Endoscope, Borescope, Microscope, USB camera 2019

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Why the screen is black?
Let's say your gadget supports the OTG function and the USB camera is compatible with ANDROID. But the screen remains black.
Why? The reason may be in that you don't choice "Use by default for this USB device" (see the figure below).
This dialog box appears when you start the application with connected USB camera.
The appearance of this dialog box is guaranteed after the gadget is restarted.