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Vance Ingalls 8548a17771 feat(hdr): GSAP transforms and border-radius masks on HDR video (#290)
## Summary

HDR video elements with GSAP animations (position, scale, rotation, opacity) and CSS border-radius rendered without any transforms applied — the video just sat at (0,0) full-size. This PR adds affine transform support and rounded-corner masking for natively-composited HDR video.

## What it does

**Affine blit with bilinear interpolation:**
- `blitRgb48leAffine()` — Takes a 4x4 DOMMatrix and maps each destination pixel back to source coordinates via the inverse transform. Bilinear interpolation between the 4 nearest source pixels produces smooth edges under rotation and non-integer scaling. Optional opacity and border-radius parameters.
- `parseTransformMatrix()` — Parses CSS `matrix(a,b,c,d,e,f)` strings into `[a,b,c,d,e,f]` tuples.

**Accumulated viewport matrix:**
- `getViewportMatrix()` — Walks the `offsetParent` chain from element to viewport, accumulating position offsets and CSS transforms at each level. Correctly handles `transform-origin` using the CSS sandwich: `translate(origin) × M × translate(-origin)`. This is critical because GSAP animates transforms on wrapper divs, not directly on the video element.

**Effective opacity:**
- `getEffectiveOpacity()` — Multiplies opacity values walking up the ancestor chain. Uses `Number.isNaN()` (not `|| 1`) so opacity:0 isn't incorrectly treated as 1.

**Border-radius masks:**
- `roundedRectAlpha()` — Per-pixel anti-aliased rounded-rectangle mask with support for independent corner radii.
- `getEffectiveBorderRadius()` — Walks ancestors for `overflow:hidden` + border-radius. Resolves percentage values (e.g., `50%` for circles) via `offsetWidth`/`offsetHeight`.

**Layout dimensions for extraction:**
- Uses `offsetWidth`/`offsetHeight` (unaffected by CSS transforms) instead of `getBoundingClientRect()` (which returns the transformed bounding box and wobbles under rotation).

## Files changed

| File | What changed |
|------|-------------|
| `packages/engine/src/utils/alphaBlit.ts` | `blitRgb48leAffine()`, `parseTransformMatrix()`, `roundedRectAlpha()`, `cornerAlpha()` |
| `packages/engine/src/services/videoFrameInjector.ts` | `getViewportMatrix()`, `getEffectiveOpacity()`, `getEffectiveBorderRadius()`, `layoutWidth`/`layoutHeight` on `ElementStackingInfo` |
| `packages/producer/src/services/renderOrchestrator.ts` | Affine blit path, extraction at layout dimensions, border-radius parameter passing |

## How to test

Render a composition with an HDR video that has GSAP scale + rotation animation and a `border-radius: 50%` wrapper (circle mask). The video should rotate smoothly with round edges — no wobble, no sharp corners.

## Stack position

**5 of 6** — Stacked on #289 (z-ordered layers). Adds transform and masking support to the HDR blit that the layer compositor uses.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
2026-04-19 17:03:29 -07:00
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2026-04-19 23:35:35 +00:00