Not every problem is about algorithms. Sometimes the camera just won't cooperate. While testing on a laptop, our C270 webcam started causing trouble in ways we didn't expect.

First, it kept changing its device number. The same camera would show up as camera 1, then silently move to camera 0, with no error and no warning. The wrong camera would open, and every position the system calculated would be wrong. We tried identifying the camera by name from the system's device list, but that list changed order within seconds. We tried its "unique ID" too, but that turned out to change depending on which USB port it was plugged into.

In the end, we went with the most reliable test of all: look at what each camera actually sees. We saved a snapshot from every camera index and checked them by eye. One showed our desk setup, one was the laptop's selfie camera, and one showed only black.

That black frame was our next mystery. The camera returned a completely black image every time, byte-for-byte identical. We ruled out other apps holding the camera, then wrote a diagnostic that tries different video formats and resolutions and prints each frame's brightness values, so we could tell a dead feed from a dark room.

We also learned that the paper needs to be kept away from anything else bright in the camera's view. Once, a monitor in the background merged with the paper, and the software couldn't find the page's edges.

Lesson learned: when hardware misbehaves, check what it's actually doing, and don't trust assumptions about what it should be doing.