FFmpeg源代码简单分析：avcodec

=====================================================

FFmpeg的库函数源代码分析文章列表：

【架构图】

FFmpeg源代码结构图 - 解码

FFmpeg源代码结构图 - 编码

【通用】

FFmpeg 源代码简单分析：av_register_all()

FFmpeg 源代码简单分析：avcodec_register_all()

FFmpeg 源代码简单分析：内存的分配和释放（av_malloc()、av_free()等）

FFmpeg 源代码简单分析：常见结构体的初始化和销毁（AVFormatContext，AVFrame等）

FFmpeg 源代码简单分析：avio_open2()

FFmpeg 源代码简单分析：av_find_decoder()和av_find_encoder()

FFmpeg 源代码简单分析：avcodec_open2()

FFmpeg 源代码简单分析：avcodec_close()

【解码】

图解FFMPEG打开媒体的函数avformat_open_input

FFmpeg 源代码简单分析：avformat_open_input()

FFmpeg 源代码简单分析：avformat_find_stream_info()

FFmpeg 源代码简单分析：av_read_frame()

FFmpeg 源代码简单分析：avcodec_decode_video2()

FFmpeg 源代码简单分析：avformat_close_input()

【编码】

FFmpeg 源代码简单分析：avformat_alloc_output_context2()

FFmpeg 源代码简单分析：avformat_write_header()

FFmpeg 源代码简单分析：avcodec_encode_video()

FFmpeg 源代码简单分析：av_write_frame()

FFmpeg 源代码简单分析：av_write_trailer()

【其它】

FFmpeg源代码简单分析：日志输出系统（av_log()等）

FFmpeg源代码简单分析：结构体成员管理系统-AVClass

FFmpeg源代码简单分析：结构体成员管理系统-AVOption

FFmpeg源代码简单分析：libswscale的sws_getContext()

FFmpeg源代码简单分析：libswscale的sws_scale()

FFmpeg源代码简单分析：libavdevice的avdevice_register_all()

FFmpeg源代码简单分析：libavdevice的gdigrab

【脚本】

FFmpeg源代码简单分析：makefile

FFmpeg源代码简单分析：configure

【H.264】

FFmpeg的H.264解码器源代码简单分析：概述

=====================================================

本文简单分析FFmpeg的avcodec_open2()函数。该函数用于初始化一个视音频编解码器的AVCodecContext。avcodec_open2()的声明位于libavcodec\avcodec.h，如下所示。

/*** Initialize the AVCodecContext to use the given AVCodec. Prior to using this* function the context has to be allocated with avcodec_alloc_context3().** The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),* avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for* retrieving a codec.** @warning This function is not thread safe!** @code* avcodec_register_all();* av_dict_set(&opts, "b", "2.5M", 0);* codec = avcodec_find_decoder(AV_CODEC_ID_H264);* if (!codec)*     exit(1);** context = avcodec_alloc_context3(codec);** if (avcodec_open2(context, codec, opts) < 0)*     exit(1);* @endcode** @param avctx The context to initialize.* @param codec The codec to open this context for. If a non-NULL codec has been*              previously passed to avcodec_alloc_context3() or*              avcodec_get_context_defaults3() for this context, then this*              parameter MUST be either NULL or equal to the previously passed*              codec.* @param options A dictionary filled with AVCodecContext and codec-private options.*                On return this object will be filled with options that were not found.** @return zero on success, a negative value on error* @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(),*      av_dict_set(), av_opt_find().*/
int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

用中文简单转述一下avcodec_open2()各个参数的含义：

avctx：需要初始化的AVCodecContext。
codec：输入的AVCodec
options：一些选项。例如使用libx264编码的时候，“preset”，“tune”等都可以通过该参数设置。

该函数最典型的例子可以参考：

最简单的基于FFMPEG的视频编码器（YUV编码为H.264）

函数调用关系图

avcodec_open2()函数调用关系非常简单，如下图所示。

avcodec_open2()

