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FFmpeg的库函数源代码分析文章列表：
【架构图】
【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：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】
【H.264】
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本文简单分析FFmpeg的avcodec_encode_video2()函数。该函数用于编码一帧视频数据。avcodec_encode_video2()函数的声明位于libavcodec\avcodec.h，如下所示。
/**
* Encode a frame of video.
*
* Takes input raw video data from frame and writes the next output packet, if
* available, to avpkt. The output packet does not necessarily contain data for
* the most recent frame, as encoders can delay and reorder input frames
* internally as needed.
*
* @param avctx codec context
* @param avpkt output AVPacket.
* The user can supply an output buffer by setting
* avpkt->data and avpkt->size prior to calling the
* function, but if the size of the user-provided data is not
* large enough, encoding will fail. All other AVPacket fields
* will be reset by the encoder using av_init_packet(). If
* avpkt->data is NULL, the encoder will allocate it.
* The encoder will set avpkt->size to the size of the
* output packet. The returned data (if any) belongs to the
* caller, he is responsible for freeing it.
*
* If this function fails or produces no output, avpkt will be
* freed using av_free_packet() (i.e. avpkt->destruct will be
* called to free the user supplied buffer).
* @param[in] frame AVFrame containing the raw video data to be encoded.
* May be NULL when flushing an encoder that has the
* CODEC_CAP_DELAY capability set.
* @param[out] got_packet_ptr This field is set to 1 by libavcodec if the
* output packet is non-empty, and to 0 if it is
* empty. If the function returns an error, the
* packet can be assumed to be invalid, and the
* value of got_packet_ptr is undefined and should
* not be used.
* @return 0 on success, negative error code on failure
*/
int avcodec_encode_video2(AVCodecContext *avctx, AVPacket *avpkt,
const AVFrame *frame, int *got_packet_ptr);
该函数每个参数的含义在注释里面已经写的很清楚了，在这里用中文简述一下：
avctx：编码器的AVCodecContext。
avpkt：编码输出的AVPacket。
frame：编码输入的AVFrame。
got_packet_ptr：成功编码一个AVPacket的时候设置为1。
函数返回0代表编码成功。
函数调用关系图
函数的调用关系如下图所示。
avcodec_encode_video2()
avcodec_encode_video2()的定义位于libavcodec\utils.c，如下所示。
int attribute_align_arg avcodec_encode_video2(AVCodecContext *avctx,
AVPacket *avpkt,
const AVFrame *frame,
int *got_packet_ptr)
{
int ret;
AVPacket user_pkt = *avpkt;
int needs_realloc = !user_pkt.data;
*got_packet_ptr = 0;
if(CONFIG_FRAME_THREAD_ENCODER &&
avctx->internal->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))
return ff_thread_video_encode_frame(avctx, avpkt, frame, got_packet_ptr);
if ((avctx->flags&CODEC_FLAG_PASS1) && avctx->stats_out)
avctx->stats_out[0] = '\0';
if (!(avctx->codec->capabilities & CODEC_CAP_DELAY) && !frame) {
av_free_packet(avpkt);
av_init_packet(avpkt);
avpkt->size = 0;
return 0;
}
//检查输入
if (av_image_check_size(avctx->width, avctx->height, 0, avctx))
return AVERROR(EINVAL);
