png2svg

png2svg: geometry-first PNG -> SVG reconstruction

You are the editor in this loop. Deterministic commands measure, render, and score; you make the design judgments; shape decomposition, layering, paint model; and iterate until the metrics converge.

This is reconstruction, not tracing. The output is the handful of shapes a designer would have drawn, not a contour fitted to pixels.

The visual loop is deliberate. check renders the current SVG and writes a 50/50 reference/render overlay.png; inspect that together with the edge and colour residuals, change the decomposition or constraints, and repeat. The library makes measurements reproducible. It cannot decide whether a meeting of two edges is a fillet, an overlap, or a coincidence.

0. Triage; is this image suitable?

Look at the image first. Proceed when it reads as designer-built geometry: flat or gradient fills, crisp edges, shapes decomposable into lines, arcs and Béziers.

Rule of thumb: if it could have been built in Figma from shapes and gradients, it is recoverable. There are three outcomes, and picking the right one up front decides which targets you are aiming at:

The middle case is the one to get right rather than refuse. A textured source disagrees with itself by more than any vector can match, so per-pixel ΔE will report failure for a reconstruction that reads correctly at a glance. check reports texture_std; around 1-2 for clean vector art, 10+ for a textured render. When it is high, reconstruct the structure and the large-scale shading, drop grain and fine detail deliberately, say so in the model notes, and judge by the low-frequency figures.

For the third case say so plainly and suggest vtracer or potrace, which do the different job of fitting contours to pixels.

1. Setup

Everything runs through one entry point: scripts/png2svg_cli.py in this skill's directory. On first run uv builds a cached environment from the script's own dependency block; there is nothing to install.

Check for uv before starting:

command -v uv
uv --version

If uv is missing, stop and ask the user to install it from https://docs.astral.sh/uv/getting-started/installation/. Do not require a separate Python installation; uv provisions a compatible interpreter.

Below, $SKILL stands for this skill's directory (the folder containing this file). Substitute the literal path: each command may run in a fresh shell, so an assignment made in one will not survive to the next.

uv run --no-project "$SKILL/scripts/png2svg_cli.py" init INPUT.png --project work/<name>
uv run --no-project "$SKILL/scripts/png2svg_cli.py" analyse work/<name>

init copies the source and estimates the background from a 2px border median. analyse writes analysis/features.json and analysis/overlay.png: components with bboxes, boundary segments (line / arc / corner / curve with fitted params and errors), and a paint probe per component:

• flat; solid colour
• linear; gradient, dominant direction given
• angular; conic sweep, centre given
• complex; layered or occluded paint; your judgment needed

Each component also carries a structure block, and analyse prints its hint line if it found anything: repeated spacings between parallel edges, exact 180°/mirror symmetry with its centre, and corners far tighter than the shape's usual radius. Read that line first; it is the fastest available answer to the route question in §2, and every number in it is one you would otherwise derive by hand. A repeated spacing or a tight corner means overlapping primitives; a symmetry centre means parameters you can delete rather than fit.

It reports only what it measured. Silence means nothing was found, not that the mark is simple.

Treat every coordinate in features.json as a ±0.5px proposal, and read overlay.png next to the source before deciding anything. analyse is fooled by watermarks and compression artefacts.

2. Decompose; and pick your route

Decide the structure before measuring anything: how many overlapping primitives, what occludes what, where the paint seams are.

A hard colour boundary inside one silhouette usually means two overlapping shapes; the seam is the top shape's edge, not a gradient stop, and it often passes through an arc centre. Getting this right is most of the work; everything downstream is arithmetic.

This decision also picks which of two routes you measure with. They are siblings, not a default and a fallback; reaching for the wrong one costs you either a pile of nodes or an afternoon:

analyse's structure hint checks the first two of these for you. Three tells, any one of which is enough:

• A corner that is an intersection, not a fillet. Walk the silhouette and

ask of each corner: did a designer round this, or did it appear because two shapes crossed? Sharp spikes and tiny radii next to generous ones mean crossings.

• Coincidences at a distance. Two edges collinear but far apart; a spacing

that repeats; 180°/mirror symmetry across the whole mark. Separate shapes do not accidentally line up; that is one primitive, duplicated.

• A colour region that is exactly an overlap. Three bands where the middle

one is the intersection of the outer two.

