What is wrong with choosing camera for video capturing?

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I am trying to learn image processing, but it is a pain because of camera selection.

In C# Winform, I am trying to get video capture by using the camera. When I use integrated webcam, everything works fine.
However, when i pick USB connected webcam, nothing happens in the app, it shows nothing, then gives an error.

The integrated webcam index as '0' and usb webcam is '1'.
The error I am getting is:

System.NullReferenceException: 'Nesne başvurusu bir nesnenin örneğine ayarlanmadı.'

C#

private void btnStart_Click(object sender, EventArgs e)
{
    if (capture == null)
    {
        //capture = new VideoCapture(0);
        capture = new VideoCapture(1);
    }

    capture.ImageGrabbed += Capture_ImageGrabbed;
    capture.Start();
}

If I choose '0' there is no problem but if I pick '1', nothing happens.

What I have tried:

I tried using AforgeDirectShow library, It works, but I want to do this with emgu CV. In principle, there should be no problem.

Also, I do not want to have an extra library for future apps which will be really memory consumer.

I go through whole web sites, there is almost the same problem, but nothing works for me.

Is there an effect of having usb 2.0 or 3.0?

Posted 9-Sep-23 8:41am

OzzieOzburn

Updated 13-Sep-23 3:22am

Deeksha Shenoy

v2

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OzzieOzburn 16-Sep-23 13:41pm

Update: I have still no solution. Any new suggestion is welcome.

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OriginalGriff · Answer 1 · 2023-09-09T09:10:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out! Most likely it's to do with the VideoCapture constructor - but we have no access to that so we can't help you.

Richard MacCutchan · Answer 2 · 2023-09-09T21:28:00

The return value from VideoCapture(1) is null, so the line:

C#

capture.ImageGrabbed += Capture_ImageGrabbed;

throws the exception, since capture does not contain a valid reference. You should check that the call to VideoCapture(1) returns a proper camera reference. However looking at the documentation at https://docs.opencv.org/3.4/d8/dfe/classcv_1_1VideoCapture.html#a5d5f5dacb77bbebdcbfb341e3d4355c1[^], you need more than a simple index value.

What is wrong with choosing camera for video capturing?

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