Can't change OpenCV video capture resolution - c++

Question

The problem I am having is that I am unable to change the resolution of an OpenCV video capture. The resolution is always 640x480, no matter what. The code I'm using is written in C++ and I'm using opencv 3.4.8. I've created a super simple program with which to do this and it just doesn't seem to work no matter what I try.
Here is the code in its entirety:
#include "opencv2/opencv.hpp"
using namespace cv;
int main(int argc, char** argv)
{
VideoCapture cap(0);
cap.set(CAP_PROP_FRAME_HEIGHT, 1080);
cap.set(CAP_PROP_FRAME_WIDTH, 1920);
// open the default camera, use something different from 0 otherwise;
// Check VideoCapture documentation.
if (!cap.open(0))
return 0;
for (;;)
{
Mat frame;
cap.read(frame);
if (frame.empty()) break; // end of video stream
imshow("this is you, smile! :)", frame);
if (waitKey(10) == 27) break; // stop capturing by pressing ESC
}
// the camera will be closed automatically upon exit
// cap.close();
return 0;
}
When I run the above code frame is always 640x480.
I've tried changing the resolution with cap.set() to smaller and higher resolutions. I am using an ImageSource camera and I know that the resolutions I am attempting to use are supported by the camera and I can view video at those resolutions in another program.
I've tried using different cameras/webcams.
I've tried explicitly changing the backend API when I create the VideoCapture object - i.e. VideoCapture cap(0, CAP_DSHOW). I tried DSHOW, FFMPEG, IMAGES, etc.
I've tried running the same program on different computers.
The result is always the same 640x480 resolution.
Is there something simple I am missing? Every other post I can seem to find on SO just points toward using the cap.set() to change the width and height.

Answer 1

It depends on what your camera backend is. As the documentation says:
Each backend supports devices properties (cv::VideoCaptureProperties)
in a different way or might not support any property at all.
Also mentioned in this documentation:
Reading / writing properties involves many layers. Some unexpected
result might happens along this chain. Effective behaviour depends
from device hardware, driver and API Backend.
It seems your camera backend is not supported by OpenCV Video I/O module.
Note: I also met such kind of cameras, some of them different resolutions are working with different numbers. For example, you may catch desired resolution by trying VideoCaptur(-1) , VideoCapture(1) , VideoCapture(2) ...

Answer 2

Turns out the error was in the "if(!cap.open(0))" line that I was trying to use to check if cap had successfully initialized.
I was under the impression open was just returning true if the video capture object was open or false otherwise. But it actually releases the video capture object if it is already open and then it re-opens it.
Long story short that means that the cap.set() calls that I was using to change the resolution were being erased when the object was re-opened with cap.open(0). At which point the resolution was set back to the default of 640x480.
The method I was looking for is cap.isOpened(), which simply returns true or false if the object is open. A simple, silly mistake.