[Libav-user] Delay to capture frames from live stream

[robertot1 at libero.it]

Hi everyone, I’m trying to capture live stream from usb camera with raspberry pi 2.I installed ffmpeg and I’m using this code to get live stream from camera.I have some problems:- the framerate decreases because every ~22 frames there is a big delay (~950ms), it’s how if the buffer become full and then it’s emptied, or another possibility is that every ~22 frames the camera adjusts some parameters for contrast, brightness, ecc…- from details camera I could capture the frames with 30fps with 640x480 resolution, but, without to consider the delay, the difference in ms between frames is 44ms, then ~23fps, why?
The usb camera is: ELP-USB30W02M-L36, I use 640x480 resolution with format YUY2.Thanks a lot for yours helps.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------#include <libavdevice/avdevice.h>
#include <libswscale/swscale.h>
#include <libavutil/pixfmt.h>
#include <libavutil/imgutils.h>
#include <sys/time.h>


typedef struct Timestamp {
	int seconds;
	int useconds;
} Timestamp;


#define ONE_SECOND_IN_USEC 1000000


int difference_timestamp(Timestamp timestamp1, Timestamp timestamp2) {
	int difference = 0; //in usec
	if (timestamp1.seconds > timestamp2.seconds) {
		difference = (timestamp1.seconds - timestamp2.seconds)
				* ONE_SECOND_IN_USEC;
		if (timestamp1.useconds > timestamp2.useconds) {
			difference += timestamp1.useconds - timestamp2.useconds;
		} else {
			difference += timestamp2.useconds - timestamp1.useconds;
			difference -= ONE_SECOND_IN_USEC;
		}
	} else {
		difference = (timestamp2.seconds - timestamp1.seconds)
				* ONE_SECOND_IN_USEC;
		if (timestamp1.useconds > timestamp2.useconds) {
			difference += timestamp1.useconds - timestamp2.useconds;
			difference -= ONE_SECOND_IN_USEC;
		} else {
			difference += timestamp2.useconds - timestamp1.useconds;
		}
	}
	return difference;
}


void get_current_time(Timestamp* timestamp) {
	struct timeval tv;
	gettimeofday(&tv, NULL);
	timestamp->seconds = (int) (tv.tv_sec);
	timestamp->useconds = (int) (tv.tv_usec);
}


int main(int argc, char *argv[]) {


	avdevice_register_all();
	avcodec_register_all();


	const char *filenameSrc = "/dev/video0";


	AVCodecContext *pCodecCtx;
	AVFormatContext *pFormatCtx = avformat_alloc_context();
	AVCodec * pCodec;
	AVInputFormat *iformat = av_find_input_format("dshow");
	AVFrame *pFrame, *pFrameRGB;
	AVCodecParameters *pCodecPrm = NULL;


	if (avformat_open_input(&pFormatCtx, filenameSrc, iformat, NULL) != 0)
		return -12;
	if (avformat_find_stream_info(pFormatCtx, NULL) < 0)
		return -13;
	av_dump_format(pFormatCtx, 0, filenameSrc, 0);
	int videoStream = 1;
	int i;
	for (i = 0; i < pFormatCtx->nb_streams; i++) {
		if (pFormatCtx->streams[i]->codecpar->codec_type
				== AVMEDIA_TYPE_VIDEO) {
			videoStream = i;
			break;
		}
	}
	// Get a pointer to the codec context for the video stream
	pCodecPrm = pFormatCtx->streams[videoStream]->codecpar;


	if (videoStream == -1)
		return -14;


	// Find the decoder for the video stream
	pCodec = avcodec_find_decoder(pCodecPrm->codec_id);


	if (pCodec == NULL)
		return -15; //codec not found


	pCodecCtx = avcodec_alloc_context3(pCodec);
	pCodecCtx->bit_rate = pCodecPrm->bit_rate;
	pCodecCtx->width = pCodecPrm->width;
	pCodecCtx->height = pCodecPrm->height;
	pCodecCtx->pix_fmt = AV_PIX_FMT_YUYV422;


	AVDictionary *codec_options = NULL;


	if (avcodec_open2(pCodecCtx, pCodec, &codec_options) < 0)
		return -16;


	pFrame = av_frame_alloc();
	pFrameRGB = av_frame_alloc();


	enum AVPixelFormat pFormat = AV_PIX_FMT_BGR24;


	int numBytes = av_image_get_buffer_size(pFormat, pCodecPrm->width,
			pCodecPrm->height, 1);
	uint8_t *buffer = NULL;
	buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));


	av_image_fill_arrays(pFrameRGB->data, pFrameRGB->linesize, buffer, pFormat,
			pCodecPrm->width, pCodecPrm->height, 1);


	int res = 0, diff;
	AVPacket packet;
	Timestamp timestamp_prec, timestamp_curr;
	get_current_time(&timestamp_prec);


	sleep(10);


	while (res >= 0) {
		res = av_read_frame(pFormatCtx, &packet);
		get_current_time(&timestamp_curr);
		diff = difference_timestamp(timestamp_prec, timestamp_curr) / 1000; //diff in ms


		printf("T_prec:%d.%d\tT_curr:%d.%d\ndiff:%d\n", timestamp_prec.seconds,
				timestamp_prec.useconds, timestamp_curr.seconds,
				timestamp_curr.useconds, diff);
		fflush(stdout);


		if (packet.stream_index == videoStream) {


			avcodec_send_packet(pCodecCtx, &packet);
			avcodec_receive_frame(pCodecCtx, pFrame);


			struct SwsContext * img_convert_ctx;
			img_convert_ctx = sws_getCachedContext(NULL, pCodecCtx->width,
					pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width,
					pCodecCtx->height, AV_PIX_FMT_BGR24, SWS_BICUBIC, NULL,
					NULL, NULL);


			sws_scale(img_convert_ctx, (uint8_t const * const *) pFrame->data,
					pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data,
					pFrameRGB->linesize);


			av_packet_unref(&packet);
			sws_freeContext(img_convert_ctx);


		}
		timestamp_prec = timestamp_curr;
	}


	av_packet_unref(&packet);
	avcodec_close(pCodecCtx);
	av_free(pFrame);
	av_free(pFrameRGB);
	avformat_close_input(&pFormatCtx);


	return 0;
}
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