A clean container around surviving pixels
PNG is a sequence of typed chunks. Each chunk carries a CRC-32 checksum, so the diagnostic can distinguish a well-formed sequence from bytes that changed after the file was written. When the browser still decodes a complete frame, a lossless rewrite makes new IHDR, IDAT and IEND chunks with correct lengths and checksums.
The rewrite preserves rendered colour and transparency. It intentionally drops ancillary text, profile and time chunks because copying untrusted chunks would copy the structural problem too.
What a checksum cannot repair
- A bad CRC proves a chunk changed; it does not reveal what the original bytes were.
- If damaged IDAT data cannot be decompressed into a complete frame, this page refuses to offer a misleading download.
- Animation chunks are flattened to the frame the browser exposes; use an APNG specialist when motion matters.
Other kinds of restoration
- Repair a JPEG fileInspect its markers, salvage a decodable frame and write a clean full-size JPG.
- RepairImageDetection, an editable mask and the fill, on one column.
- Restore an old photographOpens on fade correction, for a print that has gone pale and warm.
- What restoration cannot bring backSilvering, mould, missing faces, and the honest end of the list.