RenderThread
  owns a hidden OpenGL context
  renders simple BGRA8 motion at selected cadence
  queues async PBO readback
  publishes completed frames into SystemFrameExchange

SystemFrameExchange
  owns Free / Rendering / Completed / Scheduled slots
  drops old completed unscheduled frames when render needs space
  protects scheduled frames until DeckLink completion

DeckLinkOutputThread
  consumes completed system-memory frames
  schedules them into DeckLink up to target depth
  never renders

Startup warms up real rendered frames before DeckLink scheduled playback starts.

Current Scope

Included now:

output-only DeckLink
non-blocking startup when DeckLink output is unavailable
hidden render-thread-owned OpenGL context
simple smooth-motion renderer
BGRA8-only output
async PBO readback
latest-N system-memory frame exchange
rendered-frame warmup
background Slang compile of shaders/happy-accident
app-owned display/render layer model for shader build readiness
app-owned submission of a completed shader artifact
render-thread-owned runtime render scene for ready shader layers
shared-context GL prepare worker for runtime shader program compile/link
render-thread-only GL program swap once a prepared program is ready
manifest-driven stateless single-pass shader packages
manifest-driven stateless named-pass shader packages
atomic render-plan swap after every pass program is prepared
HTTP shader list populated from supported stateless full-frame shader packages
default float, vec2, color, boolean, enum, and trigger parameters
small JSON writer for future HTTP/WebSocket payloads
JSON serialization for cadence telemetry snapshots
background logging with log, warning, and error levels
local HTTP control server matching the OpenAPI route surface
startup config provider for config/runtime-host.json
quiet telemetry health monitor
non-GL frame-exchange tests

Intentionally not included yet:

DeckLink input
temporal/history/feedback shader storage
texture/LUT asset upload
text-parameter rasterization
runtime state
OSC/API control
preview
screenshots
persistence

Those features should be ported only after the cadence spine is stable.

V1 Feature Parity Checklist

This tracks parity with apps/LoopThroughWithOpenGLCompositing.

Stable DeckLink output cadence
BGRA8 system-memory output path
Render thread owns its primary GL context
Output startup warmup before scheduled playback
Non-blocking startup when DeckLink output is unavailable
Runtime shader package discovery
Background Slang shader compile
Shared-context GL shader/program preparation
Render-thread program swap at a frame boundary
Stateless single-pass shader rendering
Stateless named-pass shader rendering
Atomic multipass render-plan commit
Shader add/remove control path
Previous-layer texture handoff for stacked shaders
Supported shader list in HTTP/UI state
Local HTTP server
WebSocket state updates for the UI
OpenAPI document serving
Static control UI serving
Startup config loading from config/runtime-host.json
Cadence telemetry JSON
Health logging for schedule/drop/starvation events
DeckLink input capture
Input frame upload into the render scene
Live video input bound to gVideoInput
Temporal history buffers
Feedback buffers
Texture asset loading and upload
LUT asset loading and upload
Text parameter rasterization
Runtime parameter updates from controls
Layer reorder/bypass/set-shader/update-parameter/reset-parameter controls
Full runtime state store/read model
Persistent layer stack/config writes
OSC ingress
Preview output
Screenshot capture
External keying support
Full V1 health/runtime presentation model

Build

cmake --build --preset build-debug --target RenderCadenceCompositor -- /m:1

The executable is:

build\vs2022-x64-debug\Debug\RenderCadenceCompositor.exe

Run

Run from VS Code with:

Debug RenderCadenceCompositor

Or from a terminal:

build\vs2022-x64-debug\Debug\RenderCadenceCompositor.exe

Press Enter to stop.

To test a different compatible shader package:

build\vs2022-x64-debug\Debug\RenderCadenceCompositor.exe --shader solid-color

Use --no-shader to keep the simple motion fallback only.

Startup Config

On startup the app loads config/runtime-host.json through AppConfigProvider, then applies explicit CLI overrides.

