How to capture still image from webcam on linux

Question

I am trying to write a C++/Qt program for linux, where I take a still image photo from a webcam, make some transformations to a photo (cropping, resizing, etc.), and save it to a jpeg file.

But I have encountered some problems. The main problem is that standart UVC (usb video device class) linux driver currently does not support direct still image capture: http://www.ideasonboard.org/uvc/ .

So, there are two possible ways to capture still image. You can take one frame from the video stream from the camera, or you can take a separate photo, like a digital portable camera. The second way is not supported in linux uvc driver, so the first method is the only way. But the problem is, that if you want to take a frame from the video stream, the size of the photo can't be bigger than the size of video in the video preview window. So, if I want to take 2 megapixel photo, I must start videostream with the size 1600x1200, which is not so comfortable (At least, in Qt the size of the videostream depends on the videopreview window size).

I know that there is video for linux 2 API, which may be helpful in this task, but I don't know how to use it. I am currently learning gstreamer, but I can't now figure out how to do what I need using these tools.

So, I will appreciate any help. I think it is not a hard problem for people who know Linux, GStreamer, v4l2 API, and other linux-specific things.

By the way, the program will be used only with web-camera Logitech C270 HD.

Please, help me. I don't know what API or framework can help me do this. May be you know.

Answer 1

Unfortunately the C4V2 calls in opencv did not work for still image capture with any camera I have tried out of the box using the UVC driver.

To debug the issue I have been playing with trying to accomplish this with c code calling c4v2 directly.

I have been playing with the example code found here. It uses the method of pulling frames from the video stream.

You can compile it with:

gcc -O2 -Wall `pkg-config --cflags --libs libv4l2` filename.c -o filename

I have experimented with 3 logitech cameras. The best of the lot seems to be the Logitech C910. But even it has significant issues.

Here are the problems I have encountered trying to accomplish your same task with this code.

It works pretty much every time with width and height set to 1920x1080.

When I query other possibilities directly from the command line using for example:

v4l2-ctl --list-formats-ext

and I try some of the other "available" smaller sizes it hangs in the select waiting for the camera to release the buffer.

Also when I try to set other sizes directly from the command line using for example:

v4l2-ctl -v height=320 -v width=240 -v pixelformat=YUYV

Then check with

v4l2-ctl -V

I find that it returns the correct pixel format but quite often not the correct size.

Apparently this camera which is listed on the UVC site as being UVC and therefore v4l2 compatible is not up to snuff. I suspect it is just as bad for other cameras. The other two I tried were also listed as compatible on the site but had worse problems.

I did some more testing on the LogitechC910 after I posted this. I thought I would post the results in case it helps someone else out.

I wrote a script to test v4l2 grabber code mentioned above on all the formats the camera claims it supports when it is queried with v4l2 here are the results:

640x480 => Hangs on clearing buffer
160x120 => Works
176x144 => Works
320x176 => Works
320x240 => Works
432x240 => Works
352x288 => Works
544x288 => Works
640x360 =>  Works
752x416 => Hangs on clearing buffer
800x448 => Hangs on clearing buffer
864x480 => Works
960x544 => Works
1024x576 => Works
800x600 => Works
1184x656 => Works
960x720 => Works
1280x720 => Works
1392x768 => Works
1504x832 => Works
1600x896 => Works
1280x960 => Works
1712x960 => Works
1792x1008 => Works
1920x1080 => Works
1600x1200 => Works
2048x1536 => Works
2592x1944 => Hangs on clearing buffer.

It turns out that the default setting of 640x480 doesnt work and that is what trapped me and most others who have posted on message boards.

Since it is grabbing a video frame the first frame it grabs when starting up may have incorrect exposure (often black or close to it). I believe this is because since it is being used as a video camera it adjusts exposure as it goes and doesnt care about the first frames. I believe this also trapped me and other who saw the first frame as black or nearly black and thought it was some kind of error. Later frames have the correct exposure

It turns out that opencv with python wrappers works fine with this camera if you avoid the land mines listed above and ignore all the error messages. The error messages are due to the fact while the camera accepts v4l2 commands it doesnt respond correctly. So if you set the width it actually gets set correctly but it responds with an incorrect width.

To run under opencv with python wrappers you can do the following:

import cv2
import numpy

cap = cv2.VideoCapture(0)  #ignore the errors
cap.set(3, 960)        #Set the width important because the default will timeout
                       #ignore the error or false response
cap.set(4, 544)        #Set the height ignore the errors
r, frame = cap.read()
cv2.imwrite("test.jpg", frame)