一、函數功能
向濾鏡源中添加一個frame，源碼摘錄如下：
/**
* Add a frame to the buffer source.
*
* @param ctx an instance of the buffersrc filter
* @param frame frame to be added. If the frame is reference counted, this
* function will take ownership of the reference(s) and reset the frame.
* Otherwise the frame data will be copied. If this function returns an error,
* the input frame is not touched.
*
* @return 0 on success, a negative AVERROR on error.
*
* @note the difference between this function and av_buffersrc_write_frame() is
* that av_buffersrc_write_frame() creates a new reference to the input frame,
* while this function takes ownership of the reference passed to it.
*
* This function is equivalent to av_buffersrc_add_frame_flags() without the
* AV_BUFFERSRC_FLAG_KEEP_REF flag.
*/
av_warn_unused_result
int av_buffersrc_add_frame(AVFilterContext *ctx, AVFrame *frame);
/**
* Add a frame to the buffer source.
*
* By default, if the frame is reference-counted, this function will take
* ownership of the reference(s) and reset the frame. This can be controlled
* using the flags.
*
* If this function returns an error, the input frame is not touched.
*
* @param buffer_src pointer to a buffer source context
* @param frame a frame, or NULL to mark EOF
* @param flags a combination of AV_BUFFERSRC_FLAG_*
* @return >= 0 in case of success, a negative AVERROR code
* in case of failure
*/
av_warn_unused_result
int av_buffersrc_add_frame_flags(AVFilterContext *buffer_src,
AVFrame *frame, int flags);同時提供的還有個相似的接口av_buffersrc_add_frame_flags,可以指定調用的接口的方式，第三個參數（flags）的類型有下面幾種：
enum {
/**
* Do not check for format changes.
*/
AV_BUFFERSRC_FLAG_NO_CHECK_FORMAT = 1,
/**
* Immediately push the frame to the output.
*/
AV_BUFFERSRC_FLAG_PUSH = 4,
/**
* Keep a reference to the frame.
* If the frame if reference-counted, create a new reference; otherwise
* copy the frame data.
*/
AV_BUFFERSRC_FLAG_KEEP_REF = 8,
};二、使用場景
對音頻或者視頻數據解碼後，調用該接口，把frame添加進濾鏡源，然後由濾鏡對數據做一些混合或者裁剪工作。示例代碼如下：
ret = avcodec_decode_audio4(p**AudioCodecCtx, p**Frame, &frameFinished, &packet);
if (ret < 0) {
LOGD("Decoding failed ret = %d, %s", ret, av_err2str(ret));
}
if (frameFinished) {
LOGD("packet pts = %" PRId64", dts = %" PRId64", duration = %d, pos = %" PRId64", stream_index = %d, flags = %d, size = %d",
packet.pts, packet.dts, packet.duration, packet.pos, packet.stream_index, packet.flags, packet.size);
LOGD("pFrame nb_samples = %d, channel_layout = %" PRId64", channels = %d, format(sample_fmt) = %d, sample_rate = %d, pts = %" PRId64"",
pFrame->nb_samples, pFrame->channel_layout, pFrame->channels, pFrame->format, pFrame->sample_rate, pFrame->pts);
pFrame->pts = av_frame_get_best_effort_timestamp(pFrame);
ret = av_buffersrc_add_frame(buffersrc_ctx[0], pFrame);
// ret = av_buffersrc_add_frame_flags(buffersrc_ctx[0], pFrame, AV_BUFFERSRC_FLAG_NO_CHECK_FORMAT);
if (ret < 0) {
LOGD("Error while feeding the filtergraph 0 ret = %d, %s", ret, av_err2str(ret));
} else {