av_assert0(avctx->codec->encode2);
//编码
ret = avctx->codec->encode2(avctx, avpkt, frame, got_packet_ptr);
av_assert0(ret <= 0);
if (avpkt->data && avpkt->data == avctx->internal->byte_buffer) {
needs_realloc = 0;
if (user_pkt.data) {
if (user_pkt.size >= avpkt->size) {
memcpy(user_pkt.data, avpkt->data, avpkt->size);
} else {
av_log(avctx, AV_LOG_ERROR, "Provided packet is too small, needs to be %d\n", avpkt->size);
avpkt->size = user_pkt.size;
ret = -1;
}
avpkt->buf = user_pkt.buf;
avpkt->data = user_pkt.data;
#if FF_API_DESTRUCT_PACKET
FF_DISABLE_DEPRECATION_WARNINGS
avpkt->destruct = user_pkt.destruct;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
} else {
if (av_dup_packet(avpkt) < 0) {
ret = AVERROR(ENOMEM);
}
}
}
if (!ret) {
if (!*got_packet_ptr)
avpkt->size = 0;
else if (!(avctx->codec->capabilities & CODEC_CAP_DELAY))
avpkt->pts = avpkt->dts = frame->pts;
if (needs_realloc && avpkt->data) {
ret = av_buffer_realloc(&avpkt->buf, avpkt->size + FF_INPUT_BUFFER_PADDING_SIZE);
if (ret >= 0)
avpkt->data = avpkt->buf->data;
}
avctx->frame_number++;
}
if (ret < 0 || !*got_packet_ptr)
av_free_packet(avpkt);
else
av_packet_merge_side_data(avpkt);
emms_c();
return ret;
}
从函数的定义可以看出，avcodec_encode_video2()首先调用了av_image_check_size()检查设置的宽高参数是否合理，然后调用了AVCodec的encode2()调用具体的解码器。
av_image_check_size()
av_image_check_size()是一个很简单的函数，用于检查图像宽高是否正常，它的定义如下所示。
int av_image_check_size(unsigned int w, unsigned int h, int log_offset, void *log_ctx)
{
ImgUtils imgutils = { &imgutils_class, log_offset, log_ctx };
if ((int)w>0 && (int)h>0 && (w+128)*(uint64_t)(h+128) < INT_MAX/8)
return 0;
av_log(&imgutils, AV_LOG_ERROR, "Picture size %ux%u is invalid\n", w, h);
return AVERROR(EINVAL);
}
从代码中可以看出，av_image_check_size()主要是要求图像宽高必须为正数，而且取值不能太大。
AVCodec->encode2()
AVCodec的encode2()是一个函数指针，指向特定编码器的编码函数。在这里我们以libx264为例，看一下它对应的AVCodec的结构体的定义，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,
};
从ff_libx264_encoder的定义可以看出，encode2()函数指向的是X264_frame()函数。
X264_frame()
X264_frame()函数的定义位于libavcodec\libx264.c，如下所示。
static int X264_frame(AVCodecContext *ctx, AVPacket *pkt, const AVFrame *frame,
int *got_packet)
{
X264Context *x4 = ctx->priv_data;
x264_nal_t *nal;
int nnal, i, ret;
x264_picture_t pic_out = {0};
AVFrameSideData *side_data;
x264_picture_init( &x4->pic );
x4->pic.img.i_csp = x4->params.i_csp;
if (x264_bit_depth > 8)
x4->pic.img.i_csp |= X264_CSP_HIGH_DEPTH;
x4->pic.img.i_plane = avfmt2_num_planes(ctx->pix_fmt);
if (frame) {
for (i = 0; i < x4->pic.img.i_plane; i++) {
x4->pic.img.plane[i] = frame->data[i];
x4->pic.img.i_stride[i] = frame->linesize[i];
}
x4->pic.i_pts = frame->pts;
x4->pic.i_type =
frame->pict_type == AV_PICTURE_TYPE_I ? X264_TYPE_KEYFRAME :
frame->pict_type == AV_PICTURE_TYPE_P ? X264_TYPE_P :
frame->pict_type == AV_PICTURE_TYPE_B ? X264_TYPE_B :
X264_TYPE_AUTO;
if (x4->avcintra_class < 0) {
if (x4->params.b_interlaced && x4->params.b_tff != frame->top_field_first) {