Currently consumed fields:

serverPort
shaderLibrary
oscBindAddress
oscPort
oscSmoothing
inputVideoFormat
inputFrameRate
outputVideoFormat
outputFrameRate
autoReload
maxTemporalHistoryFrames
previewFps
enableExternalKeying

The loaded config is treated as a read-only startup snapshot. Subsystems that need config should receive this snapshot or a narrowed config struct from app orchestration; they should not reload files independently.

Supported CLI overrides:

--shader <shader-id>
--no-shader
--port <port>

Expected Telemetry

Startup, shutdown, shader-build, and render-thread event messages are written through the app logger. Telemetry is intentionally separate and remains a compact once-per-second cadence line.

The logger writes to the console, OutputDebugStringA, and logs/render-cadence-compositor.log by default. Render-thread log calls use the non-blocking path so diagnostics do not become cadence blockers.

HTTP Control Server

The app starts a local HTTP control server on http://127.0.0.1:8080 by default, searching nearby ports if that one is busy.

Current endpoints:

GET / and UI asset paths: serve the bundled control UI from ui/dist
GET /api/state: returns OpenAPI-shaped display data with cadence telemetry, supported shaders, output status, and a read-only current runtime layer
GET /ws: upgrades to a WebSocket and streams state snapshots when they change
GET /docs/openapi.yaml and GET /openapi.yaml: serves the OpenAPI document
GET /docs: serves Swagger UI
POST /api/layers/add, /remove, /reorder, /set-bypass, /set-shader, /update-parameter, and /reset-parameters use the shared runtime control-command path
other OpenAPI POST routes are present but return { "ok": false, "error": "Endpoint is not implemented in RenderCadenceCompositor yet." }

The HTTP server runs on its own thread. It serves static UI/docs files, samples/copies telemetry through callbacks, and translates POST bodies into runtime control commands. Command execution is app-owned, so future OSC ingress can create the same commands without depending on HTTP route code. Control commands may update the display layer model, start background shader builds, or publish an already-built render-layer snapshot, but they do not call render work or DeckLink scheduling directly.

Optional DeckLink Output

DeckLink output is an optional edge service in this app.

Startup order is:

start render thread
warm up rendered system-memory frames
try to attach DeckLink output
start telemetry and HTTP either way

If DeckLink discovery or output setup fails, the app logs a warning and continues running without starting the output scheduler or scheduled playback. This keeps render cadence, runtime shader testing, HTTP state, and logging available on machines without DeckLink hardware or drivers.

/api/state reports the output status in videoIO.statusMessage.

The app samples telemetry once per second.

Normal cadence samples are available through GET /api/state and are not printed to the console. The telemetry monitor only logs health events:

warning when DeckLink late/dropped-frame counters increase
warning when schedule failures increase
error when the app/DeckLink output buffer is starved

Healthy first-run signs:

visible DeckLink output is smooth
renderFps is close to the selected cadence
scheduleFps is close to the selected cadence after warmup
scheduled stays near 4
decklinkBuffered stays near 4 when available
late and dropped do not increase continuously
scheduleFailures does not increase
shaderCommitted becomes 1 after the background Happy Accident compile completes
shaderFailures remains 0

completedPollMisses means the DeckLink scheduling thread woke up before a completed frame was available. It is not a DeckLink playout underrun by itself. Treat it as healthy polling noise when scheduled, decklinkBuffered, late, dropped, and scheduleFailures remain stable.

Runtime Slang Shader Test

On startup the app begins compiling the selected shader package on a background thread owned by the app orchestration layer. The default is shaders/happy-accident.

The render thread keeps drawing the simple motion renderer while Slang compiles. It does not choose packages, launch Slang, or track build lifecycle. Once a completed shader artifact is published, the render-thread-owned runtime scene queues changed layers to a shared-context GL prepare worker. That worker compiles/links runtime shader programs off the cadence thread. The render thread only swaps in an already-prepared GL program at a frame boundary. If either the Slang build or GL preparation fails, the app keeps rendering the current renderer or simple motion fallback.