LOGD_DEBUG("av_buffersrc_add_frame 0 success mediatype = %d", mediatype);
}
av_packet_unref(&packet);
break;
}三、源碼分析
av_buffersrc_add_frame內部調用了av_buffersrc_add_frame_flags，且不指定任何flags。
int attribute_align_arg av_buffersrc_add_frame(AVFilterContext *ctx, AVFrame *frame)
{
return av_buffersrc_add_frame_flags(ctx, frame, 0);
}av_buffersrc_add_frame_flags函數實現如下：
int attribute_align_arg av_buffersrc_add_frame_flags(AVFilterContext *ctx, AVFrame *frame, int flags)
{
AVFrame *copy = NULL;
int ret = 0;
if (frame && frame->channel_layout &&
av_get_channel_layout_nb_channels(frame->channel_layout) != frame->channels) {
av_log(ctx, AV_LOG_ERROR, "Layout indicates a different number of channels than actually present\n");
return AVERROR(EINVAL);
}
if (!(flags & AV_BUFFERSRC_FLAG_KEEP_REF) || !frame)
return av_buffersrc_add_frame_internal(ctx, frame, flags);
if (!(copy = av_frame_alloc()))
return AVERROR(ENOMEM);
ret = av_frame_ref(copy, frame);
if (ret >= 0)
ret = av_buffersrc_add_frame_internal(ctx, copy, flags);
av_frame_free(©);
return ret;
}可以看出，如果指定了AV_BUFFERSRC_FLAG_KEEP_REF方式，會拷貝一份幀數據進行處理，否則直接使用原來幀數據進行處理。同時調用了av_buffersrc_add_frame_internal接口進行操作。
static int av_buffersrc_add_frame_internal(AVFilterContext *ctx,
AVFrame *frame, int flags)
{
BufferSourceContext *s = ctx->priv;
AVFrame *copy;
int refcounted, ret;
s->nb_failed_requests = 0;
if (!frame)
return av_buffersrc_close(ctx, AV_NOPTS_VALUE, flags);
if (s->eof)
return AVERROR(EINVAL);
refcounted = !!frame->buf[0];
if (!(flags & AV_BUFFERSRC_FLAG_NO_CHECK_FORMAT)) {
switch (ctx->outputs[0]->type) {
case AVMEDIA_TYPE_VIDEO:
CHECK_VIDEO_PARAM_CHANGE(ctx, s, frame->width, frame->height,
frame->format, frame->pts);
break;
case AVMEDIA_TYPE_AUDIO:
/* For layouts unknown on input but known on link after negotiation. */
if (!frame->channel_layout)
frame->channel_layout = s->channel_layout;
CHECK_AUDIO_PARAM_CHANGE(ctx, s, frame->sample_rate, frame->channel_layout,
frame->channels, frame->format, frame->pts);
break;
default:
return AVERROR(EINVAL);
}
}
if (!av_fifo_space(s->fifo) &&
(ret = av_fifo_realloc2(s->fifo, av_fifo_size(s->fifo) +
sizeof(copy))) < 0)
return ret;
if (!(copy = av_frame_alloc()))
return AVERROR(ENOMEM);
if (refcounted) {
av_frame_move_ref(copy, frame);
} else {
ret = av_frame_ref(copy, frame);
if (ret < 0) {
av_frame_free(©);
return ret;
}
}
if ((ret = av_fifo_generic_write(s->fifo, ©, sizeof(copy), NULL)) < 0) {
if (refcounted)
av_frame_move_ref(frame, copy);
av_frame_free(©);
return ret;
}
if ((ret = ctx->output_pads[0].request_frame(ctx->outputs[0])) < 0)
return ret;
if ((flags & AV_BUFFERSRC_FLAG_PUSH)) {
ret = push_frame(ctx->graph);
if (ret < 0)
return ret;
}
return 0;
}如果未指定AV_BUFFERSRC_FLAG_NO_CHECK_FORMAT方式，該函數會對frame的參數進行校驗，否則不進行校驗，最後添加進過濾鏡上下文（AVFilterContext）的待處理緩存中。如果指定了AV_BUFFERSRC_FLAG_PUSH方式，會緊接着交由濾鏡濾波器（filtergraph）進行處理。
AV_BUFFERSRC_FLAG_NO_CHECK_FORMAT的使用注意事項可以參考：https://blog.csdn.net/Martin_chen2/article/details/100052276