Can I edit the thumbnail image inside JFIF files?

Question

Can I edit the thumbnail image inside JPG/JFIF files?

If this is possible--how so (using what utility)?

The end result needs to be that the thumbnail image "can" be a wholly different image than the jpeg.

Thank you much, Michael

Answer 1

Typically, thumbnails are uncompressed RGB data. You locate the marker, see where the thumbnail's width/height are marked, then modify the byte stream following it. the stream is of length width*height*3 bytes.

If it's indexed, you'd have to overwrite the palette and the index entries. Just look for the APP0 marker, start modifying it.

Answer 2

A compliant EXIF thumbnail image must fit in the 64K APP1 marker and is usually compressed as JPEG (unlike what @Karthik says). The thumbnail image is independent of the main image and can easily be changed since it is inside a marker segment that doesn't affect the main image. The JPEG marker segments are basically a linked list of independent binary blobs with 2-byte identifiers (e.g. FFE1 in this case) and 2-byte lengths. You can swap out one for another and you won't "break" the file. There is no checksum or other mechanism that verifies the entire file data integrity. I'm not familiar with libraries to edit this information, but you can do it in a small amount of code that only has to parse the marker blobs type and length without knowing their contents. You can also do it the "quick and dirty" way by ensuring that your new thumbnail is no larger than the original and then you can just write it in it's place without moving the other parts of the file around. The marker length is not checked against its contents, so unused space is ignored.