Capturing frames from a webcam on Linux

Not many people are trying to capture images from their webcam using Python under Linux and blogging about it. In fact, I could find nobody who did that. I found people capturing images using Python under Windows, and people capturing images using C under Linux, and finally some people capturing images with Python under Linux but not blogging about it. This instructional post I wrote to help those people who want to start processing images from a webcam using the great Python language and a stable Linux operating system.

There is a very good library for capturing images in Windows called VideoCapture. It works, and a number of people blogged about using it. I was jealous for a long time.

There are a number of very old libraries which were meant to help with capturing images on Linux: libfg, two separate versions of pyv4l, and pyv4l2. But the first doesn’t work on my computer, the two versions of pyv4l cause segfaults because they are so old and not updated, and the last has no code written.

Finally, I learned that OpenCV has an interface to V4L/V4L2. OpenCv is Intel’s Open Source Computer Vision library. It’s excellent, extensive, and has a good community behind it. V4L is Linux’s standard abstraction for reading in video data. V4L2 is the newer version which Ubuntu also has installed.

Plus, OpenCV has very complete Python bindings. Unfortunately, these bindings and how to use them properly to capture images from a webcam are not documented. Only after careful searching on the sizable OpenCV mailing list did I finally find the answer.

Below is code that reads in up to 30 frames per second from a web cam while simultaneously displaying what it reads in. It’s very cool. It uses opencv’s camera acquisition abstraction, PIL, and pygame for speed in the looping. Note that with the images read into Python, you and I can now do arbitrary things with the image. We can flip it, track objects, draw markers, or do really anything.

This is example utility code. It is not a well structured program. Much of the code I use below is from techlists.org.


import pygame
import Image
from pygame.locals import *
import sys

import opencv
#this is important for capturing/displaying images
from opencv import highgui 

camera = highgui.cvCreateCameraCapture(0)
def get_image():
    im = highgui.cvQueryFrame(camera)
    # Add the line below if you need it (Ubuntu 8.04+)
    #im = opencv.cvGetMat(im)
    #convert Ipl image to PIL image
    return opencv.adaptors.Ipl2PIL(im) 

fps = 30.0
pygame.init()
window = pygame.display.set_mode((640,480))
pygame.display.set_caption("WebCam Demo")
screen = pygame.display.get_surface()

while True:
    events = pygame.event.get()
    for event in events:
        if event.type == QUIT or event.type == KEYDOWN:
            sys.exit(0)
    im = get_image()
    pg_img = pygame.image.frombuffer(im.tostring(), im.size, im.mode)
    screen.blit(pg_img, (0,0))
    pygame.display.flip()
    pygame.time.delay(int(1000 * 1.0/fps))