Live view

Nikon live view over PTP is pulled, not streamed. The host asks for one frame at a time with GetLiveViewImageEx on the ordinary command channel. The parser is Sources/OpenZCineCore/PTPLiveViewObject.swift. Confirmed on the ZR; Gen-1 layout confirmed on a Z 7.

Start and stop

1. StartLiveView (0x9201).
2. Poll DeviceReady (0x90C8) until OK, about 50 ms apart.
3. Loop GetLiveViewImageEx (0x9428) as fast as each transaction completes. There is no target frame rate and no pacing sleep.
4. EndLiveView (0x9202) whenever the feed is completely hidden: Command mode or a full-screen panel. It resumes when the operator can see the feed.

If live view will not start, LiveViewProhibitionCondition (0xD1A4) says why. The first frame has a 10 s deadline and later frames 6 s.

Frame layout

Each LiveViewObject is a display-info header followed by a JPEG.

Body	Header	Notes
Gen 3 (ZR, Z 8, Z 9, Z 6III and peers)	1024 bytes	Confirmed on ZR
Gen 1 (Z 5, Z 6, Z 7, Z 50)	512 bytes	Confirmed on Z 7. No sound, record, rotation or level fields

The JPEG starts at the header length (FF D8 FF); an unknown body falls back to a bounded scan for the JPEG start. The header is big-endian, unlike the little-endian PTP containers around it.

Offset	Field	Notes
12	JPEG length	u32
16 / 18	Whole width / height	u16, the AF coordinate space (6048×3400 on the ZR)
42	Focus result	2 focused, 1 not focused
43	Subject detection active	1 when subject detection drives AF
44 / 45	Box count / selected box
46	Tracking active	1 during target tracking
48	AF boxes	[w, h, cx, cy] u16 each, 8 bytes per box. Box 0 is the AF area; with subject detection, then face and eye
824–827	Sound indicator	Peak L, peak R, current L, current R, 0…14 segments
828	Recording	0 live view, 1 recording. Catches a take started on the body
831–835	Timecode	On flag, then hours, minutes, seconds, frames
839	Rotation	0 landscape, 1 grip up, 2 grip down, 3 upside down [verify-on-HW polarity]
840 / 844 / 848	Roll / pitch / yaw	Signed 16.16 fixed point, degrees = value / 65536. FFFFFFFF means unreliable

The feed is the camera’s monitoring JPEG, not the recorded image. OpenZCine treats header state as authoritative for recording, timecode and level, which is why these are not polled separately during a take.

The sound indicator is metering only. PTP live view carries no program audio, and the body advertises no PTP streaming operations (feasibility note). Use the camera’s headphone jack.

Stream presets

Two camera-side bytes shape the stream. Both are remembered per setup, and the camera access point starts on Fast.

Setting	Property	Fast	Balanced	Quality
Stream Preset (frame size)	LiveViewImageSize 0xD1AC	1, up to about 320×240	2, up to about 640×480	3, up to about 1024×768
Quality Bias (compression)	LiveViewImageCompression 0xD1BC	0 Basic, size priority	3 Normal, quality priority	5 Fine, quality priority [verify-on-HW]

The app asks for a smaller frame, never a larger one than the preset, while recording (capped to VGA), when the phone is thermally stressed, or when the camera reports a warning. Command mode and HDMI capture only need the header, so they request the smallest size.

Polls between frames

Every property read and every GetEventEx is a round trip the next frame queues behind. LiveViewPollPacing scales the poll stride by the measured round trip, so a slow router path does not turn a fixed cadence into a periodic hitch. Body-announced changes (DevicePropChanged) keep a fast cadence; the background round-robin spreads out. During a take only battery, power and warning status are polled, storage every 15 s, and lens and mode descriptors every 60 s outside a take.

Watchdog and heat

LiveViewWatchdog declares a stall after 6 s without a good frame, a streak of unparsable frames, or the same JPEG repeating for 4 s. A wedged body can keep answering with one cached frame, and a live sensor never produces the same bytes twice. A stall restarts the stream through session recovery.

Under serious or critical phone thermal state the app slows only its own feed display and scope sampling (×1.5 and ×2). It never changes the camera’s recording resolution, codec or take.

One camera, many screens

A body serves one PTP-IP initiator. Other screens get the picture from the phone that holds the camera: Share This Feed re-encodes the monitor as HEVC (JPEG fallback) and advertises Bonjour _openzcine-mon._tcp. Control is proxied by the broadcaster, never transferred. See Share This Feed and the streaming architecture.