avcodec_open2()的定义位于libavcodec\utils.c，如下所示。

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{int ret = 0;AVDictionary *tmp = NULL;//如果已经打开，直接返回if (avcodec_is_open(avctx))return 0;if ((!codec && !avctx->codec)) {av_log(avctx, AV_LOG_ERROR, "No codec provided to avcodec_open2()\n");return AVERROR(EINVAL);}if ((codec && avctx->codec && codec != avctx->codec)) {av_log(avctx, AV_LOG_ERROR, "This AVCodecContext was allocated for %s, ""but %s passed to avcodec_open2()\n", avctx->codec->name, codec->name);return AVERROR(EINVAL);}if (!codec)codec = avctx->codec;if (avctx->extradata_size < 0 || avctx->extradata_size >= FF_MAX_EXTRADATA_SIZE)return AVERROR(EINVAL);if (options)av_dict_copy(&tmp, *options, 0);ret = ff_lock_avcodec(avctx);if (ret < 0)return ret;//各种Mallocavctx->internal = av_mallocz(sizeof(AVCodecInternal));if (!avctx->internal) {ret = AVERROR(ENOMEM);goto end;}avctx->internal->pool = av_mallocz(sizeof(*avctx->internal->pool));if (!avctx->internal->pool) {ret = AVERROR(ENOMEM);goto free_and_end;}avctx->internal->to_free = av_frame_alloc();if (!avctx->internal->to_free) {ret = AVERROR(ENOMEM);goto free_and_end;}if (codec->priv_data_size > 0) {if (!avctx->priv_data) {avctx->priv_data = av_mallocz(codec->priv_data_size);if (!avctx->priv_data) {ret = AVERROR(ENOMEM);goto end;}if (codec->priv_class) {*(const AVClass **)avctx->priv_data = codec->priv_class;av_opt_set_defaults(avctx->priv_data);}}if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, &tmp)) < 0)goto free_and_end;} else {avctx->priv_data = NULL;}//将输入的AVDictionary形式的选项设置到AVCodecContextif ((ret = av_opt_set_dict(avctx, &tmp)) < 0)goto free_and_end;if (avctx->codec_whitelist && av_match_list(codec->name, avctx->codec_whitelist, ',') <= 0) {av_log(avctx, AV_LOG_ERROR, "Codec (%s) not on whitelist\n", codec->name);ret = AVERROR(EINVAL);goto free_and_end;}// only call ff_set_dimensions() for non H.264/VP6F codecs so as not to overwrite previously setup dimensionsif (!(avctx->coded_width && avctx->coded_height && avctx->width && avctx->height &&(avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_VP6F))) {if (avctx->coded_width && avctx->coded_height)ret = ff_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);else if (avctx->width && avctx->height)ret = ff_set_dimensions(avctx, avctx->width, avctx->height);if (ret < 0)goto free_and_end;}//检查宽和高if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)&& (  av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx) < 0|| av_image_check_size(avctx->width,       avctx->height,       0, avctx) < 0)) {av_log(avctx, AV_LOG_WARNING, "Ignoring invalid width/height values\n");ff_set_dimensions(avctx, 0, 0);}//检查宽高比if (avctx->width > 0 && avctx->height > 0) {if (av_image_check_sar(avctx->width, avctx->height,avctx->sample_aspect_ratio) < 0) {av_log(avctx, AV_LOG_WARNING, "ignoring invalid SAR: %u/%u\n",avctx->sample_aspect_ratio.num,avctx->sample_aspect_ratio.den);avctx->sample_aspect_ratio = (AVRational){ 0, 1 };}}/* if the decoder init function was already called previously,* free the already allocated subtitle_header before overwriting it */if (av_codec_is_decoder(codec))av_freep(&avctx->subtitle_header);if (avctx->channels > FF_SANE_NB_CHANNELS) {ret = AVERROR(EINVAL);goto free_and_end;}avctx->codec = codec;if ((avctx->codec_type == AVMEDIA_TYPE_UNKNOWN || avctx->codec_type == codec->type) &&avctx->codec_id == AV_CODEC_ID_NONE) {avctx->codec_type = codec->type;avctx->codec_id   = codec->id;}if (avctx->codec_id != codec->id || (avctx->codec_type != codec->type&& avctx->codec_type != AVMEDIA_TYPE_ATTACHMENT)) {av_log(avctx, AV_LOG_ERROR, "Codec type or id mismatches\n");ret = AVERROR(EINVAL);goto free_and_end;}avctx->frame_number = 0;avctx->codec_descriptor = avcodec_descriptor_get(avctx->codec_id);//检查编码器是否出于“实验”阶段if (avctx->codec->capabilities & CODEC_CAP_EXPERIMENTAL &&avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";AVCodec *codec2;av_log(avctx, AV_LOG_ERROR,"The %s '%s' is experimental but experimental codecs are not enabled, ""add '-strict %d' if you want to use it.