Still image on Raspberry Pi about hkcam

[arkdchst]

Tried now guide from this page: https://github.com/brutella/hkcam/blob/master/ffmpeg/README.md

Stream from Raspberry Pi to Mac:

ffmpeg -re -f video4linux2 -i /dev/video0 -map 0:0 -vcodec h264 -pix_fmt yuv420p -f rawvideo
    -tune zerolatency -b:v 1500k -bufsize 1500k -payload_type 99 -ssrc 16132552 -f rtp
    -srtp_out_suite AES_CM_128_HMAC_SHA1_80 -srtp_out_params omz31e5SiZSneUySvSsIaFfu+NW2uWUl9+FHs3HD 
    "srtp://192.168.0.217:58536?rtcpport=58536&localrtcpport=58536&pkt_size=1378"

On Mac:

ffplay -i in.sdp -protocol_whitelist file,udp,rtp

Video is updating every 3 seconds (but is better than freezes 30-60 seconds long when I use hkcam). When I add -vsync vfr to sender, framerate increases to ~7 fps.

Why I achieve so low fps using hkcam and receiving it on iPhone?