avcodec_open2 用于初始化的音视频编解码器AVCodecContext，声明位于 libavcodec\utils.c 中，函数原型如下：
int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)- avctx：需要初始化的 AVCodecContext
- codec：输入的AVCodec
- options：包含AVCodecContext和编解码器专用的字典
我们就具体看下这个函数的实现，代码很长，我就直接把贴出来，代码里加上自己理解的注释
int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{
int ret = 0;
int codec_init_ok = 0;
AVDictionary *tmp = NULL;
const AVPixFmtDescriptor *pixdesc;
AVCodecInternal *avci;
//判断编解码器是否打开,!!(非零)=1 ，!!(零)=0
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);
ff_lock_avcodec(avctx, codec);
avci = av_mallocz(sizeof(*avci));
if (!avci) {
ret = AVERROR(ENOMEM);
goto end;
}
avctx->internal = avci;
avci->to_free = av_frame_alloc();
avci->compat_decode_frame = av_frame_alloc();
avci->compat_encode_packet = av_packet_alloc();
avci->buffer_frame = av_frame_alloc();
avci->buffer_pkt = av_packet_alloc();
avci->es.in_frame = av_frame_alloc();
avci->ds.in_pkt = av_packet_alloc();
avci->last_pkt_props = av_packet_alloc();
if (!avci->compat_decode_frame || !avci->compat_encode_packet ||
!avci->buffer_frame || !avci->buffer_pkt ||
!avci->es.in_frame || !avci->ds.in_pkt ||
!avci->to_free || !avci->last_pkt_props) {
ret = AVERROR(ENOMEM);
goto free_and_end;
}
avci->skip_samples_multiplier = 1;
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 free_and_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AVCodecContext
if ((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 \'%s\'\n", codec->name, avctx->codec_whitelist);
ret = AVERROR(EINVAL);
goto free_and_end;
}
// only call ff_set_dimensions() for non H.264/VP6F/DXV codecs so as not to overwrite previously setup dimensions
if (!(avctx->coded_width && avctx->coded_height && avctx->width && avctx->height &&
(avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_VP6F || avctx->codec_id == AV_CODEC_ID_DXV))) {
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_size2(avctx->coded_width, avctx->coded_height, avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx) < 0
|| av_image_check_size2(avctx->width, avctx->height, avctx->max_pixels, AV_PIX_FMT_NONE, 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 || avctx->channels < 0) {
av_log(avctx, AV_LOG_ERROR, "Too many or invalid channels: %d\n", avctx->channels);
ret = AVERROR(EINVAL);
goto free_and_end;
}
if (avctx->sample_rate < 0) {
av_log(avctx, AV_LOG_ERROR, "Invalid sample rate: %d\n", avctx->sample_rate);
ret = AVERROR(EINVAL);
goto free_and_end;
}
if (avctx->block_align < 0) {
av_log(avctx, AV_LOG_ERROR, "Invalid block align: %d\n", avctx->block_align);
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);
if ((avctx->codec->capabilities & AV_CODEC_CAP_EXPERIMENTAL) &&
avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";
const 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 & AV_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 && av_codec_is_encoder(avctx->codec)) {
ff_unlock_avcodec(codec); //we will instantiate a few encoders thus kick the counter to prevent false detection of a problem
ret = ff_frame_thread_encoder_init(avctx, options ? *options : NULL);
ff_lock_avcodec(avctx, codec);
if (ret < 0)
goto free_and_end;
}
if (av_codec_is_decoder(avctx->codec)) {
ret = ff_decode_bsfs_init(avctx);
if (ret < 0)
goto free_and_end;
}
if (HAVE_THREADS
&& !(avci->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 & AV_CODEC_CAP_AUTO_THREADS))
avctx->thread_count = 1;
if (avctx->codec->max_lowres < avctx->lowres || avctx->lowres < 0) {
av_log(avctx, AV_LOG_WARNING, "The maximum value for lowres supported by the decoder is %d\n",
avctx->codec->max_lowres);
avctx->lowres = avctx->codec->max_lowres;
}
//如果是编码器检查输入参数
if (av_codec_is_encoder(avctx->codec)) {
int i;
#if FF_API_CODED_FRAME
FF_DISABLE_DEPRECATION_WARNINGS
avctx->coded_frame = av_frame_alloc();
if (!avctx->coded_frame) {
ret = AVERROR(ENOMEM);
goto free_and_end;
}
FF_ENABLE_DEPRECATION_WARNINGS
#endif
//对于视频，检查帧的时间戳信息
if (avctx->time_base.num <= 0 || avctx->time_base.den <= 0) {
av_log(avctx, AV_LOG_ERROR, "The encoder timebase is not set.\n");
ret = AVERROR(EINVAL);
goto free_and_end;
}
//对于音频，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++) {
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;
}
}
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;
}
}