\n",codec_string, codec->name, FF_COMPLIANCE_EXPERIMENTAL);codec2 = av_codec_is_encoder(codec) ? avcodec_find_encoder(codec->id) : avcodec_find_decoder(codec->id);if (!(codec2->capabilities & CODEC_CAP_EXPERIMENTAL))av_log(avctx, AV_LOG_ERROR, "Alternatively use the non experimental %s '%s'.\n",codec_string, codec2->name);ret = AVERROR_EXPERIMENTAL;goto free_and_end;}if (avctx->codec_type == AVMEDIA_TYPE_AUDIO &&(!avctx->time_base.num || !avctx->time_base.den)) {avctx->time_base.num = 1;avctx->time_base.den = avctx->sample_rate;}if (!HAVE_THREADS)av_log(avctx, AV_LOG_WARNING, "Warning: not compiled with thread support, using thread emulation\n");if (CONFIG_FRAME_THREAD_ENCODER) {ff_unlock_avcodec(); //we will instanciate a few encoders thus kick the counter to prevent false detection of a problemret = ff_frame_thread_encoder_init(avctx, options ? *options : NULL);ff_lock_avcodec(avctx);if (ret < 0)goto free_and_end;}if (HAVE_THREADS&& !(avctx->internal->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))) {ret = ff_thread_init(avctx);if (ret < 0) {goto free_and_end;}}if (!HAVE_THREADS && !(codec->capabilities & CODEC_CAP_AUTO_THREADS))avctx->thread_count = 1;if (avctx->codec->max_lowres < avctx->lowres || avctx->lowres < 0) {av_log(avctx, AV_LOG_ERROR, "The maximum value for lowres supported by the decoder is %d\n",avctx->codec->max_lowres);ret = AVERROR(EINVAL);goto free_and_end;}#if FF_API_VISMVif (avctx->debug_mv)av_log(avctx, AV_LOG_WARNING, "The 'vismv' option is deprecated, ""see the codecview filter instead.\n");
#endif//检查输入参数是否符合【编码器】要求if (av_codec_is_encoder(avctx->codec)) {int i;//如果包含采样率参数（表明是音频），检查采样率是否符合要求if (avctx->codec->sample_fmts) {//遍历编码器支持的所有采样率for (i = 0; avctx->codec->sample_fmts[i] != AV_SAMPLE_FMT_NONE; i++) {//如果设置的采样率==编码器支持的采样率，跳出循环。if (avctx->sample_fmt == avctx->codec->sample_fmts[i])break;if (avctx->channels == 1 &&av_get_planar_sample_fmt(avctx->sample_fmt) ==av_get_planar_sample_fmt(avctx->codec->sample_fmts[i])) {avctx->sample_fmt = avctx->codec->sample_fmts[i];break;}}//再检查一下采样率取值是否正确//注意，此时的i值没有变化if (avctx->codec->sample_fmts[i] == AV_SAMPLE_FMT_NONE) {char buf[128];snprintf(buf, sizeof(buf), "%d", avctx->sample_fmt);av_log(avctx, AV_LOG_ERROR, "Specified sample format %s is invalid or not supported\n",(char *)av_x_if_null(av_get_sample_fmt_name(avctx->sample_fmt), buf));ret = AVERROR(EINVAL);goto free_and_end;}}//检查像素格式if (avctx->codec->pix_fmts) {for (i = 0; avctx->codec->pix_fmts[i] != AV_PIX_FMT_NONE; i++)if (avctx->pix_fmt == avctx->codec->pix_fmts[i])break;if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_NONE&& !((avctx->codec_id == AV_CODEC_ID_MJPEG || avctx->codec_id == AV_CODEC_ID_LJPEG)&& avctx->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL)) {char buf[128];snprintf(buf, sizeof(buf), "%d", avctx->pix_fmt);av_log(avctx, AV_LOG_ERROR, "Specified pixel format %s is invalid or not supported\n",(char *)av_x_if_null(av_get_pix_fmt_name(avctx->pix_fmt), buf));ret = AVERROR(EINVAL);goto free_and_end;}if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ420P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ411P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ422P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ440P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ444P)avctx->color_range = AVCOL_RANGE_JPEG;}//检查采样率if (avctx->codec->supported_samplerates) {for (i = 0; avctx->codec->supported_samplerates[i] != 0; i++)if (avctx->sample_rate == avctx->codec->supported_samplerates[i])break;if (avctx->codec->supported_samplerates[i] == 0) {av_log(avctx, AV_LOG_ERROR, "Specified sample rate %d is not supported\n",avctx->sample_rate);ret = AVERROR(EINVAL);goto free_and_end;}}//检查声道布局if (avctx->codec->channel_layouts) {if (!avctx->channel_layout) {av_log(avctx, AV_LOG_WARNING, "Channel layout not specified\n");} else {for (i = 0; avctx->codec->channel_layouts[i] != 0; i++)if (avctx->channel_layout == avctx->codec->channel_layouts[i])break;if (avctx->codec->channel_layouts[i] == 0) {char buf[512];av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);av_log(avctx, AV_LOG_ERROR, "Specified channel layout '%s' is not supported\n", buf);ret = AVERROR(EINVAL);goto