Colour and background on digit captchas
2026-02-26 · Numeric captchas
Colour is part of the image. Do not threshold it to black and white before you send it.
Blue digits on a yellow band are still a numeric captcha. The model sees colour. A client-side threshold can delete a light digit and leave a stub that becomes a wrong reliable read.
Send the file as the portal sent it. PNG if they sent PNG. JPEG if they sent JPEG. Re-encoding through a screenshot tool adds blur.
A transparent GIF is fine. The decoder handles the formats listed in the API guide. An exotic camera raw file is not on that list and will be invalid_image.
If you must convert, convert to PNG without scaling up.
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