvid_enc_create.height = x->ysize = avctx->height; //圖像高度

??? /* XviD can determine the proper profile to use */
???? xvid_enc_create.profile = XVID_PROFILE_S_L3; //? 編碼設定檔次和級別 //壓縮級別，MPEG4-ASP最高壓縮級別

??? /* We don't use zones or threads */
??? xvid_enc_create.zones = NULL;
??? xvid_enc_create.num_zones = 0;
??? xvid_enc_create.num_threads = 0;

??? xvid_enc_create.plugins = plugins;
??? xvid_enc_create.num_plugins = 0;

??? /* Initialize Buffers */
??? x->twopassbuffer = NULL;
??? x->old_twopassbuffer = NULL;
??? x->twopassfile = NULL;

??? if( xvid_flags & CODEC_FLAG_PASS1 ) {
? ?
??????? }
??????? x->twopassbuffer[0] = x->old_twopassbuffer[0] = 0;

??????? plugins[xvid_enc_create.num_plugins].func = xvid_ff_2pass;
??????? plugins[xvid_enc_create.num_plugins].param = &rc2pass1;
??????? xvid_enc_create.num_plugins++;
??? } else if( xvid_flags & CODEC_FLAG_PASS2 ) {//第二次編碼，需要第一次編碼生成的文件才可以完成第二次編碼，不適用實時編碼
??????? memset(&rc2pass2, 0, sizeof(xvid_plugin_2pass2_t));
??????? rc2pass2.version = XVID_VERSION;
??????? rc2pass2.bitrate = avctx->bit_rate;

????? ?

??????? close(fd);
??????? rc2pass2.filename = x->twopassfile;
??????? plugins[xvid_enc_create.num_plugins].func = xvid_plugin_2pass2;
??????? plugins[xvid_enc_create.num_plugins].param = &rc2pass2;
??????? xvid_enc_create.num_plugins++;
??? } else if( !(xvid_flags & CODEC_FLAG_QSCALE) ) {
??????? /* Single Pass Bitrate Control! */
??????? memset(&single, 0, sizeof(xvid_plugin_single_t));
??????? single.version = XVID_VERSION;
??????? single.bitrate = avctx->bit_rate;

??????? plugins[xvid_enc_create.num_plugins].func = xvid_plugin_single;
??????? plugins[xvid_enc_create.num_plugins].param = &single;
??????? xvid_enc_create.num_plugins++;
??? }

??? /* Luminance Masking */
??? if( 0.0 != avctx->lumi_masking ) {
??????? plugins[xvid_enc_create.num_plugins].func = xvid_plugin_lumimasking;
??????? plugins[xvid_enc_create.num_plugins].param = NULL;
??????? xvid_enc_create.num_plugins++;
??? }

??? /* Frame Rate and Key Frames */
??? xvid_correct_framerate(avctx);
??? xvid_enc_create.fincr = avctx->time_base.num;
??? xvid_enc_create.fbase = avctx->time_base.den;
??? if( avctx->gop_size > 0 )
??????? xvid_enc_create.max_key_interval = avctx->gop_size; //圖像組的長度設定
??? else
??????? xvid_enc_create.max_key_interval = 240; /* XviD's best default */

??? /* Quants */
??? if( xvid_flags & CODEC_FLAG_QSCALE ) x->qscale = 1; ///encodes[i]->codec.vcc->flags|=CODEC_FLAG_QSCALE; 選擇常量化/ //質量控制=量化參數，1~31，數值越小質量越高
??? else x->qscale = 0;

??? xvid_enc_create.min_quant[0] = avctx->qmin;
??? xvid_enc_create.min_quant[1] = avctx->qmin;
??? xvid_enc_create.min_quant[2] = avctx->qmin;
??? xvid_enc_create.max_quant[0] = avctx->qmax;
??? xvid_enc_create.max_quant[1] = avctx->qmax;
??? xvid_enc_create.max_quant[2] = avctx->qmax;

??? /* Quant Matrices */
??? x->intra_matrix = x->inter_matrix = NULL;
??? if( avctx->mpeg_quant ) //允許MPEG量化 量化矩陣
?????? x->vol_flags |= XVID_VOL_MPEGQUANT;
??? if( (avctx->intra_matrix || avctx->inter_matrix) ) { //可以自己設定量化矩陣avctx->intra_matrix，avctx->inter_matrix
?????? x->vol_flags |= XVID_VOL_MPEGQUANT;

?????? if( avctx->intra_matrix ) {
?????????? intra = avctx->intra_matrix;
?????????? x->intra_matrix = av_malloc(sizeof(unsigned char) * 64);
?????? } else
?????????? intra = NULL;
?????? if( avctx->inter_matrix ) {
?????????? inter = avctx->inter_matrix;
?????????? x->inter_matrix = av_malloc(sizeof(unsigned char) * 64);
?????? } else
?????????? inter = NULL;

?????? for( i = 0; i < 64; i++ ) {
?????????? if( intra )
?????????????? x->intra_matrix[i] = (unsigned char)intra[i];
?????????? if( inter )
?????????????? x->inter_matrix[i] = (unsigned char)inter[i];
?????? }
??? }

??? /* Misc Settings */
??? xvid_enc_create.frame_drop_ratio = 0; //丟幀率；0~100
??? xvid_enc_create.global = 0;
??? if( xvid_flags & CODEC_FLAG_CLOSED_GOP )
??????? xvid_enc_create.global |= XVID_GLOBAL_CLOSED_GOP;

??? /* Determines which codec mode we are operating in */
??? avctx->extradata = NULL;
??? avctx->extradata_size = 0;
??? if( xvid_flags & CODEC_FLAG_GLOBAL_HEADER ) {
??????? /* In this case, we are claiming to be MPEG4 */
??????? x->quicktime_format = 1;
??????? avctx->codec_id = CODEC_ID_MPEG4;
??? } else {
??????? /* We are claiming to be XviD */
??????? x->quicktime_format = 0;
??????? if(!avctx->codec_tag)
??????????? avctx->codec_tag = ff_get_fourcc("xvid");
??? }

??? /* Bframes */
??? xvid_enc_create.max_bframes = avctx->max_b_frames;
??? xvid_enc_create.bquant_offset = 100 * avctx->b_quant_offset;
??? xvid_enc_create.bquant_ratio = 100 * avctx->b_quant_factor;
??? if( avctx->max_b_frames > 0? && !x->quicktime_format ) xvid_enc_create.global |= XVID_GLOBAL_PACKED;

??? /* Create encoder context */
??? xerr = xvid_encore(NULL, XVID_ENC_CREATE, &xvid_enc_create, NULL); //建立編碼上下文
??? if( xerr ) {
??????? av_log(avctx, AV_LOG_ERROR, "XviD: Could not create encoder reference\n");
??????? return -1;
??? }

??? x->encoder_handle = xvid_enc_create.handle;
??? avctx->coded_frame = &x->encoded_picture;

??? return 0;
}

int ff_xvid_encode_frame(AVCodecContext *avctx,

???????????????????????? unsigned char *frame, int buf_size, void *data) { 開始編碼一幀

?xerr = xvid_encore(x->encoder_handle, XVID_ENC_ENCODE,
??????? &xvid_enc_frame, &xvid_enc_stats); //開始編碼一幀

｝

XVID的量化可以有三種方式：常量化，用戶自己設定量化矩陣，調用默認的量化矩陣

總結

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