IntentumDiff — Perceptual asset diff

For images (and, on the roadmap, other binary media), IntentumDiff shows a perceptual comparison instead of a text diff. The hard architectural rule: all decode/compute happens in the Rust engine; the webview does no image processing — it renders engine-produced PNG artifacts and draws lightweight SVG/CSS overlays on top.

Engine side (crates/rust-core-host/src/asset_diff.rs)

diff_asset_image_value (and the git-driven diff_git_assets) produce, for a before/after pair, all of:

• Metrics: MAE, RMSE, pixels-changed.
• Layers/artifacts (PNG): before, after, diff, mask, overlay, heatmap.
• hotspots: changed regions with pixel bbox, normalized centroid, and severity; optional hotspot_navigation.order.
• histograms: 16-bin per-channel (R/G/B/brightness, +alpha) delta arrays.
• comparison_dimensions: { width, height } — the reference frame for normalizing hotspot bboxes into the viewer.

Decode cost is bounded (max_decoded_pixels) so large/adversarial media fails fast. Never send media to a cloud LLM — asset diff is fully on-device.

Webview side (plugins/vscode/src/reviewWebviewModel.ts)

assetModeViewer(...) renders four modes and the overlays. All are pure DOM/CSS/SVG:

• Side-by-side / Onion (opacity + blink) / Swipe (curtain) / Difference (heatmap).
• Lasso (marching ants): an SVG rect per hotspot with animated stroke-dashoffset, drawn over both panes so the same change is locatable on LHS and RHS. Rectangles come from hotspot.bbox normalized by comparison_dimensions (left=x/W, top=y/H, …).
• Blink comparator: setInterval toggling the onion top layer opacity 0↔1.
• Interactive hotspots: list items + on-image markers are clickable; selection pulses the lasso, dims others; prev/next + arrow keys cycle; uses hotspot_navigation.order when present.
• Histograms: per-channel SVG bar charts from the delta arrays (theme-tinted).

Pure helpers to reuse: assetLasso(hotspots, compDims, options), assetHistogramBars(histograms), assetHotspot, assetMarker. Client interactions follow the existing setAssetMode / setAssetOpacity delegation pattern (no round-trip to the engine).

Threading gotcha: comparison_dimensions must be parsed in assetDiffFromMetadata and threaded from the call site into assetModeViewer — the model historically dropped it, which breaks overlay alignment. Wrap each <img> in an inline-block box sized to the image so inset:0 overlays align over object-fit:contain.

Routing & lifecycle

Image-extension files route to asset review instead of the text semantic diff (isImageLikePath, and the engine's content_type category). Clicking an image's Evidence must open the asset viewer, not an intentumdiff-base: text diff.

Performance

The perceptual pipeline is compute-heavy (PNG deflate encoding + per-pixel passes). It is ~20–50× slower on a debug core — always build with maturin develop --release (a 1050×700 image drops from ~6 s to ~0.3 s). The VS Code compare is also non-blocking: the preview shows instantly and upgrades when the engine result is ready. See intentumdiff-architecture (build) and docs/PERCEPTUAL_ASSET_DIFF.md.

Roadmap: video / audio / other binary (docs/BACKLOG.md)

Extend the pattern by content-type routing, keeping all decode/compute in Rust and on-device:

• Video: sample keyframes, run the per-frame image pipeline, add a timeline scrubber with changed-frame markers (needs a vetted ffmpeg-backed decode dependency).
• Audio: waveform + spectrogram comparison (prefer pure-Rust symphonia + rustfft).
• Other binary: a routed fallback (size/format delta) for formats with no perceptual comparator. Bound decode cost like the image max_decoded_pixels cap.

Tests / verify

test/reviewWebviewModel.test.ts asserts the Swipe tab, blink controls, lasso <svg> with per-hotspot data-asset-hotspot, histogram <svg> bars, and that the panel script executes. For interaction, use the panel-render harness (see intentumdiff-vscode) with a real image injected as every artifact, served for the Claude Preview MCP; resize narrow (~760px) to confirm no overflow regression. Rust side: cargo test -p rust-core-host.