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How does video work?

e14 Contributor

So the Zedboard is using the FPGA to do some basic video processing.  But if I wanted to use v4l how would I go about doing that?  I want to integrate a frame grabber with some video processing code, but I can't figure out how to capture the video stream live.  Thoughts?

  • 0

    jayjay71,

    My experience with Video4Linux is limited, however I do believe I know enough to point you in the correct direction.

    Here is the how-to guide for writing a driver for a device to make it V4L compatible:

    http://www.linuxtv.org/downloads/legacy/video4linux/v4l2dwgNew.html

    You will want to write a driver for your custom logic that will be compiled against the version of the Kernel you are using.  This driver must be in line with the aforementioned rules set by the V4L folks.

    I would start by setting up a FIFO within your PL, and fill that with video data.  The other side of the FiFo would be butted up to one of the AXI interfaces.  Depending on how much image data you are pushing through it may have to be the High Performance AXI interface.  Then, you driver would connect the read, write, open, close, ioctr, and mmap functions to specific memory read and writes that corresponding to the address(es) that EDK assigned your peripheral.

    I know that my response requires you to know a lot about a lot but does that make sense?  If not we can try and break it down piece by piece.

  • 0

    I appreciate the quick response.  I guess I should elaborate on my situation.

    First of all, I somewhat understand what you're saying, but I've never written a driver before so it's mostly new to me.  Breaking it down would certainly be helpful, but I'm used to scavenging tutorials and binding them together to get what I want eventually.

    So basically what I'm trying to do is get a USB frame grabber to work with OpenCV on the Zedboard.  Using the software that came with the frame grabber (sensoray 2255s) I'm able to get a live stream.  However, this is NOT using v4l and as far as I can tell there's no obvious way to integrate it with OpenCV.  At the moment I'd be happy just have to a driver or a module that will allow me to see the frame grabber as a webcam and then OpenCV can do the rest, although ultimately I'll probably need to do a lot more low-level code to optimize things using VHDL since speed and resources are a big concern.  I had bad luck recompiling the full Digilent kernel (I tried twice and both times I couldn't get anything to boot - it was something with the /dev directory and/or initrd and/or initramfs).  If you know that the Digilent kernel works with v4l (and I assume a full kernel does) I can give that another go.  But like I said, in the end I'll *probably* need to build the driver like you suggested, so any details are appreciated.

    Alternatively, if somebody here knows a way to modify highgui in OpenCV to access non-standard video buffers (somebody who's actually done it) that would work for me as well.

    Again, I appreciate the quick response.

  • 0 in reply to e14 Contributor

    Although I'm not sure, I think the issue is not the driver but rather a missing v4l module within the kernel.  Hopefully I'll know by the end of the week and I'll probably report back whether I was successful or not.

  • 0 in reply to e14 Contributor

    Yes, that all makes sense.  I think it's gonna be a while before I get anything put into hardware though.  I'm still very much at the proof-of-concept stage for my project.