JFIF APP0 segment

[Leonard WHYTE]

Does anyone know what the 'AMPF' string is all about?  It's in the last four characters of the v1.01 JFIF APP0 segment that apple includes within JPEG images generated on an iPhone12?
And while we're at it, why are the x and y resolutions within the JFIF (300dpi) different to the resolutions within the TIFF IFD (72dpi)?.  Len

[Phil Harvey]

I have no idea what the AMPF is about.  This is not part of the JFIF specification.

The resolutions are largely irrelevant.  They are meaningless for pictures taken by a digital camera.

- Phil

[Leonard WHYTE]

Phil,  Your comment regarding resolutions is noted.  But I'm on the track of the other AMPF matter.  I'm guessing that it is an apple extension, indicating that there is Apple MPF  (Multi Picture Format) data in the file.   And in truth, there ARE two MPF images in the iPhone 12 Pro Max image that I was focussed upon.  The first is the main image that the JPEG ordinarily carries, but there is an MPImage2 (as per your exiftool -s output) which is a smaller, apparently black and white version of the main image.  This is VERY VERY curious.  I'm still researching this......   Len

[Phil Harvey]

Hi Len,

I've seen a lot of B&W depth-data images written by various phones.

- Phil