JohannesItten
68 lines
2.7 KiB
Markdown
68 lines
2.7 KiB
Markdown
# DeckLink Output Node — Architecture Plan
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## What you'll build
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- `shared/DeckLinkSender.hpp` — analogous to `DeckLinkReceiver.hpp`
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- `nodes/decklinkout/main.cpp` + `CMakeLists.txt`
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## The key difference: scheduled playback
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DeckLink output uses a **pull model** — the card calls you when it needs frames,
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rather than you reading when a frame arrives.
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Steps:
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1. `EnableVideoOutput(mode, bmdVideoOutputFlagDefault)`
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2. `EnableAudioOutput(48kHz, int32, channels, bmdAudioOutputStreamContinuous)`
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3. Allocate a pool of ~3 `IDeckLinkMutableVideoFrame` objects via `CreateVideoFrame`
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4. Pre-fill and `ScheduleVideoFrame` those 3 frames to prime the pipeline
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5. `StartScheduledPlayback(0, fps_num, 1.0)`
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6. The card fires `ScheduledFrameCompleted` callback when a frame has been displayed —
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fill it with the next grain and re-schedule it
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Audio is pushed separately:
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`ScheduleAudioSamples(buf, count, stream_time, timescale, &written)` — no callback,
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just keep it full.
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## DeckLinkSender.hpp public API
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Use a condition variable the same way as the receiver, but inverted — the callback
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signals that a frame slot is free:
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```
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DeckLinkSender:
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start_output(device_index, width, height, fps_num, fps_den, channels)
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submit_frame(const uint8_t* src, uint32_t stride) // blocks until a slot is free
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submit_audio(const float* planar, int samples) // non-blocking push
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```
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## Implementation steps
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1. **`nodes/decklinkout/CMakeLists.txt`** — copy from `decklinkin`, rename target to `dmf-node-decklinkout`
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2. **`shared/DeckLinkSender.hpp`**
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- `enumerate_devices()` — identical to receiver
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- `start_output()`: `EnableVideoOutput` → `EnableAudioOutput` → allocate 3 frames →
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pre-fill with black → `StartScheduledPlayback`
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- `OutputCallback` (nested private class) implementing
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`IDeckLinkVideoOutputCallback::ScheduledFrameCompleted` — pushes freed frame back
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to a queue, signals a CV
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- `submit_frame()`: waits for a free frame from the queue, `memcpy` src → frame,
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call `ScheduleVideoFrame`
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- `submit_audio()`: convert float32 planar → int32 interleaved (reverse of
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receiver), then call `ScheduleAudioSamples`
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3. **`nodes/decklinkout/main.cpp`** — create MXL flow readers, detect format, create
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sender, loop reading grains and calling `submit_frame` / `submit_audio`
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## Gotcha: stream time
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The `stream_time` passed to `ScheduleVideoFrame` must be monotonically increasing in
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units of `fps_num` (the timescale passed to `StartScheduledPlayback`).
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Easiest approach: keep a counter and pass `frame_count * fps_den` as the stream time.
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```cpp
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ScheduleVideoFrame(frame, frame_count * fps_den, fps_den, fps_num);
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frame_count++;
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```
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