if (avctx->sample_rate < 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);
}
if (avctx->channels < 0) {
av_log(avctx, AV_LOG_ERROR, "Specified number of channels %d is not supported\n",
avctx->channels);
ret = AVERROR(EINVAL);
goto free_and_end;
}
if(avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
pixdesc = av_pix_fmt_desc_get(avctx->pix_fmt);
//检查视频数据比特数
if ( avctx->bits_per_raw_sample < 0
|| (avctx->bits_per_raw_sample > 8 && pixdesc->comp[0].depth <= 8)) {
av_log(avctx, AV_LOG_WARNING, "Specified bit depth %d not possible with the specified pixel formats depth %d\n",
avctx->bits_per_raw_sample, pixdesc->comp[0].depth);
avctx->bits_per_raw_sample = pixdesc->comp[0].depth;
}
//检查宽高
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 %"PRId64" is extremely low, maybe you mean %"PRId64"k\n", avctx->bit_rate, avctx->bit_rate);
}
if (!avctx->rc_initial_buffer_occupancy)
avctx->rc_initial_buffer_occupancy = avctx->rc_buffer_size * 3LL / 4;
if (avctx->ticks_per_frame && avctx->time_base.num &&
avctx->ticks_per_frame > INT_MAX / avctx->time_base.num) {
av_log(avctx, AV_LOG_ERROR,
"ticks_per_frame %d too large for the timebase %d/%d.",
avctx->ticks_per_frame,
avctx->time_base.num,
avctx->time_base.den);
goto free_and_end;
}
if (avctx->hw_frames_ctx) {
AVHWFramesContext *frames_ctx = (AVHWFramesContext*)avctx->hw_frames_ctx->data;
if (frames_ctx->format != avctx->pix_fmt) {
av_log(avctx, AV_LOG_ERROR,
"Mismatching AVCodecContext.pix_fmt and AVHWFramesContext.format\n");
ret = AVERROR(EINVAL);
goto free_and_end;
}
if (avctx->sw_pix_fmt != AV_PIX_FMT_NONE &&
avctx->sw_pix_fmt != frames_ctx->sw_format) {
av_log(avctx, AV_LOG_ERROR,
"Mismatching AVCodecContext.sw_pix_fmt (%s) "
"and AVHWFramesContext.sw_format (%s)\n",
av_get_pix_fmt_name(avctx->sw_pix_fmt),
av_get_pix_fmt_name(frames_ctx->sw_format));
ret = AVERROR(EINVAL);
goto free_and_end;
}
avctx->sw_pix_fmt = frames_ctx->sw_format;
}
}
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;
if ( !CONFIG_GRAY && avctx->flags & AV_CODEC_FLAG_GRAY
&& avctx->codec_descriptor->type == AVMEDIA_TYPE_VIDEO)
av_log(avctx, AV_LOG_WARNING,
"gray decoding requested but not enabled at configuration time\n");
if (avctx->flags2 & AV_CODEC_FLAG2_EXPORT_MVS) {
avctx->export_side_data |= AV_CODEC_EXPORT_DATA_MVS;
}
//所有检查无误后，调用编解码器初始化函数，根据AVCodecContext中的设置来初始化编码器
if ( avctx->codec->init && (!(avctx->active_thread_type&FF_THREAD_FRAME)
|| avci->frame_thread_encoder)) {
ret = avctx->codec->init(avctx);
if (ret < 0) {
codec_init_ok = -1;
goto free_and_end;
}
codec_init_ok = 1;
}
ret=0;
//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->bits_per_coded_sample < 0) {
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_ICONV
iconv_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);
#else
av_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_TIMEBASE
if (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
}
if (codec->priv_data_size > 0 && avctx->priv_data && codec->priv_class) {
av_assert0(*(const AVClass **)avctx->priv_data == codec->priv_class);
}
end:
ff_unlock_avcodec(codec);
if (options) {
av_dict_free(options);
*options = tmp;
}
return ret;
free_and_end:
if (avctx->codec && avctx->codec->close &&
(codec_init_ok > 0 || (codec_init_ok < 0 &&
avctx->codec->caps_internal & FF_CODEC_CAP_INIT_CLEANUP)))
avctx->codec->close(avctx);
if (HAVE_THREADS && avci->thread_ctx)
ff_thread_free(avctx);
if (codec->priv_class && avctx->priv_data)
av_opt_free(avctx->priv_data);
av_opt_free(avctx);
if (av_codec_is_encoder(avctx->codec)) {
#if FF_API_CODED_FRAME
FF_DISABLE_DEPRECATION_WARNINGS
av_frame_free(&avctx->coded_frame);
FF_ENABLE_DEPRECATION_WARNINGS
#endif
av_freep(&avctx->extradata);
avctx->extradata_size = 0;
}
av_dict_free(&tmp);
av_freep(&avctx->priv_data);
av_freep(&avctx->subtitle_header);
av_frame_free(&avci->to_free);
av_frame_free(&avci->compat_decode_frame);
av_frame_free(&avci->buffer_frame);
av_packet_free(&avci->compat_encode_packet);
av_packet_free(&avci->buffer_pkt);
av_packet_free(&avci->last_pkt_props);
av_packet_free(&avci->ds.in_pkt);
av_frame_free(&avci->es.in_frame);
av_bsf_free(&avci->bsf);
av_buffer_unref(&avci->pool);
av_freep(&avci);
avctx->internal = NULL;
avctx->codec = NULL;
goto end;
}
总结起来就是做了：
- 为各种结构体分配内存
- 将输入的AVDictionary形式的选项设置到AVCodecContext
- 针对是否是音频，是否为编&解码器，做一些参数的检查
- 调用AVCodec中的init来初始化具体的编解码器