x4->params.b_tff = frame->top_field_first;
x264_encoder_reconfig(x4->enc, &x4->params);
}
if (x4->params.vui.i_sar_height != ctx->sample_aspect_ratio.den ||
x4->params.vui.i_sar_width != ctx->sample_aspect_ratio.num) {
x4->params.vui.i_sar_height = ctx->sample_aspect_ratio.den;
x4->params.vui.i_sar_width = ctx->sample_aspect_ratio.num;
x264_encoder_reconfig(x4->enc, &x4->params);
}
if (x4->params.rc.i_vbv_buffer_size != ctx->rc_buffer_size / 1000 ||
x4->params.rc.i_vbv_max_bitrate != ctx->rc_max_rate / 1000) {
x4->params.rc.i_vbv_buffer_size = ctx->rc_buffer_size / 1000;
x4->params.rc.i_vbv_max_bitrate = ctx->rc_max_rate / 1000;
x264_encoder_reconfig(x4->enc, &x4->params);
}
if (x4->params.rc.i_rc_method == X264_RC_ABR &&
x4->params.rc.i_bitrate != ctx->bit_rate / 1000) {
x4->params.rc.i_bitrate = ctx->bit_rate / 1000;
x264_encoder_reconfig(x4->enc, &x4->params);
}
if (x4->crf >= 0 &&
x4->params.rc.i_rc_method == X264_RC_CRF &&
x4->params.rc.f_rf_constant != x4->crf) {
x4->params.rc.f_rf_constant = x4->crf;
x264_encoder_reconfig(x4->enc, &x4->params);
}
if (x4->params.rc.i_rc_method == X264_RC_CQP &&
x4->cqp >= 0 &&
x4->params.rc.i_qp_constant != x4->cqp) {
x4->params.rc.i_qp_constant = x4->cqp;
x264_encoder_reconfig(x4->enc, &x4->params);
}
if (x4->crf_max >= 0 &&
x4->params.rc.f_rf_constant_max != x4->crf_max) {
x4->params.rc.f_rf_constant_max = x4->crf_max;
x264_encoder_reconfig(x4->enc, &x4->params);
}
}
side_data = av_frame_get_side_data(frame, AV_FRAME_DATA_STEREO3D);
if (side_data) {
AVStereo3D *stereo = (AVStereo3D *)side_data->data;
int fpa_type;
switch (stereo->type) {
case AV_STEREO3D_CHECKERBOARD:
fpa_type = 0;
break;
case AV_STEREO3D_COLUMNS:
fpa_type = 1;
break;
case AV_STEREO3D_LINES:
fpa_type = 2;
break;
case AV_STEREO3D_SIDEBYSIDE:
fpa_type = 3;
break;
case AV_STEREO3D_TOPBOTTOM:
fpa_type = 4;
break;
case AV_STEREO3D_FRAMESEQUENCE:
fpa_type = 5;
break;
default:
fpa_type = -1;
break;
}
if (fpa_type != x4->params.i_frame_packing) {
x4->params.i_frame_packing = fpa_type;
x264_encoder_reconfig(x4->enc, &x4->params);
}
}
}
do {
if (x264_encoder_encode(x4->enc, &nal, &nnal, frame? &x4->pic: NULL, &pic_out) < 0)
return -1;
ret = encode_nals(ctx, pkt, nal, nnal);
if (ret < 0)
return -1;
} while (!ret && !frame && x264_encoder_delayed_frames(x4->enc));
pkt->pts = pic_out.i_pts;
pkt->dts = pic_out.i_dts;
switch (pic_out.i_type) {
case X264_TYPE_IDR:
case X264_TYPE_I:
ctx->coded_frame->pict_type = AV_PICTURE_TYPE_I;
break;
case X264_TYPE_P:
ctx->coded_frame->pict_type = AV_PICTURE_TYPE_P;
break;
case X264_TYPE_B:
case X264_TYPE_BREF:
ctx->coded_frame->pict_type = AV_PICTURE_TYPE_B;
break;
}
pkt->flags |= AV_PKT_FLAG_KEY*pic_out.b_keyframe;
if (ret)
ctx->coded_frame->quality = (pic_out.i_qpplus1 - 1) * FF_QP2LAMBDA;
*got_packet = ret;
return 0;
}
有关X264编码的代码在以后分析X264的时候再进行详细分析。在这里我们可以我们可以简单看出该函数中有一个do while循环，其中调用了x264_encoder_encode()完成了编码的工作。
雷霄骅
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020