Frame output keys¶

Every instrument publishes each acquired frame to zero or more outputs, built at startup from these keys. The two outputs are independent: either, both or neither can be enabled.

FITS¶

Writes one FITS file per frame. A queue and a dedicated writer thread keep the readout thread from ever blocking on disk.

Key	Default	Meaning
`FITS_ENABLED`	`no`	Enable the FITS writer
`FITS_OUTPUT_DIR`	`/tmp/images`	Base directory. Must already exist.
`FITS_AUTODIR`	`no`	Write into a `YYYYMMDD` subdirectory of `FITS_OUTPUT_DIR`
`FITS_BASENAME`	`tracking`	Base filename
`FITS_QUEUE_SIZE`	`32`	Frames buffered for the writer thread. The oldest is dropped when full.
`FITS_DRAIN_TIMEOUT_MS`	`5000`	How long to keep draining the queue at shutdown before giving up

Warning

Dropping frames is the designed behaviour, not a failure mode: disk is slower than acquisition can be, and stalling acquisition to wait for a write would be worse. If you need every frame, size FITS_QUEUE_SIZE for the burst and watch the dropped count in output_status().

Shared memory¶

Publishes each frame as an ImageStreamIO stream, which AO frameworks such as cacao read directly. Needs a build with -DENABLE_SHM_OUTPUT=ON.

Key	Default	Meaning
`SHM_ENABLED`	`no`	Enable the shared-memory writer
`SHM_SEGMENT_NAME`	`camera`	ImageStreamIO stream name
`SHM_RING_BUFFER_SIZE`	`4`	Depth of the internal history ring buffer (`CBsize`). Separate from the live frame a real-time reader sees.
`SHM_DIR`	unset	Directory ImageStreamIO writes into. Unset falls back to `MILK_SHM_DIR`, then `/milk/shm`. If set, it must exist and be writable.

Frame geometry is deliberately not a key. An ImageStreamIO stream’s geometry is fixed for its lifetime, so the writer recreates the stream whenever the geometry changes from what is allocated.

Unlike the FITS writer, this output posts a semaphore per frame, so it is the right attachment point for anything that has to wake up on every frame. The semaphores live inside the segment, so a reader opens it read-write; segments are created 0666 so any local user can attach.

Readers¶

camerad-shm-reader prints geometry, keywords and pixel statistics once, for diagnostics:

camerad-shm-reader <segment-name> [shm-dir]

python/examples/shm_read_frames.py is a streaming consumer that blocks on the semaphore and flags frames it missed:

python python/examples/shm_read_frames.py --segment camera --count 10

It needs numpy and ImageStreamIOWrap, the Python wrapper from the ImageStreamIO source tree built with -DPYTHON_WRAPPER=ON. The wrapper is not on PyPI.