Tracing in the second case is not merely bigger, it throws the structure away: the constraints that made the artwork regular are exactly what a contour fit cannot see.

3a. Measure by tracing

Copy scripts/measure_template.py next to your work and edit the marked sections; it carries the whole pipeline and writes project.json directly. (scripts/build_template.py is the split-file variant, worth it only when hand measurement dominates and you want analysis/measurements.json as a separate artefact.)

For each region you decided on above:

C = subpixel_contour(field, region)            # ordered subpixel boundary
prims = segment_outline(C, tol=0.4)            # lines / arcs / cubics
prims, notes = snap_outline(prims, contour=C)  # constraints, each verified
segments = to_segments(prims)                  # model path segments

That is a complete outline in four calls, and on a clean logo it lands within a fifth of a pixel. Read notes; it says which constraints were kept and which were rejected for making the fit worse, which tells you how the shape is built.

subpixel_contour treats its offset as a maximum and finds the connected background gap for each ray. A nearby component can no longer become the scan start and pull the contour across the gap.

tol is the one dial. It trades segment count against deviation; sweep it (0.25 to 0.6 suits most sources) and take the knee. If the fit wants far more segments than the artwork plausibly has, the tolerance is chasing noise. On a textured source, set it near the texture scale; fitting tighter than the grain fits the grain.

3b. Measure by fitting primitives

Declare the decomposition as a function of a parameter vector and solve for the parameters against every contour point at once:

def build(p):                                  # -> [(vertices, radius), ...]
    ang, cx, cy, a, b, g, k, r = p
    ...                                        # your construction, in code
    return [(rect_1, r), (rect_2, r), (rect_3, r)]

fit = primitives.fit_union(contour, build, p0)  # signed distance -> zero
print(fit.summary())                            # mean/rms/p95/max, in pixels
d = primitives.paths(build(fit.params))         # emit the exact fitted shapes

The residual is a real distance in pixels, so it reads straight against the edge targets in section 5. A radius may be one value or one value per corner. The fitter, raster mask, ink bounds and path emitter all use that same representation, so mixed corner styles cannot drift between stages.

Let the symmetry remove parameters. This is where the route pays. On one mark, three rounded parallelograms with 180° symmetry came to eight numbers; and the offset between primitives and each one's width were consequences of the symmetry rather than things to fit. Eight numbers, mean residual 0.08px, against ~40 nodes for the traced equivalent. Every parameter you can derive instead of fit is one that was absorbing noise.

Use fit_union(..., trim=0.05) when the union has sharp spikes: rasterising a narrow tip rounds it, so those points sit ~1px inside the model through no fault of the parameters. Check fit.worst_points() before believing a trim; it reports original contour coordinates as well as residuals. Trimmed points anywhere other than a tip mean the decomposition is wrong, not the data.

3c. Then measure by hand what no fitter can know

A tangency, a shared centre, a radius that is exactly a half-width: these are design decisions, and each one you confirm removes a free parameter. This is where the reconstruction goes from good to exact.

Read [references/conventions.md](references/conventions.md) before writing any hand measurement. The two that cost the most hours:

• Scan rays must start in background and run into the shape. A ray

starting inside the foreground returns None. A light counter inside dark ink is background too; its rays run outward from the interior.

• Never trust a traced vertex. Sample each edge, fit a line, and take

vertices from the intersections.

Constrain before you fit: a round cap between parallel sides has radius exactly the half-width, and a bowl meeting a flat edge is tangent to it. When a constrained fit beats the free one, the constraint was the designer's.

3d. Paint

fit_linear_gradient(rgb, region, trim=0.12)   # axis, stop positions, colours
flat_colour(rgb, region)                       # median, ignores overlays

Pass trim whenever anything is painted on top of the fill; a shadow where a shape crosses itself, a glow, a watermark. Without it the overlay drags the whole fit and the paint looks like something exotic; with it the same paint reads as the plain two-stop ramp it is. Raise the trimmed pixels as their own shape rather than pretending they belong to the gradient.

A ring cannot carry a linear gradient; a ramp fitted across one runs from one side, through the hole in the middle, to the other. Rings want their median colour, or a shape-following paint.

