You are right, the default pixel format of OpenCV is BGR.
The equivalent format on the ffmpeg side would be BGR24, so you don't need to convert it to YUV420p if you don't want to.
This post shows how to use a python application to capture frames from the webcam and write the frames to stdout. The purpose is to invoke this app on the cmd-line and pipe the result directly to the ffmpeg application, which stores the frames on the disk. Quite clever indeed!
capture.py:
import cv, sys
cap = cv.CaptureFromCAM(0)
if not cap:
sys.stdout.write("failed CaptureFromCAM")
while True :
if not cv.GrabFrame(cap) :
break
frame = cv.RetrieveFrame(cap)
sys.stdout.write( frame.tostring() )
And the command to be executed on the shell is:
python capture.py | ffmpeg -f rawvideo -pix_fmt bgr24 -s 640x480 -r 30 -i - -an -f avi -r 30 foo.avi
Where:
- -r gives the frame rate coming off the camera
- -an says "don't encode audio"
I tested this solution on my Mac OS X with OpenCV 2.4.2.
EDIT:
In case you haven't tried to record from the camera and use OpenCV to write the video to an mp4 file on the disk, here we go:
import cv, sys
cap = cv.CaptureFromCAM(0) # 0 is for /dev/video0
if not cap:
sys.stdout.write("!!! Failed CaptureFromCAM")
sys.exit(1)
frame = cv.RetrieveFrame(cap)
if not frame:
sys.stdout.write("!!! Failed to retrieve first frame")
sys.exit(1)
# Unfortunately, the following instruction returns 0
#fps = cv.GetCaptureProperty(cap, cv.CV_CAP_PROP_FPS)
fps = 25.0 # so we need to hardcode the FPS
print "Recording at: ", fps, " fps"
frame_size = cv.GetSize(frame)
print "Video size: ", frame_size
writer = cv.CreateVideoWriter("out.mp4", cv.CV_FOURCC('F', 'M', 'P', '4'), fps, frame_size, True)
if not writer:
sys.stdout.write("!!! Error in creating video writer")
sys.exit(1)
while True :
if not cv.GrabFrame(cap) :
break
frame = cv.RetrieveFrame(cap)
cv.WriteFrame(writer, frame)
cv.ReleaseVideoWriter(writer)
cv.ReleaseCapture(cap)
I've tested this with Python 2.7 on Mac OS X and OpenCV 2.4.2.