free_and_end;}}}//检查声道数if (avctx->channel_layout && avctx->channels) {int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);if (channels != avctx->channels) {char buf[512];av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);av_log(avctx, AV_LOG_ERROR,"Channel layout '%s' with %d channels does not match number of specified channels %d\n",buf, channels, avctx->channels);ret = AVERROR(EINVAL);goto free_and_end;}} else if (avctx->channel_layout) {avctx->channels = av_get_channel_layout_nb_channels(avctx->channel_layout);}//检查宽高if(avctx->codec_type == AVMEDIA_TYPE_VIDEO) {if (avctx->width <= 0 || avctx->height <= 0) {av_log(avctx, AV_LOG_ERROR, "dimensions not set\n");ret = AVERROR(EINVAL);goto free_and_end;}}//检查码率if (   (avctx->codec_type == AVMEDIA_TYPE_VIDEO || avctx->codec_type == AVMEDIA_TYPE_AUDIO)&& avctx->bit_rate>0 && avctx->bit_rate<1000) {av_log(avctx, AV_LOG_WARNING, "Bitrate %d is extremely low, maybe you mean %dk\n", avctx->bit_rate, avctx->bit_rate);}if (!avctx->rc_initial_buffer_occupancy)avctx->rc_initial_buffer_occupancy = avctx->rc_buffer_size * 3 / 4;}avctx->pts_correction_num_faulty_pts =avctx->pts_correction_num_faulty_dts = 0;avctx->pts_correction_last_pts =avctx->pts_correction_last_dts = INT64_MIN;//关键：//一切检查都无误之后，调用编解码器初始化函数if (   avctx->codec->init && (!(avctx->active_thread_type&FF_THREAD_FRAME)|| avctx->internal->frame_thread_encoder)) {ret = avctx->codec->init(avctx);if (ret < 0) {goto free_and_end;}}ret=0;#if FF_API_AUDIOENC_DELAYif (av_codec_is_encoder(avctx->codec))avctx->delay = avctx->initial_padding;
#endif//【解码器】//解码器的参数大部分都是由系统自动设定而不是由用户设定，因而不怎么需要检查if (av_codec_is_decoder(avctx->codec)) {if (!avctx->bit_rate)avctx->bit_rate = get_bit_rate(avctx);/* validate channel layout from the decoder */if (avctx->channel_layout) {int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);if (!avctx->channels)avctx->channels = channels;else if (channels != avctx->channels) {char buf[512];av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);av_log(avctx, AV_LOG_WARNING,"Channel layout '%s' with %d channels does not match specified number of channels %d: ""ignoring specified channel layout\n",buf, channels, avctx->channels);avctx->channel_layout = 0;}}if (avctx->channels && avctx->channels < 0 ||avctx->channels > FF_SANE_NB_CHANNELS) {ret = AVERROR(EINVAL);goto free_and_end;}if (avctx->sub_charenc) {if (avctx->codec_type != AVMEDIA_TYPE_SUBTITLE) {av_log(avctx, AV_LOG_ERROR, "Character encoding is only ""supported with subtitles codecs\n");ret = AVERROR(EINVAL);goto free_and_end;} else if (avctx->codec_descriptor->props & AV_CODEC_PROP_BITMAP_SUB) {av_log(avctx, AV_LOG_WARNING, "Codec '%s' is bitmap-based, ""subtitles character encoding will be ignored\n",avctx->codec_descriptor->name);avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_DO_NOTHING;} else {/* input character encoding is set for a text based subtitle* codec at this point */if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_AUTOMATIC)avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_PRE_DECODER;if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_PRE_DECODER) {
#if CONFIG_ICONViconv_t cd = iconv_open("UTF-8", avctx->sub_charenc);if (cd == (iconv_t)-1) {ret = AVERROR(errno);av_log(avctx, AV_LOG_ERROR, "Unable to open iconv context ""with input character encoding \"%s\"\n", avctx->sub_charenc);goto free_and_end;}iconv_close(cd);
#elseav_log(avctx, AV_LOG_ERROR, "Character encoding subtitles ""conversion needs a libavcodec built with iconv support ""for this codec\n");ret = AVERROR(ENOSYS);goto free_and_end;
#endif}}}#if FF_API_AVCTX_TIMEBASEif (avctx->framerate.num > 0 && avctx->framerate.den > 0)avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
#endif}
end:ff_unlock_avcodec();if (options) {av_dict_free(options);*options = tmp;}return ret;
free_and_end:av_dict_free(&tmp);if (codec->priv_class && codec->priv_data_size)av_opt_free(avctx->priv_data);av_freep(&avctx->priv_data);if (avctx->internal) {av_frame_free(&avctx->internal->to_free);av_freep(&avctx->internal->pool);}av_freep(&avctx->internal);avctx->codec = NULL;goto end;
}

