v4l2grab – Grabbing JPEGs from V4L2 devices

A friend of mine, wanted to use a standard USB WebCam as an IPCam. He used an old Media-Receiver Box (T-Online S100) and installed Gentoo Linux on an CF Card.

Since Kernel 2.6.27 most of the USB webcams are supported through the Linux UVC drivers, which provides an Video4Linux2 Interface to those cams. We found some programs who claimed being able to grab JPEG images from V4L2 Cams, but most of them either didn’t compile, supported only V4L Version 1 or just brought up some other errors.

So I checked up the Video for Linux Two API Specification and found there in Appendix B a small sample of howto communicate with the Cam. I adapted it to our needs and included an converter from YUV colorspace to RGB. Finally I added some code to support the export of JPEG images.

You can download it here: v4l2grab Version 0.1

The whole program is written in C and only needs libjpeg to compile. There are different ways to communicate with the camera, if you want to support all, you should go best with:

gcc v4l2grab.c -o v4l2grab -Wall -ljpeg -DIO_READ -DIO_MMAP -DIO_USERPTR

If you compiled the program, you can get some images with:

./v4l2grab -o image.jpg

With -W width -H height its possible to adjust the size of the image. If the camera doesn’t support the resolution the programs adjusts it automatically and prints an information on stderr.

The current version of V4L2grab can be found on github.

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April 23, 2009
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9 Responses to “v4l2grab – Grabbing JPEGs from V4L2 devices”

v4l2grab Version 0.2 | #1
June 9, 2009 19:33

[...] few weeks ago, I published my v4l2grab program to grab JPEGs from V4l2 [...]
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Roby | #2
October 2, 2010 19:00

I’m trying to use a webcam in V4L2_PIX_FMT_MJPEG format (not V4L2_PIX_FMT_YUYV) and grab image into jpeg files, some ideas on how do it?
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Frank | #3
July 13, 2011 19:25

Hoi,

Great work! It would be a good idea to add the compile statement to the readme file on github.
Works like a charm!

Groeten,
Frank.
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Jemiah | #4
July 19, 2011 16:37

I just built this for the DM3730 Logic SOM running Linux built from the 2.6.32 OMAP PSP_03.00.01.06 kernel from TI. I used libjpeg62-dev. Worked right out of the box. Great work.

Thanks,
Jemiah
.
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tommy liu | #5
August 17, 2011 04:50

I was running v4l2grab on ARM with logitech C200.
Even set the format by using VIDIOC_S_FMT, I still get 176×144 size of image.
I saw the notes in source as follow:
‘Note VIDIOC_S_FMT may change width and height.’
Could you please share the more detail about this?
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Anders | #6
October 4, 2011 11:17

Hi,

I’m trying to compile v4l2grab for an ARM processor and can see that some of you have already succeded in that. Could you please give me a step by step guide on how to accomplish this task? Thanks in advanced

/Anders
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Alexw | #7
October 28, 2011 11:56

@Anders

On v5l ARM machine I have no compilation issue:

gcc v4l2grab.c -o v4l2grab -Wall -ljpeg -DIO_READ -DIO_MMAP -DIO_USERPTR

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Jonathan Andrews | #8

February 4, 2012 18:17

This is a very useful little utility, nice code, but has a stutter.

I’ve added some debugging text

static void imageProcess(const void* p)
{
        unsigned char* src = (unsigned char*)p;
        unsigned char* dst = malloc(width*height*3*sizeof(char));

        // convert from YUV422 to RGB888
printf("Munge\n"); fflush(stdout);
        YUV422toRGB888(width,height,src,dst);

        // write jpeg
printf("Write\n"); fflush(stdout);
        jpegWrite(dst);

        // free temporary image
        free(dst);
}

# ./v4l2grab -m -o test.jpg
1Open
Init
Start
Frameread
Frameread
Munge
Write
Frameread
Munge
Write

Should it not read a frame once, process it once, and write t once ?

Thanks,
Jon

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twam | #9
February 6, 2012 15:57

Jonathan Andrews:
Should it not read a frame once, process it once, and write t once ?

The program is reading the frame again, when the read returned an EAGAIN error.

twam.info