# DeckLink Output Node — Architecture Plan

## What you'll build

- `shared/DeckLinkSender.hpp` — analogous to `DeckLinkReceiver.hpp`
- `nodes/decklinkout/main.cpp` + `CMakeLists.txt`

## The key difference: scheduled playback

DeckLink output uses a **pull model** — the card calls you when it needs frames,
rather than you reading when a frame arrives.

Steps:
1. `EnableVideoOutput(mode, bmdVideoOutputFlagDefault)`
2. `EnableAudioOutput(48kHz, int32, channels, bmdAudioOutputStreamContinuous)`
3. Allocate a pool of ~3 `IDeckLinkMutableVideoFrame` objects via `CreateVideoFrame`
4. Pre-fill and `ScheduleVideoFrame` those 3 frames to prime the pipeline
5. `StartScheduledPlayback(0, fps_num, 1.0)`
6. The card fires `ScheduledFrameCompleted` callback when a frame has been displayed —
   fill it with the next grain and re-schedule it

Audio is pushed separately:
`ScheduleAudioSamples(buf, count, stream_time, timescale, &written)` — no callback,
just keep it full.

## DeckLinkSender.hpp public API

Use a condition variable the same way as the receiver, but inverted — the callback
signals that a frame slot is free:

```
DeckLinkSender:
  start_output(device_index, width, height, fps_num, fps_den, channels)
  submit_frame(const uint8_t* src, uint32_t stride)   // blocks until a slot is free
  submit_audio(const float* planar, int samples)       // non-blocking push
```

## Implementation steps

1. **`nodes/decklinkout/CMakeLists.txt`** — copy from `decklinkin`, rename target to `dmf-node-decklinkout`

2. **`shared/DeckLinkSender.hpp`**
   - `enumerate_devices()` — identical to receiver
   - `start_output()`: `EnableVideoOutput` → `EnableAudioOutput` → allocate 3 frames →
     pre-fill with black → `StartScheduledPlayback`
   - `OutputCallback` (nested private class) implementing
     `IDeckLinkVideoOutputCallback::ScheduledFrameCompleted` — pushes freed frame back
     to a queue, signals a CV
   - `submit_frame()`: waits for a free frame from the queue, `memcpy` src → frame,
     call `ScheduleVideoFrame`
   - `submit_audio()`: convert float32 planar → int32 interleaved (reverse of
     receiver), then call `ScheduleAudioSamples`

3. **`nodes/decklinkout/main.cpp`** — create MXL flow readers, detect format, create
   sender, loop reading grains and calling `submit_frame` / `submit_audio`

## Gotcha: stream time

The `stream_time` passed to `ScheduleVideoFrame` must be monotonically increasing in
units of `fps_num` (the timescale passed to `StartScheduledPlayback`).

Easiest approach: keep a counter and pass `frame_count * fps_den` as the stream time.

```cpp
ScheduleVideoFrame(frame, frame_count * fps_den, fps_den, fps_num);
frame_count++;
```