avcodec_open2()的源代码量是非常长的，但是它的调用关系非常简单——它只调用了一个关键的函数，即AVCodec的init()，后文将会对这个函数进行分析。
我们可以简单梳理一下avcodec_open2()所做的工作，如下所列：

（1）为各种结构体分配内存（通过各种av_malloc()实现）。
（2）将输入的AVDictionary形式的选项设置到AVCodecContext。
（3）其他一些零零碎碎的检查，比如说检查编解码器是否处于“实验”阶段。
（4）如果是编码器，检查输入参数是否符合编码器的要求
（5）调用AVCodec的init()初始化具体的解码器。

前几步比较简单，不再分析。在这里我们分析一下第4步和第5步。

检查输入参数是否符合编码器要求

在这里简单分析一下第4步，即“检查输入参数是否符合编码器的要求”。这一步中检查了很多的参数，在这里我们随便选一个参数pix_fmts（像素格式）看一下，如下所示。

//检查像素格式if (avctx->codec->pix_fmts) {for (i = 0; avctx->codec->pix_fmts[i] != AV_PIX_FMT_NONE; i++)if (avctx->pix_fmt == avctx->codec->pix_fmts[i])break;if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_NONE&& !((avctx->codec_id == AV_CODEC_ID_MJPEG || avctx->codec_id == AV_CODEC_ID_LJPEG)&& avctx->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL)) {char buf[128];snprintf(buf, sizeof(buf), "%d", avctx->pix_fmt);av_log(avctx, AV_LOG_ERROR, "Specified pixel format %s is invalid or not supported\n",(char *)av_x_if_null(av_get_pix_fmt_name(avctx->pix_fmt), buf));ret = AVERROR(EINVAL);goto free_and_end;}if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ420P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ411P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ422P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ440P ||avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ444P)avctx->color_range = AVCOL_RANGE_JPEG;}

可以看出，该代码首先进入了一个for()循环，将AVCodecContext中设定的pix_fmt与编码器AVCodec中的pix_fmts数组中的元素逐一比较。
先简单介绍一下AVCodec中的pix_fmts数组。AVCodec中的pix_fmts数组存储了该种编码器支持的像素格式，并且规定以AV_PIX_FMT_NONE（AV_PIX_FMT_NONE取值为-1）为结尾。例如，libx264的pix_fmts数组的定义位于libavcodec\libx264.c，如下所示。

static const enum AVPixelFormat pix_fmts_8bit[] = {AV_PIX_FMT_YUV420P,AV_PIX_FMT_YUVJ420P,AV_PIX_FMT_YUV422P,AV_PIX_FMT_YUVJ422P,AV_PIX_FMT_YUV444P,AV_PIX_FMT_YUVJ444P,AV_PIX_FMT_NV12,AV_PIX_FMT_NV16,AV_PIX_FMT_NONE
};

从pix_fmts_8bit的定义可以看出libx264主要支持的是以YUV为主的像素格式。
现在回到“检查输入pix_fmt是否符合编码器的要求”的那段代码。如果for()循环从AVCodec->pix_fmts数组中找到了符合AVCodecContext->pix_fmt的像素格式，或者完成了AVCodec->pix_fmts数组的遍历，都会跳出循环。如果发现AVCodec->pix_fmts数组中索引为i的元素是AV_PIX_FMT_NONE（即最后一个元素，取值为-1）的时候，就认为没有找到合适的像素格式，并且最终提示错误信息。

AVCodec->init()

avcodec_open2()中最关键的一步就是调用AVCodec的init()方法初始化具体的编码器。AVCodec的init()是一个函数指针，指向具体编解码器中的初始化函数。这里我们以libx264为例，看一下它对应的AVCodec的定义。libx264对应的AVCodec的定义位于libavcodec\libx264.c，如下所示。

AVCodec ff_libx264_encoder = {.name             = "libx264",.long_name        = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),.type             = AVMEDIA_TYPE_VIDEO,.id               = AV_CODEC_ID_H264,.priv_data_size   = sizeof(X264Context),.init             = X264_init,.encode2          = X264_frame,.close            = X264_close,.capabilities     = CODEC_CAP_DELAY | CODEC_CAP_AUTO_THREADS,.priv_class       = &x264_class,.defaults         = x264_defaults,.init_static_data = X264_init_static,
};

可以看出在ff_libx264_encoder中init()指向X264_init()。X264_init()的定义同样位于libavcodec\libx264.c，如下所示。

