[英]How to extract frames at 30 fps using FFMPEG APIs on Android?
我们正在开发一个使用FFMPEG
库在 Android 平台上提取视频帧的项目。
在 Windows 上,我们观察到:
ffmpeg -i input.flv -vf fps=1 out%d.png
以 30 fps 提取帧。但是当我们直接在 Android 上使用 FFMPEG API 时(请参阅硬件详细信息),我们得到以下结果:
我们还没有在 Android 上测试 Xuggler/CLI。
理想情况下,我们应该能够在恒定时间(大约 30 毫秒/帧)内获取数据。
我们如何在 Android 上获得 30 fps?
Android 上使用的代码:
if (avformat_open_input(&pFormatCtx, pcVideoFile, NULL, NULL)) {
iError = -1; //Couldn't open file
}
if (!iError) {
//Retrieve stream information
if (avformat_find_stream_info(pFormatCtx, NULL) < 0)
iError = -2; //Couldn't find stream information
}
//Find the first video stream
if (!iError) {
for (i = 0; i < pFormatCtx->nb_streams; i++) {
if (AVMEDIA_TYPE_VIDEO
== pFormatCtx->streams[i]->codec->codec_type) {
iFramesInVideo = pFormatCtx->streams[i]->nb_index_entries;
duration = pFormatCtx->streams[i]->duration;
begin = pFormatCtx->streams[i]->start_time;
time_base = (pFormatCtx->streams[i]->time_base.num * 1.0f)
/ pFormatCtx->streams[i]->time_base.den;
pCodecCtx = avcodec_alloc_context3(NULL);
if (!pCodecCtx) {
iError = -6;
break;
}
AVCodecParameters params = { 0 };
iReturn = avcodec_parameters_from_context(¶ms,
pFormatCtx->streams[i]->codec);
if (iReturn < 0) {
iError = -7;
break;
}
iReturn = avcodec_parameters_to_context(pCodecCtx, ¶ms);
if (iReturn < 0) {
iError = -7;
break;
}
//pCodecCtx = pFormatCtx->streams[i]->codec;
iVideoStreamIndex = i;
break;
}
}
}
if (!iError) {
if (iVideoStreamIndex == -1) {
iError = -3; // Didn't find a video stream
}
}
if (!iError) {
// Find the decoder for the video stream
pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
if (pCodec == NULL) {
iError = -4;
}
}
if (!iError) {
// Open codec
if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0)
iError = -5;
}
if (!iError) {
iNumBytes = av_image_get_buffer_size(AV_PIX_FMT_RGB24, pCodecCtx->width,
pCodecCtx->height, 1);
// initialize SWS context for software scaling
sws_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height,
pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height,
AV_PIX_FMT_RGB24,
SWS_BILINEAR,
NULL,
NULL,
NULL);
if (!sws_ctx) {
iError = -7;
}
}
clock_gettime(CLOCK_MONOTONIC_RAW, &end);
delta_us = (end.tv_sec - start.tv_sec) * 1000000
+ (end.tv_nsec - start.tv_nsec) / 1000;
start = end;
//LOGI("Starting_Frame_Extraction: %lld", delta_us);
if (!iError) {
while (av_read_frame(pFormatCtx, &packet) == 0) {
// Is this a packet from the video stream?
if (packet.stream_index == iVideoStreamIndex) {
pFrame = av_frame_alloc();
if (NULL == pFrame) {
iError = -8;
break;
}
// Decode video frame
avcodec_decode_video2(pCodecCtx, pFrame, &iFrameFinished,
&packet);
if (iFrameFinished) {
//OUR CODE
}
av_frame_free(&pFrame);
pFrame = NULL;
}
av_packet_unref(&packet);
}
}
您需要来自libavfilter
一些结构和函数。
vf
选项的意思是“视频过滤器”。 命令行ffmpeg -i input -vf fps=30 out%d.png
将输出video_length_in_seconds * 30
与原始视频 fps 无关。 这意味着如果视频是 25 fps,你会得到一些重复的帧。 而如果视频超过 30 fps,您将丢失一些帧。
为此,您必须初始化一些过滤器上下文。 请参阅 ffmpeg 源中的filtering_video.c
示例。
AVFilter* buffersrc = avfilter_get_by_name("buffer");
AVFilter* buffersink = avfilter_get_by_name("buffersink");
AVFilterInOut* outputs = avfilter_inout_alloc();
AVFilterInOut* inputs = avfilter_inout_alloc();
AVRational time_base = p_format_ctx->streams[video_stream]->time_base;
enum AVPixelFormat pix_fmts[] = { p_codec_ctx->pix_fmt, AV_PIX_FMT_NONE };
filter_graph = avfilter_graph_alloc();
if (!outputs || !inputs || !filter_graph) {
// failed, goto cleanup
}
char args[512];
snprintf(args, sizeof(args),
"video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
p_codec_ctx->width, p_codec_ctx->height, p_codec_ctx->pix_fmt,
time_base.num, time_base.den,
p_codec_ctx->sample_aspect_ratio.num, p_codec_ctx->sample_aspect_ratio.den);
int ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
args, NULL, filter_graph);
if (ret < 0) {
LOG(ERROR) << "Cannot create buffer source";
avfilter_inout_free(&inputs);
avfilter_inout_free(&outputs);
return false;
}
ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
NULL, NULL, filter_graph);
if (ret < 0) {
// failed... blabla
}
ret = av_opt_set_int_list(buffersink_ctx, "pix_fmts", pix_fmts,
AV_PIX_FMT_NONE, AV_OPT_SEARCH_CHILDREN);
if (ret < 0) {
// failed... blabla
}
outputs->name = av_strdup("in");
outputs->filter_ctx = buffersrc_ctx;
outputs->pad_idx = 0;
outputs->next = NULL;
inputs->name = av_strdup("out");
inputs->filter_ctx = buffersink_ctx;
inputs->pad_idx = 0;
inputs->next = NULL;
const char* filter_description[256] = "fps=fps=30";
if ((ret = avfilter_graph_parse_ptr(filter_graph, filters_descr.c_str(),
&inputs, &outputs, NULL)) < 0) {
// failed...
}
if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0) {
// failed...
}
好的,这是所有需要的初始化。
并在解码部分添加一些代码:
avcodec_decode_video2(p_codec_ctx, p_frame, &got_frame, &packet);
if (*got_frame) {
p_frame->pts = av_frame_get_best_effort_timestamp(p_frame);
if (av_buffersrc_add_frame_flags(buffersrc_ctx, p_frame, AV_BUFFERSRC_FLAG_KEEP_REF) < 0) {
// failed... blabla
}
while (1) {
int ret = av_buffersink_get_frame(buffersink_ctx, p_frame_stage);
// p_frame_stage is a AVFrame struct. Same size as p_frame. Need to allocated before.
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
break;
if (ret < 0) {
// something wrong. filter failed.
}
// Do something with p_frame_stage here.
}
}
请看一下https://gitter.im/mobile-ffmpeg/Lobby?at=5c5bb384f04ef00644f1bb4e下面几行,他们提到了加速过程的选项,例如... -preset ultrafast, -threads 10, -tune零延迟,-x264-params sliced-threads=1
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