API reference/POST /mask

Alpha matte only

Returns the grayscale alpha matte — black background, white subject, soft gradient on edges. Drop into your own compositing pipeline (Photoshop, OpenCV, ffmpeg).

POSThttps://useknockout--api.modal.run/mask

Parameters

Send as multipart/form-data unless noted otherwise.

filerequiredfile—Image to process. JPG, PNG, WebP, HEIC. Up to 10 MB and 4096×4096.
formatstringpngOutput format. png or jpg.

Request

curl -X POST "https://useknockout--api.modal.run/mask" \
  -H "Authorization: Bearer $TOKEN" \
  -F "file=@cat.jpg" \
  -o out.png

Response

HTTP/1.1 200 OK
content-type: image/png
x-knockout-latency: 178

Errors

401unauthorized—Missing or invalid token.
402payment_required—Free tier exhausted. Add a card to continue.
413payload_too_large—Image exceeds 10 MB or 4096×4096.
422no_subject_detected—Foreground could not be isolated from background.
429rate_limit_exceeded—Slow down. Retry-After header tells you when.
500internal_error—Something broke on our side. Include request_id when reporting.
Every error response also includes a request_id in the JSON body. Quote it when reporting issues.

Notes

  • Use this when you want the matte but plan to do your own compositing — for example, applying a custom feather or threshold.
  • Output is single-channel grayscale. Some image libraries convert to RGB on load — explicitly read as L mode.
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