static av_cold int X264_init(AVCodecContext *avctx)
{X264Context *x4 = avctx->priv_data;int sw,sh;if (avctx->global_quality > 0)av_log(avctx, AV_LOG_WARNING, "-qscale is ignored, -crf is recommended.\n");x264_param_default(&x4->params);x4->params.b_deblocking_filter         = avctx->flags & CODEC_FLAG_LOOP_FILTER;if (x4->preset || x4->tune)if (x264_param_default_preset(&x4->params, x4->preset, x4->tune) < 0) {int i;av_log(avctx, AV_LOG_ERROR, "Error setting preset/tune %s/%s.\n", x4->preset, x4->tune);av_log(avctx, AV_LOG_INFO, "Possible presets:");for (i = 0; x264_preset_names[i]; i++)av_log(avctx, AV_LOG_INFO, " %s", x264_preset_names[i]);av_log(avctx, AV_LOG_INFO, "\n");av_log(avctx, AV_LOG_INFO, "Possible tunes:");for (i = 0; x264_tune_names[i]; i++)av_log(avctx, AV_LOG_INFO, " %s", x264_tune_names[i]);av_log(avctx, AV_LOG_INFO, "\n");return AVERROR(EINVAL);}if (avctx->level > 0)x4->params.i_level_idc = avctx->level;x4->params.pf_log               = X264_log;x4->params.p_log_private        = avctx;x4->params.i_log_level          = X264_LOG_DEBUG;x4->params.i_csp                = convert_pix_fmt(avctx->pix_fmt);OPT_STR("weightp", x4->wpredp);if (avctx->bit_rate) {x4->params.rc.i_bitrate   = avctx->bit_rate / 1000;x4->params.rc.i_rc_method = X264_RC_ABR;}x4->params.rc.i_vbv_buffer_size = avctx->rc_buffer_size / 1000;x4->params.rc.i_vbv_max_bitrate = avctx->rc_max_rate    / 1000;x4->params.rc.b_stat_write      = avctx->flags & CODEC_FLAG_PASS1;if (avctx->flags & CODEC_FLAG_PASS2) {x4->params.rc.b_stat_read = 1;} else {if (x4->crf >= 0) {x4->params.rc.i_rc_method   = X264_RC_CRF;x4->params.rc.f_rf_constant = x4->crf;} else if (x4->cqp >= 0) {x4->params.rc.i_rc_method   = X264_RC_CQP;x4->params.rc.i_qp_constant = x4->cqp;}if (x4->crf_max >= 0)x4->params.rc.f_rf_constant_max = x4->crf_max;}if (avctx->rc_buffer_size && avctx->rc_initial_buffer_occupancy > 0 &&(avctx->rc_initial_buffer_occupancy <= avctx->rc_buffer_size)) {x4->params.rc.f_vbv_buffer_init =(float)avctx->rc_initial_buffer_occupancy / avctx->rc_buffer_size;}OPT_STR("level", x4->level);if (avctx->i_quant_factor > 0)x4->params.rc.f_ip_factor         = 1 / fabs(avctx->i_quant_factor);if (avctx->b_quant_factor > 0)x4->params.rc.f_pb_factor         = avctx->b_quant_factor;if (avctx->chromaoffset)x4->params.analyse.i_chroma_qp_offset = avctx->chromaoffset;if (avctx->me_method == ME_EPZS)x4->params.analyse.i_me_method = X264_ME_DIA;else if (avctx->me_method == ME_HEX)x4->params.analyse.i_me_method = X264_ME_HEX;else if (avctx->me_method == ME_UMH)x4->params.analyse.i_me_method = X264_ME_UMH;else if (avctx->me_method == ME_FULL)x4->params.analyse.i_me_method = X264_ME_ESA;else if (avctx->me_method == ME_TESA)x4->params.analyse.i_me_method = X264_ME_TESA;if (avctx->gop_size >= 0)x4->params.i_keyint_max         = avctx->gop_size;if (avctx->max_b_frames >= 0)x4->params.i_bframe             = avctx->max_b_frames;if (avctx->scenechange_threshold >= 0)x4->params.i_scenecut_threshold = avctx->scenechange_threshold;if (avctx->qmin >= 0)x4->params.rc.i_qp_min          = avctx->qmin;if (avctx->qmax >= 0)x4->params.rc.i_qp_max          = avctx->qmax;if (avctx->max_qdiff >= 0)x4->params.rc.i_qp_step         = avctx->max_qdiff;if (avctx->qblur >= 0)x4->params.rc.f_qblur           = avctx->qblur;     /* temporally blur quants */if (avctx->qcompress >= 0)x4->params.rc.f_qcompress       = avctx->qcompress; /* 0.0 => cbr, 1.0 => constant qp */if (avctx->refs >= 0)x4->params.i_frame_reference    = avctx->refs;else if (x4->level) {int i;int mbn = FF_CEIL_RSHIFT(avctx->width, 4) * FF_CEIL_RSHIFT(avctx->height, 4);int level_id = -1;char *tail;int scale = X264_BUILD < 129 ? 384 : 1;if (!strcmp(x4->level, "1b")) {level_id = 9;} else if (strlen(x4->level) <= 3){level_id = av_strtod(x4->level, &tail) * 10 + 0.5;if (*tail)level_id = -1;}if (level_id <= 0)av_log(avctx, AV_LOG_WARNING, "Failed to parse level\n");for (i = 0; i<x264_levels[i].level_idc; i++)if (x264_levels[i].level_idc == level_id)x4->params.i_frame_reference = av_clip(x264_levels[i].dpb / mbn / scale, 1, x4->params.i_frame_reference);}if (avctx->trellis >= 0)x4->params.analyse.i_trellis    = avctx->trellis;if (avctx->me_range >= 0)x4->params.analyse.i_me_range   = avctx->me_range;if (avctx->noise_reduction >= 0)x4->params.analyse.i_noise_reduction = avctx->noise_reduction;if (avctx->me_subpel_quality >= 0)x4->params.analyse.i_subpel_refine   = avctx->me_subpel_quality;if (avctx->b_frame_strategy >= 0)x4->params.i_bframe_adaptive = avctx->b_frame_strategy;if (avctx->keyint_min >= 0)x4->params.i_keyint_min = avctx->keyint_min;if (avctx->coder_type >= 0)x4->params.b_cabac = avctx->coder_type == FF_CODER_TYPE_AC;if (avctx->me_cmp >= 0)x4->params.analyse.b_chroma_me = avctx->me_cmp & FF_CMP_CHROMA;if (x4->aq_mode >= 0)x4->params.rc.i_aq_mode = x4->aq_mode;if (x4->aq_strength >= 0)x4->params.rc.f_aq_strength = x4->aq_strength;PARSE_X264_OPT("psy-rd", psy_rd);PARSE_X264_OPT("deblock", deblock);PARSE_X264_OPT("partitions", partitions);PARSE_X264_OPT("stats", stats);if (x4->psy >= 0)x4->params.analyse.b_psy  = x4->psy;if (x4->rc_lookahead >= 0)x4->params.rc.i_lookahead = x4->rc_lookahead;if (x4->weightp >= 0)x4->params.analyse.i_weighted_pred = x4->weightp;if (x4->weightb >= 0)x4->params.analyse.b_weighted_bipred = x4->weightb;if (x4->cplxblur >= 0)x4->params.rc.f_complexity_blur = x4->cplxblur;if (x4->ssim >= 0)x4->params.analyse.b_ssim = x4->ssim;if (x4->intra_refresh >= 0)x4->params.b_intra_refresh = x4->intra_refresh;if (x4->bluray_compat >= 0) {x4->params.b_bluray_compat = x4->bluray_compat;x4->params.b_vfr_input = 0;}if (x4->avcintra_class >= 0)
