If you are having trouble using multiple USB cameras with OpenPnP, make sure that you are using the OpenPnpCaptureCamera driver, and not OpenCvCamera. OpenPnpCaptureCamera may in some instances overcome the issues described below. For more information about correctly configuring the driver for multiple cameras, visit the OpenPnpCaptureCamera page. If you must use OpenCvCamera, continue reading for more information about why you might run into issues.

Put each camera on it's own USB host controller. This normally means that each camera needs it's own dedicated USB port but even that doesn't always solve the problem. If you look in your computer's USB device tree and all of your ports go back to one USB host controller you might be able to add an extra PCIe USB card, otherwise you will need a different computer.

Think of a USB bus like a hose connected to the faucet. You can only get as much water out as will flow through the faucet. Even if you hook a bunch of splitters to the end of the hose you aren't getting any more water. The faucet is the host controller. The hub is the splitter.

Most systems will be able to use a hub to run one camera and some other low bandwidth devices. For instance, running a Smoothieboard + one camera on a hub is known to work if you hook your second camera directly to the computer on another USB port.

Some USB cameras default to uncompressed YUV video mode, which is very high bandwidth. When the camera enumerates on the USB bus it tells the bus it needs something like 90% of the bandwidth, leaving no bandwidth for other cameras or devices. When you try to connect a second camera it will fail to enumerate or it will enumerate but then when you try to open it it will fail. This usually results in a black image.

If you have USB 3 specific cameras these should work. USB 2 cameras on a USB 3 port, or a USB 3 hub, will not help. USB 2 devices on a USB 3 bus share a USB 2 bus over the USB 3 cable, so they have the same limitation.

Most cameras on the market also support MJPEG compressed video. This is much lower bandwidth and you can have 2 or more cameras on a port. Unfortunately, OpenCV does not have a way to tell the camera to switch to MJPEG mode so it is stuck at YUV. By using the OpenPnpCaptureCamera instead you can get around this issue.

For some cameras even MJPEG does not help. They simply still use too much bandwidth. A typical example is the ELP Full HD camera, as opposed to the recommended ELP 720p camera. The FullHD model provides double the resolution (number of pixels) at the same 30fps or double the fps (60fps) at the same 720p resolution. It is logical that you need double the bandwidth.

ONVIF IP cameras have none of these limitations. If you use these you should be able to use as many as your network will support. Note: Links to some good ONVIF cameras here would be appreciated.