SVG has no native gradient-along-path fill. For paint that follows a bent letter or ribbon, split the travel into straight and turning pieces: use linear paints on the straight runs and conic paints on the turns. Give every piece a span of one global 0..1 ramp:

top = map_ramp(top_linear, 0.24, 0.00, ramp)
left_turn = map_ramp(left_conic, 0.24, 0.45, ramp)
middle = map_ramp(middle_linear, 0.45, 0.64, ramp)

map_ramp emits only the global knots that fall inside each span, including reversed runs and direction-aware hard stops at piece boundaries. That keeps colours continuous through turns without copying a dense set of sampled stops into every piece.

4. Model

Keep the model a pure function of the measurements; no hand-typed coordinates; so that changing one measurement moves the whole model consistently, and a rerun reproduces it exactly.

Schema, paint types, conic-gradient compilation and the winding rule for counters: references/model.md.

5. Iterate; the core loop

uv run --no-project "$SKILL/scripts/png2svg_cli.py" check work/<name> --label r1
uv run --no-project "$SKILL/scripts/png2svg_cli.py" residuals work/<name> --label r1

check writes comparisons/r1/: reference.png, render.png, overlay.png, difference.png (4x gain), deltaE.png (CIEDE2000 heatmap), edge-difference.png (red = reference boundary, green = render, white = coincident) and metrics.json.

Read the numbers, then look at the images. residuals clusters colour errors and edge misses into bboxes; fix the model where the clusters are, bump the label, repeat.

Targets: IoU ≥ 0.995, edge mean ≲ 0.2px, edge max ≤ ~1.4px, ΔE2000 mean ≤ 3.0, p95 ≤ 8.0. A well-converged simple logo reaches IoU ~0.997, edge mean under 0.15, ΔE mean under 1.

On a textured source those colour targets are unreachable by anything. switch to deltaE_lowfreq_mean/p95 and accept an IoU nearer 0.93. Check texture_std before concluding a reconstruction failed.

Know the noise floors (conventions.md); resampling ringing, renderer quarter-pixel quantisation and single-pixel edge quantisation are not yours to fix. Judge by p95, never by max. Before rebuilding a model that scores badly, confirm the fit is actually bad: measured residuals of a fifth of a pixel alongside a poor score means the instrument, not the model.

Keep the model small. Prefer one measured arc over many fitted nodes. A higher pixel score never justifies path soup: if a change adds nodes or layers for less than 0.1 IoU, revert it.

6. Finish

uv run --no-project "$SKILL/scripts/png2svg_cli.py" validate work/<name>
uv run --no-project "$SKILL/scripts/png2svg_cli.py" export work/<name> -o out.svg
uv run --no-project "$SKILL/scripts/png2svg_cli.py" export work/<name> -o animated.svg --profile animation
uv run --no-project "$SKILL/scripts/png2svg_cli.py" export work/<name> -o cropped.svg --tight --padding 2

validate must pass everything and reports bytes and element counts for all three export profiles. alpha_mid_fraction must decrease across 1x / 4x / 16x; that is the operational definition of geometrically sharp. Export refuses raster, script and external references.

export defaults to compact: no authoring ids, no unnecessary width/height, compact paths and whitespace. semantic keeps stable logical shape ids. animation wraps every logical model shape in <g id="shape-id">, so a fill stack and stroke animate as one unit. --tight solves exact fill bounds and safe stroke bounds, then changes only the viewBox. No raster crop or hand edit is involved.

Colour variants

CLI="$SKILL/scripts/png2svg_cli.py"
uv run --no-project "$CLI" recolor work/<name> -o work/<name>-alt --rotate 140
uv run --no-project "$CLI" recolor work/<name> -o work/<name>-alt --map "#294952=#4a2952,#94d49a=#d4a394"
uv run --no-project "$CLI" build work/<name>-alt
uv run --no-project "$CLI" export work/<name>-alt -o alt.svg

--map applies each colour's nearest anchor's Lab delta, so sampled gradient stops move coherently with their anchor. Geometry is untouched and seams stay matched because the transform is uniform.

Reference material

• references/conventions.md; the measurement

rules and the noise floors. Read before measuring.

• references/model.md; project.json schema, paint

types, conic wedge compilation, validation checks.

• references/examples.md; three complete

reconstructions with the reasoning that got there.