#if X264_BUILD >= 142x4->params.i_avcintra_class = x4->avcintra_class;
#elseav_log(avctx, AV_LOG_ERROR,"x264 too old for AVC Intra, at least version 142 needed\n");
#endifif (x4->b_bias != INT_MIN)x4->params.i_bframe_bias              = x4->b_bias;if (x4->b_pyramid >= 0)x4->params.i_bframe_pyramid = x4->b_pyramid;if (x4->mixed_refs >= 0)x4->params.analyse.b_mixed_references = x4->mixed_refs;if (x4->dct8x8 >= 0)x4->params.analyse.b_transform_8x8    = x4->dct8x8;if (x4->fast_pskip >= 0)x4->params.analyse.b_fast_pskip       = x4->fast_pskip;if (x4->aud >= 0)x4->params.b_aud                      = x4->aud;if (x4->mbtree >= 0)x4->params.rc.b_mb_tree               = x4->mbtree;if (x4->direct_pred >= 0)x4->params.analyse.i_direct_mv_pred   = x4->direct_pred;if (x4->slice_max_size >= 0)x4->params.i_slice_max_size =  x4->slice_max_size;else {/** Allow x264 to be instructed through AVCodecContext about the maximum* size of the RTP payload. For example, this enables the production of* payload suitable for the H.264 RTP packetization-mode 0 i.e. single* NAL unit per RTP packet.*/if (avctx->rtp_payload_size)x4->params.i_slice_max_size = avctx->rtp_payload_size;}if (x4->fastfirstpass)x264_param_apply_fastfirstpass(&x4->params);/* Allow specifying the x264 profile through AVCodecContext. */if (!x4->profile)switch (avctx->profile) {case FF_PROFILE_H264_BASELINE:x4->profile = av_strdup("baseline");break;case FF_PROFILE_H264_HIGH:x4->profile = av_strdup("high");break;case FF_PROFILE_H264_HIGH_10:x4->profile = av_strdup("high10");break;case FF_PROFILE_H264_HIGH_422:x4->profile = av_strdup("high422");break;case FF_PROFILE_H264_HIGH_444:x4->profile = av_strdup("high444");break;case FF_PROFILE_H264_MAIN:x4->profile = av_strdup("main");break;default:break;}if (x4->nal_hrd >= 0)x4->params.i_nal_hrd = x4->nal_hrd;if (x4->profile)if (x264_param_apply_profile(&x4->params, x4->profile) < 0) {int i;av_log(avctx, AV_LOG_ERROR, "Error setting profile %s.\n", x4->profile);av_log(avctx, AV_LOG_INFO, "Possible profiles:");for (i = 0; x264_profile_names[i]; i++)av_log(avctx, AV_LOG_INFO, " %s", x264_profile_names[i]);av_log(avctx, AV_LOG_INFO, "\n");return AVERROR(EINVAL);}x4->params.i_width          = avctx->width;x4->params.i_height         = avctx->height;av_reduce(&sw, &sh, avctx->sample_aspect_ratio.num, avctx->sample_aspect_ratio.den, 4096);x4->params.vui.i_sar_width  = sw;x4->params.vui.i_sar_height = sh;x4->params.i_timebase_den = avctx->time_base.den;x4->params.i_timebase_num = avctx->time_base.num;x4->params.i_fps_num = avctx->time_base.den;x4->params.i_fps_den = avctx->time_base.num * avctx->ticks_per_frame;x4->params.analyse.b_psnr = avctx->flags & CODEC_FLAG_PSNR;x4->params.i_threads      = avctx->thread_count;if (avctx->thread_type)x4->params.b_sliced_threads = avctx->thread_type == FF_THREAD_SLICE;x4->params.b_interlaced   = avctx->flags & CODEC_FLAG_INTERLACED_DCT;x4->params.b_open_gop     = !(avctx->flags & CODEC_FLAG_CLOSED_GOP);x4->params.i_slice_count  = avctx->slices;x4->params.vui.b_fullrange = avctx->pix_fmt == AV_PIX_FMT_YUVJ420P ||avctx->pix_fmt == AV_PIX_FMT_YUVJ422P ||avctx->pix_fmt == AV_PIX_FMT_YUVJ444P ||avctx->color_range == AVCOL_RANGE_JPEG;if (avctx->colorspace != AVCOL_SPC_UNSPECIFIED)x4->params.vui.i_colmatrix = avctx->colorspace;if (avctx->color_primaries != AVCOL_PRI_UNSPECIFIED)x4->params.vui.i_colorprim = avctx->color_primaries;if (avctx->color_trc != AVCOL_TRC_UNSPECIFIED)x4->params.vui.i_transfer  = avctx->color_trc;if (avctx->flags & CODEC_FLAG_GLOBAL_HEADER)x4->params.b_repeat_headers = 0;if(x4->x264opts){const char *p= x4->x264opts;while(p){char param[256]={0}, val[256]={0};if(sscanf(p, "%255[^:=]=%255[^:]", param, val) == 1){OPT_STR(param, "1");}elseOPT_STR(param, val);p= strchr(p, ':');p+=!!p;}}if (x4->x264_params) {AVDictionary *dict    = NULL;AVDictionaryEntry *en = NULL;if (!av_dict_parse_string(&dict, x4->x264_params, "=", ":", 0)) {while ((en = av_dict_get(dict, "", en, AV_DICT_IGNORE_SUFFIX))) {if (x264_param_parse(&x4->params, en->key, en->value) < 0)av_log(avctx, AV_LOG_WARNING,"Error parsing option '%s = %s'.\n",en->key, en->value);}av_dict_free(&dict);}}// update AVCodecContext with x264 parametersavctx->has_b_frames = x4->params.i_bframe ?x4->params.i_bframe_pyramid ? 2 : 1 : 0;if (avctx->max_b_frames < 0)avctx->max_b_frames = 0;avctx->bit_rate = x4->params.rc.i_bitrate*1000;x4->enc = x264_encoder_open(&x4->params);if (!x4->enc)return -1;avctx->coded_frame = av_frame_alloc();if (!avctx->coded_frame)return AVERROR(ENOMEM);if (avctx->flags & CODEC_FLAG_GLOBAL_HEADER) {x264_nal_t *nal;uint8_t *p;int nnal, s, i;s = x264_encoder_headers(x4->enc, &nal, &nnal);avctx->extradata = p = av_malloc(s);for (i = 0; i < nnal; i++) {/* Don't put the SEI in extradata. */if (nal[i].i_type == NAL_SEI) {av_log(avctx, AV_LOG_INFO, "%s\n", nal[i].p_payload+25);x4->sei_size = nal[i].i_payload;x4->sei      = av_malloc(x4->sei_size);memcpy(x4->sei, nal[i].p_payload, nal[i].i_payload);continue;}memcpy(p, nal[i].p_payload, nal[i].i_payload);p += nal[i].i_payload;}avctx->extradata_size = p - avctx->extradata;}return 0;
}

