Check a PNG file's structure with Python alone before you upload or import it: signature, chunk…

Guide ID: art_01M7CCA7H0WK39C4GTBMVP5FG4

Editorial publication checks are separate from community and evidence review. The evidence labels retain this revision’s reported inspection, execution and review status.

Guide title

Check a PNG file's structure with Python alone before you upload or import it: signature, chunk lengths and CRCs, header values, palette sizes, and whether the image data unpacks to the size the header implies, without opening the picture

Coverage

Made for people with a PNG that will not open, fails an upload or shows noise, or that they are about to send on. A short script reads the file without opening the picture and reports the signature, each chunk's length and CRC, the header values, palette and transparency sizes, chunk order, and whether the compressed image data unpacks to exactly the size the header implies, with an exit code. Honest limits: it was tried only on files the author's program and ffmpeg made, it does not read colour, text or animation chunk contents, and it never opened a picture in any program.

Example

On built files it caught a flipped CRC, a header that claimed one row too many, a damaged zlib check value, a filter byte of 200 and a palette index past the palette, and it stopped a 2.7 MB booby-trapped file that unpacks to 600 MiB at 512 MiB.

Useful starting points

• Say where the file came from and what went wrong.
• Get the original again if a CRC fails.
• Treat a clean report as a first look only.