Files
hyperframes/packages/producer
Vance Ingalls a21a62b574 feat(engine): add HDR two-pass compositing — DOM layer + native HLG video (#288)
## Summary

Compositions with HDR video AND DOM overlays (text, graphics, SDR video) couldn't render both correctly — either HDR data was lost (Chrome captures sRGB only) or DOM overlays were missing (FFmpeg pass-through skips Chrome). This PR adds in-memory alpha compositing that combines both.

## What it does

**Per-frame two-pass capture:**
1. **DOM pass** — Chrome screenshots the page with a transparent background (CDP alpha). HDR videos are hidden, leaving transparent holes where they go.
2. **HDR pass** — Pre-extracted native HLG/PQ frames (16-bit PNG from FFmpeg) are read from disk.
3. **Composite** — DOM pixels (sRGB RGBA8) are alpha-composited over HDR pixels (rgb48le) in Node.js memory, with sRGB→HLG/PQ conversion via a 256-entry lookup table.

**Key components:**
- `decodePng()` / `decodePngToRgb48le()` — Pure Node.js PNG decoders (no native dependencies). Support all 5 PNG filter types.
- `blitRgba8OverRgb48le()` — Alpha composite with per-pixel sRGB→HDR LUT conversion. Fast paths for alpha=0 (skip) and alpha=255 (overwrite).
- `initTransparentBackground()` + `captureAlphaPng()` — Split CDP transparent background setup (once) from per-frame screenshot capture (eliminates 2 CDP round-trips per frame).
- Single-pass FFmpeg extraction — All HDR frames extracted in one sequential FFmpeg run (avoids duplicate frames from per-frame `-ss` fast seek).

## Key design decisions

| Decision | Why |
|----------|-----|
| In-memory compositing (not FFmpeg overlay) | Eliminates ~2400 process spawns + temp files per render. Pure pixel math is 10x faster. |
| 16-bit PNG intermediate | Raw `-f rawvideo` loses color metadata, causing moiré artifacts. PNG is self-describing. |
| sRGB→HLG LUT (256 entries) | DOM content is sRGB. Without conversion, it appears orange-shifted in HLG stream. |
| Native HDR detection before extraction | `extractAllVideoFrames` converts SDR→HDR. Pre-extraction probe identifies original HDR sources so only truly-HDR videos get native extraction. |

## Files changed

| File | What changed |
|------|-------------|
| `packages/engine/src/utils/alphaBlit.ts` | **NEW** — PNG decode, sRGB→HDR LUT, alpha compositing (14 tests) |
| `packages/engine/src/services/screenshotService.ts` | Transparent background CDP, `captureAlphaPng()` |
| `packages/engine/src/services/videoFrameInjector.ts` | `hideVideoElements()` / `showVideoElements()` |
| `packages/engine/src/services/streamingEncoder.ts` | Input color space tags for rgb48le |
| `packages/producer/src/services/renderOrchestrator.ts` | Two-pass HDR capture loop, native HDR detection |

## How to test

Render a composition with an HDR video background and text overlays. Both should be visible — HDR video at full quality, text crisp with correct colors (not orange-shifted).

## Stack position

**3 of 6** — Stacked on #265 (HDR output pipeline). This is the foundation for all layered compositing that follows.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
2026-04-19 16:26:54 -07:00
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@hyperframes/producer

Full HTML-to-video rendering pipeline: capture frames with Chrome's BeginFrame API, encode with FFmpeg, mix audio — all in one call.

Install

npm install @hyperframes/producer

Requirements: Node.js >= 22, Chrome/Chromium (auto-downloaded), FFmpeg

Usage

Render a video

import { createRenderJob, executeRenderJob } from "@hyperframes/producer";

const job = createRenderJob({
  inputPath: "./my-composition.html",
  outputPath: "./output.mp4",
  width: 1920,
  height: 1080,
  fps: 30,
});

const result = await executeRenderJob(job, (progress) => {
  console.log(`${Math.round(progress.percent * 100)}%`);
});

console.log(result.outputPath); // ./output.mp4

Run as an HTTP server

The producer can also run as a render server, accepting render requests over HTTP:

import { startServer } from "@hyperframes/producer";

await startServer({ port: 8080 });
// POST /render with a RenderConfig body

Configuration

RenderConfig controls the render pipeline:

Option Default Description
inputPath Path to the HTML composition
outputPath Output video file path
width 1920 Frame width in pixels
height 1080 Frame height in pixels
fps 30 Frames per second (24, 30, or 60)
quality "standard" Encoder preset ("draft", "standard", "high")

How it works

  1. Serve — spins up a local file server for the HTML composition
  2. Capture — opens the page in headless Chrome, seeks frame-by-frame via HeadlessExperimental.beginFrame, captures screenshots
  3. Encode — pipes frames through FFmpeg (with GPU encoder detection and chunked concat)
  4. Mix — extracts <audio> elements and mixes them into the final video
  5. Finalize — applies faststart for streaming-friendly MP4

Documentation

Full documentation: hyperframes.heygen.com/packages/producer