Current runtime shader support is deliberately limited to stateless full-frame packages:

one or more named passes
one sampled source input per pass
named intermediate outputs routed by the pass manifest
final visible output must be named layerOutput
no temporal history
no feedback storage
no texture/LUT assets yet
no text parameters yet
manifest defaults are used for parameters
the first layer receives a small fallback source texture until DeckLink input is added
stacked layers receive the previous ready layer output through both gVideoInput and gLayerInput

The /api/state shader list uses the same support rules as runtime shader compilation and reports only packages this app can run today. Unsupported manifest feature sets such as temporal, feedback, texture-backed, font-backed, or text-parameter shaders are hidden from the control UI for now.

Runtime shaders are exposed through RuntimeLayerModel as display layers with manifest parameter defaults. The model also records whether each layer has a render-ready artifact. Add/remove POST controls mutate this app-owned model and may start background shader builds.

When a layer becomes render-ready, the app publishes the ready render-layer snapshot to the render thread. The render thread owns the GL-side RuntimeRenderScene, diffs that snapshot at a frame boundary, queues new or changed pass programs to the shared-context prepare worker, swaps in a full prepared render plan only after every pass is ready, removes obsolete GL programs, and renders ready layers in order. Stacked stateless full-frame shaders render through internal ping-pong targets so each layer can sample the previous layer through gLayerInput; multipass shaders route named intermediate outputs through their manifest-declared pass inputs, and the final ready layer renders to the output target.

Successful handoff signs:

telemetry shows shaderCommitted=1
output changes from the simple motion pattern to the Happy Accident shader
render/schedule cadence remains near 60 fps during and after the handoff
DeckLink buffer remains stable

Baseline Result

Date: 2026-05-12

User-visible result:

output was smooth
DeckLink held a 4-frame buffer

Representative telemetry:

renderFps=59.9 scheduleFps=59.9 free=8 completed=0 scheduled=4 completedPollMisses=30 scheduleFailures=0 completions=720 late=0 dropped=0 decklinkBuffered=4 scheduleCallMs=1.2
renderFps=59.8 scheduleFps=59.8 free=7 completed=1 scheduled=4 completedPollMisses=36 scheduleFailures=0 completions=1080 late=0 dropped=0 decklinkBuffered=4 scheduleCallMs=4.7
renderFps=59.9 scheduleFps=59.9 free=7 completed=1 scheduled=4 completedPollMisses=86 scheduleFailures=0 completions=1381 late=0 dropped=0 decklinkBuffered=4 scheduleCallMs=2.1

Read:

render cadence and DeckLink schedule cadence both held roughly 60 fps
app scheduled depth stayed at 4
actual DeckLink buffered depth stayed at 4
no late frames, dropped frames, or schedule failures were observed
completed poll misses were benign because playout remained fully fed

Tests

cmake --build --preset build-debug --target RenderCadenceCompositorFrameExchangeTests -- /m:1
ctest --test-dir build\vs2022-x64-debug -C Debug -R RenderCadenceCompositorFrameExchangeTests --output-on-failure

Relationship To The Probe

apps/DeckLinkRenderCadenceProbe proved the timing model in one compact file.

This app keeps the same core behavior but splits it into modules that can grow:

frames/: system-memory handoff
platform/: COM/Win32/hidden GL context support
render/: cadence thread, clock, and simple renderer
render/readback/: PBO-backed BGRA8 readback and completed-frame publication
render/runtime/RuntimeRenderScene: render-thread-owned GL scene for ready runtime shader layers
render/runtime/RuntimeShaderPrepareWorker: shared-context runtime shader program compile/link worker
runtime/: app-owned shader layer readiness model, runtime Slang build bridge, and completed artifact handoff
control/: control action results and runtime-state JSON presentation
control/http/: local HTTP API, static UI serving, OpenAPI serving, and WebSocket updates
json/: compact JSON serialization helpers
video/: DeckLink output wrapper and scheduling thread
telemetry/: cadence telemetry
telemetry/TelemetryHealthMonitor: quiet health event logging from telemetry samples
app/: startup/shutdown orchestration
app/AppConfigProvider: startup config loading and CLI overrides

Next Porting Steps

Only after this app matches the probe's smooth output:

replace SimpleMotionRenderer with a render-scene interface
port shader package rendering
port runtime snapshots/live state
add control services
add preview/screenshot from system-memory frames
add DeckLink input as a CPU latest-frame mailbox