X264_init()的代码以后研究X264的时候再进行细节的分析，在这里简单记录一下它做的两项工作：

（1）设置X264Context的参数。X264Context主要完成了libx264和FFmpeg对接的功能。可以看出代码主要在设置一个params结构体变量，该变量的类型即是x264中存储参数的结构体x264_param_t。
（2）调用libx264的API进行编码器的初始化工作。例如调用x264_param_default()设置默认参数，调用x264_param_apply_profile()设置profile，调用x264_encoder_open()打开编码器等等。

最后附上X264Context的定义，位于libavcodec\libx264.c，如下所示。

typedef struct X264Context {AVClass        *class;x264_param_t    params;x264_t         *enc;x264_picture_t  pic;uint8_t        *sei;int             sei_size;char *preset;char *tune;char *profile;char *level;int fastfirstpass;char *wpredp;char *x264opts;float crf;float crf_max;int cqp;int aq_mode;float aq_strength;char *psy_rd;int psy;int rc_lookahead;int weightp;int weightb;int ssim;int intra_refresh;int bluray_compat;int b_bias;int b_pyramid;int mixed_refs;int dct8x8;int fast_pskip;int aud;int mbtree;char *deblock;float cplxblur;char *partitions;int direct_pred;int slice_max_size;char *stats;int nal_hrd;int avcintra_class;char *x264_params;
} X264Context;

雷霄骅
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020