image-compare

Two image files in, a verdict out. No baselines, no manifest, no browser.

CLI binary is blazediff-cli (npm i -g @blazediff/cli).

Not this skill: capturing a running app, managing baselines, or re-running a visual regression suite → use the blazediff skill instead (it owns .blazediff/, blazediff-agent, and the check/rewrite loop). If the working directory has a .blazediff/manifest.json and the user is asking about routes rather than files, you are in the wrong skill.

Be terse

• Pass --json on every call; parse fields. Do not echo CLI output.
• Never send whole images to a model when interpret has located the regions — see Send crops, not pages below. This is the point of the skill.
• One final summary line: N regions | <severity> | <one-clause verdict>.

Pick the command by the question

The user is asking	Command	Read from it
"what changed?" / "describe the differences"	interpret	regions[] — bbox, changeType, position
"are these identical?"	core-native	different pixels (0 = byte-identical after decode)
"how different, perceptually?"	ssim	SSIM score, 1.0 = identical
"did quality degrade?" (compression, resize, generative output)	msssim	multi-scale score, 1.0 = identical
"where are the structural differences?"	gmsd -o map.png	deviation, 0.0 = identical

interpret is the default. The others answer how much; only interpret answers what.

blazediff-cli interpret a.png b.png --json

Exit codes: 0 no regions, 1 regions found, 2 error. Safe to branch on.

Picking --source

• pixel (default) — pixel-aligned renders: screenshots, PNG exports, chart renders, PDF rasterizations. Anything produced twice by the same renderer.
• ms-ssim / ssim — images with resampling or compression noise: JPEGs, camera captures, generative-model output, anything that went through a lossy round-trip. Per-pixel deltas are everywhere in these, so pixel returns one giant region; the metric sources threshold a similarity map instead. Their boxes are coarser (map-grid resolution), but the pixel counts inside them are still exact.

Add -a when the pair is text-heavy and the only differences are antialiasing fringes. Raise -t (default 0.1) when the renderer has known per-run jitter.

Reading the result

{
  "summary": "Moderate visual change detected (1.87% of image, 4 regions).",
  "severity": "medium",           // low | medium | high
  "diffCount": 30421,             // changed pixels
  "diffPercentage": 1.87,
  "totalRegions": 4,
  "width": 1328, "height": 1228,
  "regions": [{ "bbox": {...}, "changeType": "...", "position": "...", "shape": "...",
                "pixelCount": 0, "percentage": 0, "confidence": 0 /* + stats blocks */ }]
}

changeType — addition, deletion, content-change, color-change, shift, rendering-noise. position — top-left…bottom-right, top/bottom/left/right/center. shape — solid-region, mixed-region, contour-frame, sparse-distributed, edge-dominated.

Each region also carries chroma, colorDelta, gradient, shapeStats, and signals blocks. Do not put these in your context or forward them to a model — they are classifier inputs, not answers. Project each region down to {changeType, position, bbox, percentage} before you reason about it. The full JSON for a 4-region diff is ~7.5 KB; the projection is ~200 tokens.

Interpreting changeType:

• rendering-noise — subpixel/antialiasing jitter. Report as "no meaningful change" unless the user is specifically hunting rendering differences.
• color-change with signals.lowColorDelta and gradient.edgeCorrelation near 1.0 — same content, recoloured. Theme change, not content change.
• shift — content moved, not edited. Say what moved and by roughly how much; don't describe it as added and removed.
• addition / deletion / content-change — genuine content differences. These are the ones worth cropping and reading.

Send crops, not pages

When the user wants each change described (not just located), crop the regions and send only those. Never send the two full images.

Two reasons, and the second matters more than the first:

1. Cost. A pair of full-page screenshots is thousands of visual tokens, and nearly all of them show pixels that are identical in both images.
2. Legibility. Claude downscales any image that exceeds the model's limits. A full-page screenshot gets crushed by ~6×, which destroys the exact detail you are trying to compare — and images over 8000 px on an edge are rejected outright.

Measured on this repo's fixtures (Claude Opus 5, high-resolution tier: 2576 px max edge, 4784 max visual tokens; cost is ⌈w/28⌉ × ⌈h/28⌉ per image):

	A: fixtures/page — 3598×16384 page, 2 regions, 0.09% px	B: fixtures/blazediff/3 — 1328×1228, 4 regions, 1.87% px
Both full images, as-is	rejected — 16384 px exceeds the 8000 px limit	2112 ×2 = 4224 tokens
Both pre-resized to fit	566×2576 → 1932 ×2 = 3864 tokens — but the 711×104 changed region is now 112×16 px, and the 212×42 one is 33×7 px. Unreadable.	n/a, already fits
Crops + projected region JSON	408 + ~128 = 536	1178 + ~206 = 1384
	7.2× cheaper, at native resolution	3.1× cheaper (−67%)

The saving scales with how little of the image changed, which for real diffs is almost always "very little". Case A changed 0.09% of its pixels; sending both pages would have spent ~99.9% of the budget on identical content.

The crop step

Pad each bbox by 32 px so the change has surrounding context, clamp to the image bounds, and crop the same box from both images — the pair only reads as a before/after if the framing is identical.

# one region, both sides. x/y/w/h come from regions[i].bbox, padded and clamped.
node -e '
const sharp = require("sharp"), [src, out, x, y, w, h] = process.argv.slice(1);
sharp(src).extract({left:+x, top:+y, width:+w, height:+h}).toFile(out);
' "$SRC" "$OUT" "$X" "$Y" "$W" "$H"

sharp ships with @blazediff/cli via @blazediff/codec-sharp. If it does not resolve (global install, no local node_modules), fall back to sips -c on macOS or magick crop where ImageMagick is present.

Then send, per region, in one message: the region's projected JSON, its before crop, its after crop. Label them — Region 1 (content-change, bottom): before / after — so follow-up questions can name a region without resending anything.

When not to crop

Send the full (resized) pair instead when any of these hold:

• totalRegions is 0 — nothing to crop; answer from the metric score.
• The regions cover most of the frame (diffPercentage above ~25%, or one region spanning most of the bbox area). A full-frame rewrite is best read whole.
• Summed crop tokens exceed the full-pair cost. With many small scattered regions, per-crop 28-px padding overhead adds up — compute both, pick the smaller.
• The question is about layout or global composition rather than local content. Crops destroy spatial relationships between regions.

Answering

Lead with the verdict, then the regions, in the user's terms:

4 regions changed (medium).
1. content-change, bottom (590×479 at 356,651) — <what the crops show>
2. addition, right (95×154 at 992,614) — <what the crops show>
...

If every region is rendering-noise, say the images are visually equivalent and give the SSIM score as backing. Don't manufacture a difference to report.

Failure modes

• Different dimensions — interpret requires matching sizes. Say so and ask whether to resize or compare a common crop; don't silently resize, since that changes what "different" means.
• pixel returns one region covering everything — the pair has resampling or compression noise. Re-run with --source ms-ssim.
• Huge diffCount but the images look the same — check for an alpha or colour-profile difference; compare with ssim (structure-only) to confirm.
• gmsd output labels are inverted — the CLI prints (0=different, 1=identical) and a similarity: N% line, but GMSD is a deviation: identical images score 0.000000. Read the number as distance, and ignore the printed label.