how can I set VideoCapture attributes with set?

asked 2017-01-25 01:43:52 -0600

j0h
106 ●4 ●7 ●13

updated 2017-01-25 07:39:05 -0600

I am trying to adjust the gain in a simple camera function

void imgGet(){

    cv::VideoCapture cap(0);
    double k=0.99;
    cap.set(CAP_PROP_EXPOSURE,1); 
    Mat frame;
    cap >> frame;
    string fileName = getFName();
    imwrite(fileName, frame);
    }

Ive tried a variety of things, but the error I get is:

VIDIOC_S_CTRL: Invalid argument

I don't know what that means, how am I supposed to set the VideoCapture::set object? I have seen the videocapture documentation

but I didnt understand it in a useful way. I am on a Linux PC. despite the error, an image is still taken, but the image quality is unchanged.

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It should be

cap.set(CV_CAP_PROP_GAIN, k ); //value from 0 to 1

LBerger ( 2017-01-25 01:47:57 -0600 )edit

It is not an opencv message. May be CV_CAP_PROP_GAIN cannot be set on your camera...

LBerger ( 2017-01-25 03:05:58 -0600 )edit

I am guessing the second answer of @LBerger is right. Your camera has to support the programmatically setting of these parameters, and OpenCV must be using a camera interface that supports the set function. So please provide us your CMAKE output (to know which interfaces are enabled) and the type of your camera.

StevenPuttemans ( 2017-01-25 03:37:30 -0600 )edit

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answered 2017-01-25 03:35:44 -0600

pklab

4284 ●7 ●30 ●60

from 3.2 doc VideoCapture::set()

Even if it returns true this doesn't ensure that the property value has been accepted by the capture device

VideoCapture::get()

Reading / writing properties involves many layers. Some unexpected result might happens along this chain.

VideoCapture -> API Backend -> Operating System -> Device Driver -> Device Hardware

...Effective behaviour depends from device driver and API Backend

BRIGHTNESS, GAIN, EXPOSURE often doesn't works

in your case, you are working with Video4Linux backend using User Controls. Even if V4L2_CID_GAIN is integer, OpenCV normalizes/translates the accepted range to 0..1 as 0%..100% of accepted value by V4L thus @LBerger comment is right.

But maybe the camera doesn't accept the control, try if changing the GAIN using the v4l command line utils it works

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how can I set VideoCapture attributes with set?

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