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WINCODEC_ERR_UNKNOWNIMAGEFORMAT
WINCODEC_ERR_UNKNOWNIMAGEFORMAT means that WIC could not identify a registered decoder for the supplied bytes. It does not prove that the filename extension is wrong: decoder discovery is based on identifying patterns in the image data and the available codec registrations.
Diagnosis
- Verify that the stream starts at the beginning of the file and contains the complete image header rather than an HTTP error page, truncated download, or wrapper format.
- Check the image signature independently and confirm that a decoder for the format is installed on the target Windows version.
- For a custom codec, inspect its registration and identifying pattern so WIC can discover it at run time.
If a decoder is found but later rejects its content, the more specific WINCODEC_ERR_BADHEADER, WINCODEC_ERR_BADIMAGE, or stream errors can be more useful.
Microsoft: WIC discovery and arbitration
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