ffmpeg webcam to device driver , output 8 bit grayscale

Question

With the following command i'm trying to capture 10fps and send them the device driver. Now i want 8 bit gray raw frames (640x480= 307200 bytes per frame) to be send to the device driver. I can't figure out how to set the output with ffmpeg or the intput with v4l2 to this format.

ffmpeg -f v4l2 -r 25 -s 640x480 -i /dev/video0 -f rawvideo "/dev/devicedriver"

ffmpeg doesn't seem to support 8-bit gray output. And with v4l2 i can't figure out how to set it. It seems it doesn't recoginize V4L2_PIX_FMT_GREY.

v4l2-ctl --set-fmt-video-out=width=640,height=480,pixelformat=V4L2_PIX_FMT_GREY

I came up with a solution combining python, opencv and ffmpeg:

import cv
import sys
import os
import time
camera_index = 0 
capture = cv.CaptureFromCAM(camera_index)
count = 0
def repeat():
    global capture
    global camera_index
    global count
    frame = cv.GetMat(cv.QueryFrame(capture))
    framegray = cv.CreateMat(480, 640, cv.CV_8UC1)
    cv.CvtColor(frame, framegray, cv.CV_BGR2GRAY)
    sys.stdout.write(framegray.tostring())
    c = cv.WaitKey(1)
    if c == 27:
        print count
        sys.exit()

while True:
    repeat()

and then pipe it to ffmpeg

python capturewebcam.py | ffmpeg -f rawvideo -pix_fmt gray -s 640x480 -i - -an -f rawvideo -r 10 "/dev/null"

But i think it really has to be possible with just ffmpeg and v4l2, and i can't figure out how. My head hurts from reading the documentation :p.

score 1 · Accepted Answer · answered Aug 11 '13 at 00:28

First, check pixel formats are supported by your output device driver:

v4l2-ctl --list-formats -d /dev/devicedriver

the pixelformat you want to pass to the v4l2-ctl command line is the fourcc shown in the result, eg:

Pixel Format  : 'YUYV'

in this case your command line would be:

v4l2-ctl --set-fmt-video-out=width=640,height=480,pixelformat=YUYV

If you need V4L2_PIX_FMT_GREY probably the fourcc will be 'GREY' (I guess from videodev2.h, can't check)

If it is not in the result of the list-formats command it is not supported by the driver, so you'll need some conversion from the source (input/camera) format to the output.

ffmpeg webcam to device driver , output 8 bit grayscale

1 Answers1