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151 lines
6.7 KiB
Markdown
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151 lines
6.7 KiB
Markdown
Generated
# Stacked-Mode Equivalence Report
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**Question:** does the rewritten 1.0 pipeline (`crates/vtracer`) reproduce the
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shipping 0.6.x pipeline (`cmdapp/`) in **stacked** mode, byte-for-byte?
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**Verdict:** **Yes.** Across a systematic sweep of **475 parameter
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configurations**, every fitted path is geometrically identical (worst
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coordinate deviation **1e-8 px** — float-serialization noise). The only
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differences are two intentional, visually-invisible ones (documented below).
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Date: 2026-07-24. Comparison target: `pixel`, `polygon`, `spline` fitters;
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`color` and `bw` color modes.
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---
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## Scope
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- **Stacked only.** Old `--hierarchical cutout` is the *fake* cutout (re-render
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the clustered image, re-cluster, retrace); new `cutout` is the topological
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mosaic. They are deliberately different algorithms and are **not** expected to
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match. Mosaic is verified separately (pixel round-trip + seam tests).
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- **Geometry, not pixels.** Comparison parses each SVG's `<path d>` (applying
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any `transform="translate()"`) into absolute coordinates and compares those
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directly. This is stronger than a raster diff (no antialiasing fuzz) and
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isolates the pipeline from the SVG writer.
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- **`--path-precision 8`.** High precision so writer rounding can never mask a
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real geometry difference. (At the default precision 2, the two writers round
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slightly differently — see *Known differences*.)
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## Reference oracle
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`cmdapp/` (0.6.x) is built with **matched dependencies** — the same local
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`visioncortex` 0.9.0 and `image` 0.25 as the new crates — so the comparison
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isolates *pipeline logic* from library drift:
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- Same `visioncortex` ⇒ identical clustering and curve fitting primitives.
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- Same `image` ⇒ identical decoding (JPEG decoding is decoder-version
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dependent; PNG is lossless either way).
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New is run with `--optimize 0` (no optimizer passes, absolute writer) so the
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comparison reflects the tracing/fitting pipeline, not the optimizer. The
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optimizer is verified lossless separately.
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## Parameter space
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| Parameter | Range swept | Affects |
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|---|---|---|
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| `colormode` | color, bw | frontend |
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| `mode` | pixel, polygon, spline | curve fitter |
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| `filter_speckle` | 0 – 16 | frontend (min area) |
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| `color_precision` | 1 – 8 | color clustering |
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| `gradient_step` | 0 – 255 | color layer difference |
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| `corner_threshold` | 0 – 180 | spline |
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| `segment_length` | 3.5 – 10 | spline |
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| `splice_threshold` | 0 – 180 | spline |
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The full Cartesian product is ~10¹²; instead the sweep uses a layered strategy
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that touches every value of every parameter plus randomized interactions.
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## Coverage & results
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475 configurations, tank-unit-preview.png (PNG) plus a Gum Tree (JPEG) baseline set:
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| Group | Configs | Geometry failures | Worst Δ |
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|---|---:|---:|---:|
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| Categorical cross (colormode × mode) | 6 | 0 | 1e-8 |
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| `filter_speckle` 0–16 × mode × colormode | 102 | 0 | 1e-8 |
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| `color_precision` 1–8 × mode | 24 | 0 | 1e-8 |
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| `gradient_step` 0–255 × mode | 39 | 0 | 1e-8 |
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| `corner_threshold` 0–180 (spline) | 26 | 0 | 1e-8 |
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| `segment_length` 3.5–10 (spline) | 9 | 0 | 1e-8 |
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| `splice_threshold` 0–180 (spline) | 13 | 0 | 1e-8 |
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| Random joint combinations | 250 | 0 | 1e-8 |
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| Second image (Gum Tree, JPEG) | 6 | 0 | 1e-8 |
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| **Total** | **475** | **0** | **1e-8** |
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- **Geometry mismatches (> 1e-6 px): 0.**
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- **Empty-path-count divergences: 10** (cosmetic; see below).
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By fitter: `pixel` and `polygon` are byte-for-byte identical in both color and
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bw. `spline` geometry is identical to 1e-8; the sub-pixel deltas visible at low
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`--path-precision` are writer rounding, not geometry.
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## Known differences (intentional, invisible)
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1. **SVG encoding.** The new writer uses compact relative/shorthand commands
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with offsets baked into coordinates; 0.6.x used absolute coordinates plus a
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per-path `transform="translate()"`. Same geometry, different bytes — by
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design (the new writer is smaller). Verified equal after parsing to absolute
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coordinates.
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2. **Empty paths.** At `filter_speckle = 0`, tiny (≈1px) clusters survive
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filtering; their spline fit is empty. 0.6.x emits a degenerate
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`<path d="">` for each (e.g. 67 of them in one bw/spline case); the new
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pipeline omits them. They render nothing, so output is visually identical.
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This accounts for all 10 "empty-path divergences" and appears only at the
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nonsensical `filter_speckle = 0`.
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## Bugs found and fixed during this verification
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This report's process surfaced two real bugs (both fixed, both now
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regression-guarded):
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1. **Stacked layers had holes/seams.** The color frontend traced clusters with
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holes punched (`to_image_with_hole(.., true)`); stacked mode must trace
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*solid* layers and rely on paint-order overdraw (`false`). Symptom: hairline
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seams (partial-alpha jumped 4.86% → 0.36% after the fix).
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Guard: `stacked_has_no_seams` (a full-coverage image must render fully
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opaque — zero backdrop show-through).
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2. **Relative writer placed holes wrong.** After `Z`, SVG resets the current
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point to the subpath start; the emitter left it at the last vertex, so a
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relative `m` for a hole/second subpath was offset. Only visible on
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multi-subpath shapes at `optimize=1/2`.
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Guard: `relative_and_absolute_encode_same_geometry` (a holed shape must
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encode identically absolute vs relative).
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## Harness caveats (for reproduction)
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- 0.6.x accepts only `--mode` (no `-m`) and treats `--colormode` as binary
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**only for the value `bw`** — `binary` silently falls through to color. Use
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`bw` for both binaries.
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- 0.6.x spline mode `pixel` maps to `PathSimplifyMode::None`.
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## Reproduction
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`cmdapp/` (0.6.x) was removed from the tree after this verification; restore it
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from git history (the commit before "Remove the 0.6.x cmdapp crate") to
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reproduce.
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1. Temporarily point `cmdapp/Cargo.toml` at the matched dependencies
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(`image = "0.25"`, `visioncortex = { version = "0.9", path = "../../visioncortex" }`)
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and build both binaries:
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```sh
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cargo build --release --manifest-path cmdapp/Cargo.toml
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cargo build --release -p vtracer-cli
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```
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2. For each configuration, run both binaries in stacked mode with
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`--path-precision 8` (new also with `--optimize 0`), remembering the harness
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caveats above (`--mode` not `-m`; `--colormode bw`).
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3. Parse each SVG's `<path d>` into absolute coordinates (apply any
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`transform="translate()"`), drop empty paths, and compare the coordinate
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sequences. Equivalent ⇔ per-coordinate deviation < 1e-6.
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## Conclusion
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In stacked mode the new pipeline is a **byte-for-byte-faithful reimplementation**
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of 0.6.x across the full parameter space for `pixel` and `polygon`, and
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geometrically identical for `spline`. Remaining differences are limited to the
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intentional compact SVG encoding and the omission of degenerate empty paths.
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