v4l2 Python - streaming video - mapping buffers

Question

I'm working on a video capture script for Python in Raspbian (Raspberry Pi 2) and I'm having trouble using the Python bindings for v4l2, since I have no success on memory-maping the buffers.

What I need:

• Capture video from a HD-WebCam (will be later 2 of them at the same time).
• Be able to stream that video over WLAN (compromise between network load and processing speed).
• In the future, be able to apply filters to the image before streaming (not obligatory).

What I've tried:

• Use OpenCV (cv2). It's very easy to use, but it adds a lot of processing load as it converts the JPEG frames of the web cam to raw images and then I had to convert them back to JPEG before sending them over WLAN.
• Read directly from '/dev/video0'. It would be great, as the webcam sends the frames already compressed and I could just read and send them, but it seems that my camera doesn't support that.
• Use v4l2 bindings for Python. This is by now the most promising option, but I got stuck when I had to map the video buffers. I have found no way to overcome the "memory pointers / mappings" that this stuff seems to require.

What I've read:

• This guide: http://www.jayrambhia.com/blog/capture-v4l2/
• v4l2 documentation (some of it).
• This example in C: https://linuxtv.org/downloads/v4l-dvb-apis/capture-example.html
• Some other examples in C/C++. I've found no examples which make direct use of v4l2 bindings on Python.

My questions:

1. Is there a better way to do this? or if not...
2. Could I tell OpenCV to not decompress the image? It would be nice to use OpenCV in order to apply future extensions. I found here that it's not allowed.
3. How could I resolve the mapping step in Python? (any working example?)

Here is my (slowly) working example with OpenCV:

import cv2
import time

video = cv2.VideoCapture(0)

print 'Starting video-capture test...'

t0 = time.time()
for i in xrange(100):
    success, image = video.read()
    ret, jpeg = cv2.imencode('.jpg',image)

t1 = time.time()
t = ( t1 - t0 ) / 100.0
fps = 1.0 / t

print 'Test finished. ' + str(t) + ' sec. per img.'
print str( fps ) + ' fps reached'

video.release()

And here what I've done with v4l2:

FRAME_COUNT = 5

import v4l2
import fcntl
import mmap

def xioctl( fd, request, arg):

    r = 0

    cond = True
    while cond == True:
        r = fcntl.ioctl(fd, request, arg)
        cond = r == -1
        #cond = cond and errno == 4

    return r

class buffer_struct:
    start  = 0
    length = 0

# Open camera driver
fd = open('/dev/video1','r+b')

BUFTYPE = v4l2.V4L2_BUF_TYPE_VIDEO_CAPTURE
MEMTYPE = v4l2.V4L2_MEMORY_MMAP

# Set format
fmt = v4l2.v4l2_format()
fmt.type = BUFTYPE
fmt.fmt.pix.width       = 640
fmt.fmt.pix.height      = 480
fmt.fmt.pix.pixelformat = v4l2.V4L2_PIX_FMT_MJPEG
fmt.fmt.pix.field       = v4l2.V4L2_FIELD_NONE # progressive

xioctl(fd, v4l2.VIDIOC_S_FMT, fmt)

buffer_size = fmt.fmt.pix.sizeimage
print "buffer_size = " + str(buffer_size)

# Request buffers
req = v4l2.v4l2_requestbuffers()

req.count  = 4
req.type   = BUFTYPE
req.memory = MEMTYPE

xioctl(fd, v4l2.VIDIOC_REQBUFS, req)

if req.count < 2:
    print "req.count < 2"
    quit()

n_buffers = req.count

buffers = list()
for i in range(req.count):
    buffers.append( buffer_struct() )

# Initialize buffers. What should I do here? This doesn't work at all.
# I've tried with USRPTR (pointers) but I know no way for that in Python.
for i in range(n_buffers):

    buf = v4l2.v4l2_buffer()

    buf.type      = BUFTYPE
    buf.memory    = MEMTYPE
    buf.index     = i

    xioctl(fd, v4l2.VIDIOC_QUERYBUF, buf)

    buffers[i].length = buf.length
    buffers[i].start  = mmap.mmap(fd.fileno(), buf.length,
                                  flags  = mmap.PROT_READ,# | mmap.PROT_WRITE,
                                  prot   = mmap.MAP_SHARED,
                                  offset = buf.m.offset )

I will appreciate any help or advice. Thanks a lot!

Answer 1

I found for myself the answer as part of the code in another question. It was not the main topic of the question, but in this source code you can see how he uses the mmap in Python (line 159). Furthermore I found that I didn't need the write permissions.