Image align

POST /api/image-align3 quota units per call · cache hits free

Align a source image to a reference image using ORB keypoint matching and a RANSAC-fitted homography. Useful for registering scans, before/after photos, document captures, and image stacks.

Geometrically aligns the input `image` to match the layout of the supplied `reference`. Internally: 1. ORB keypoints + binary descriptors on both images. 2. Brute-force Hamming match, sorted by distance. 3. RANSAC-fitted homography from the top matches. 4. Warp `image` onto the reference's canvas. Useful for: - Registering scanned documents against a clean template. - Aligning before/after photos for visual diffing. - Stacking burst-shot photos before composing. When the two images have too little visual overlap (or the scene differs entirely), the homography fit fails and the tool returns 422 with a structured `code: no-homography` so callers can branch cleanly. ORB / BFMatcher availability is required — if missing from this build the tool returns 501.

Inputs

Name	Type	Default	Description
image*	file	—	Source image to be aligned.
reference*	file	—	Reference image whose layout `image` should match.
nfeatures	number (200…8000)	2000	ORB keypoint cap. Higher = slower + more robust.
matchRatio	number (0.05…1)	0.25	Fraction of best-distance matches kept for the homography fit.
format	enum (png \| jpeg)	"jpeg"	Output image format.

Response

Modes: binary, stream, json. Cache: yes (24h TTL).

Large outputs stream: responses over the 4 MiB cache limit are piped through as they are produced instead of buffered. They carry x-cache-skip: size and are never cached — identical follow-up calls recompute.

Every response carries x-cache (HIT / MISS / BYPASS), x-cache-signature (stable across identical inputs), and the rate-limit headers listed below.

Quota & limits

Cost per call	3 units against the 10,000-unit monthly quota. Cache hits are free — only cache misses decrement.
Burst limit	200 requests per rolling minute, shared across all tools.
Max request size	`25 MiB` (base64 inflates file payloads ~33% — the limit applies to the decoded bytes).
Max response size	`25 MiB`
Timeout	20s wall clock.
Rate-limit headers	`X-RateLimit-Limit`, `X-RateLimit-Remaining`, `X-RateLimit-Burst-Limit` on every response; `X-Quota-Warning` once 80% of the monthly quota is spent; `Retry-After` on 429s.

Errors

Errors are JSON with an error string; schema failures add a details object (Zod's flattened field errors).

Status	When	Body
400	Input fails the tool's schema (wrong type, missing required field, malformed base64) or the file bytes can't be parsed as the expected format.	{ "error": "invalid inputs", "details": { "fieldErrors": { … } } }
401	Missing or invalid X-Api-Key / X-Account-Id headers (or no session when called from the browser).	{ "error": "unauthorized" }
404	Unknown tool id, or calling /api/* on the website hostname instead of api.xtract.bot.	{ "error": "not found" }
413	Request body over the tool's max request size, or an image header declaring more megapixels than the tool's pixel budget.	{ "error": "…exceeds maxRequestBytes…" }
415	Content-Type isn't application/json on the json-body transport.	{ "error": "unsupported content-type; expected application/json" }
429	Monthly quota or the 200/min burst limit exhausted. Check Retry-After and the X-RateLimit-* headers.	{ "error": "monthly quota exceeded", "monthRemaining": 0, … }
5xx	Conversion engine failure. Safe to retry; if it persists, the input is hitting a bug — please report it.	{ "error": "…" }

Code samples

Built from the self-align example.

# Download or substitute the example input:
#   curl -O https://xtract.bot/examples/image-align/sample.png
IMAGE=$(base64 -w0 < sample.png)
# Download or substitute the example input:
#   curl -O https://xtract.bot/examples/image-align/sample.png
REFERENCE=$(base64 -w0 < sample.png)

curl -X POST https://api.xtract.bot/api/image-align \
  -H "Content-Type: application/json" \
  -H "Accept: application/octet-stream" \
  -H "X-Account-Id: $XTRACT_ACCOUNT_ID" \
  -H "X-Api-Key: $XTRACT_API_KEY" \
  -d '{
  "nfeatures": 2000,
  "matchRatio": 0.25,
  "format": "jpeg",
  "image": "'"$IMAGE"'",
  "reference": "'"$REFERENCE"'"
}'