Capturing H264 stream with OpenCV - c++

Question

So. I have been trying to get my Raspberry Pi 2 to capture H264 stream with OpenCV from my Logitech C920 for quite some time now. I have been scavenging the internet for info, but with no luck.
A short system description:
Raspberry Pi 2, running Raspbian, Kernel 3.18
Logitech HD Pro Webcam c920
OpenCV 2.4.11
boneCV - Credits to Derek Molloy (https://github.com/derekmolloy/boneCV)
libx264 and FFMPEG (built with x264 support)
libv4l-dev, v4l-utils, qv4l2, v4l2ucp
I know OpenCV forces format to BGR24 (MJPG). This is specified in cap_libv4l.cpp. It looks like this(line 692->):
/* libv4l will convert from any format to V4L2_PIX_FMT_BGR24 */
CLEAR (capture->form);
capture->form.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
capture->form.fmt.pix.pixelformat = V4L2_PIX_FMT_BGR24;
capture->form.fmt.pix.field = V4L2_FIELD_ANY;
capture->form.fmt.pix.width = capture->width;
capture->form.fmt.pix.height = capture->height;
I can set the pixelformat manualy with v4l2-ctl --set-fmt-video
pi#raspberrypi ~/boneCV$ v4l2-ctl --set-fmt-video=width=1920,height=1080,pixelformat=H264
pi#raspberrypi ~/boneCV$ v4l2-ctl --get-fmt-video
Format Video Capture:
Width/Height : 1920/1080
Pixel Format : 'H264'
Field : None
Bytes per Line: 3840
Size Image : 4147200
Colotspace : SRGB
And if I now run "./boneCV" - A very simple capture program that captures a picture and does a canny edge detection. (I'll add the code in the end). I get this:
pi#raspberrypi ~/boneCV$ ./boneCV
pi#raspberrypi ~/boneCV$ v4l2-ctl --get-fmt-video
Format Video Capture:
Width/Height : 1920/1080
Pixel Format : 'MJPG'
Field : None
Bytes per Line: 0
Size Image : 4147200
Colorspace : SRGB
As you can se the "Pixelformat" and the "Bytes per Line" changes. The "Field" stays at None and the "Colourspace" stays at SRGB.
Then I tried to replace every "V4L2_PIX_FMT_BGR24" with "V4L2_PIX_FMT_H264" in cap_lib4vl.cpp and rebuilded OpenCV. When I then ran the "./boneCV" my two .png images are only black with one or two stripes of white color.
To find out if it is libv4l or OpenCV I ran "./capture" script that follow Derek Molloys boneCV. It uses libv4l directly and captures an H264 video stream with no problems at all. I then have to use "./raw2mpg4" to be able to watch it. The .mp4 file is 1920x1080 at 30 fps with no glitches. And after this I checked "v4l2-ctl --get-fmt-video" again and got this:
pi#raspberrypi ~/boneCV$ v4l2-ctl --get-fmt-video
Format Video Capture:
Width/Height : 1920/1080
Pixel Format : 'H264'
Field : None
Bytes per Line: 3840
Size Image : 4147200
Colotspace : SRGB
Exactly the same as when I did set everything manualy.
I have come to the conclusion that if I want OpenCV to be able to capture raw H264 streams I'll have to change the cap_libv4l.cpp, but I have no idea how. I think it may be because the difference in bits per frame and/or colorspace.
Do anybody know how to do this or how to make an workaround so that I stil can use OpenCVs "VideoCapture" function?
I know alot of Raspberry Pi and BeagleboneBlack users would be ever so gratefull if there was any solution to this problem.
I have tried to cover everything that I think is relevant, if there is anything more I could provide to paint the picture better, please say so.
Her some links to the mentioned scripts and programs:
(edit. I tried to post the links to each of the programs, but I didn't have enough reputation. Go to Derek Molloys github page and you'll find boneCV there.)
And no I can not use the "CV_FOURCC('H','2','6','4');" because this function is not implemented for linux yet.