NI-IMAQ's imgSessionSaveBufferEx function does not save any PNG file as intended

[shiquan35]

int main()

{
	INTERFACE_ID interfaceID;

	SESSION_ID sessionID;

	unsigned int bitsPerPixel, plotFlag;

	if (imgInterfaceOpen("img0", &interfaceID) == IMG_ERR_GOOD) // "img0" is interfacename, opens an interface named img0

	{
		std::cout << "Interface is open" << std::endl;
		if (imgSessionOpen(interfaceID, &sessionID) == IMG_ERR_GOOD)

		{
			std::cout << "Session is open" << std::endl;

			// 30 sept 2021: try GRAB
			// 1. imgInterfaceOpen (DONE)
			// 2. imgSessionOpen (DONE)
			// 3. imgGrabSetup (DONE)
			// 4. imgSessionStartAcquisition (DONE)
			// 5. imgGrab (DONE)
			// 6. User specific functions
			// 7. imgSessionStop Acquisition (DONE)
			// 8. imgClose (DONE)

			//3. imgGrabSetup
			imgGrabSetup(sessionID, TRUE); //manually start acquisition with imgSessionStartAcquisition

			//4. imgSessionStartAcquisition
			imgSessionStartAcquisition(sessionID);

			//5. imgGrab (sessionID, buffer address, uint32 waitForNext)
			imgGrab(sessionID, NULL, TRUE);

			//6. imgSessionSaveBufferEx to save a buffer of a session to PNG
			imgSessionSaveBufferEx(sessionID, NULL, reinterpret_cast<Int8*>(const_cast<char*>("test.png")));
			std::cout << "Image saved" << std::endl;

			//7. imgSessionStop Acquisition
			imgSessionStopAcquisition(sessionID);
			std::cout << "Stop acquisition" << std::endl;

			imgClose(sessionID, FALSE);	// closes the session using the imgClose function
			std::cout << "Session is closed" << std::endl;

		}

		imgClose(interfaceID, FALSE);	// closes the interface using the imgClose function
		std::cout << "Interface is closed" << std::endl;

	}
}

I have a frame grabber PCIe-1437. I used to work with it using LABVIEW but now I'm trying to work with it using C++.

I'm exploring the function imgGrab() and I'm trying to see what kind of output I would get by obtaining a PNG file with imgSessionSaveBufferEx().

According to the NI-IMAQ function reference manual, I should get a PNG file. However, no such file was produced.

Also, my end goal is to produce an interference signal shown below.

Hence, my enquiries are these:

1. Did I misunderstand about how to implement imgGrab() and imgSessionSaveBufferEx() ?

2. Is there any advice on how can I achieve my goal?