VideoCapture resolution fallback behavior across V4L2 and GStreamer backends

OpenCV's VideoCapture::set does not guarantee that the requested resolution will be honored. When a requested resolution is unsupported, the backend typically falls back to the nearest supported resolution or the device's default, often doing so silently without returning an error.

Detecting Resolution Mismatches

Since there is no backend-agnostic event or callback to notify resolution changes, the only reliable pattern to detect a mismatch is to manually query the device properties immediately after attempting to set them.

1. Open the capture device using cap.open().
2. Set the desired dimensions using cap.set(CAP_PROP_FRAME_WIDTH, width) and cap.set(CAP_PROP_FRAME_HEIGHT, height).
3. Wait a brief interval (milliseconds) to allow the driver/pipeline to complete negotiation or initialization.
4. Query the actual dimensions using cap.get(CAP_PROP_FRAME_WIDTH) and cap.get(CAP_PROP_FRAME_HEIGHT).
5. Compare the returned values against your requested variables.

Querying Supported Resolutions

There is no cross-platform API within OpenCV to enumerate supported resolutions. To discover valid values without writing platform-specific ioctl or GStreamer code, you must use system-level tools:

• V4L2 (Linux): Use v4l2-ctl -d /dev/video0 --list-formats- to see all supported pixel formats and resolutions.
• GStreamer: Use gst-inspect-1.0 vl2src to view the caps supported by the hardware source element.

Backend Differences

The behavior during the fallback depends heavily on how the underlying backend handles the request:

• V4L2: If the driver does not support dynamic resolution changes, set() may be ignored entirely, or the driver may select the closest hardware-supported mode.
• GStreamer: The backend uses caps negotiation. If you provide a resolution in a custom pipeline string, the pipeline will fail to start if the resolution is unavailable. If using standard CAP_PROP, GStreamer will negotiate the best compatible format based on available elements.

Note: Neither OpenCV backend exposes a callback or property change notification. Polling the properties is the only supported method.