Embedded images in EXIF

[Changed the subject in order not to hijack the main EXIF discussion]

On 2 January 2017 at 18:20, John Bowler wrote:
> In that file there is a second smaller (1616x1080) JPEG after the main one;
> I don't understand what that is.  The full dump file is attached (this is
> from my own program and that isn't very up-to-date), I'm happy to upload the
> images to one of the binary dump sites.  (They're copyright either me or my
> wife, I can't remember who took the photo, it's not very good ;-)

There are three types of images:

1). The thumbnail image, part of standard EXIF metadata.
2). The preview image, part of the MakerNotes.
3). The main image.

Tools like IJFR ("Instant JPEG From Raw") can extract the preview
image. You can, too, with the following exiftool command:

$ exiftool -b -PreviewImage INFILE > OUTFILE

And you can extract the thumbnail image with the following command:

$ exiftool -b -ThumbnailImage INFILE > OUTFILE

So the Raw images have both, and, believe or not... the JPEG images
have both, also. (At least for the handful of digicams that I own,
this is true). I kid you not: there are three JPEGs images embedded in
an EXIF-JPEG: there is the thumbnail (very small), the preview
(small), and the original (large) JPEG image.

I discovered, in my experiments, the importance of the preview image.
The EXIF thumbnails are uselessly small (160x120px). The preview
images are what tools like XnView actually use to display as
thumbnails. I deleted the preview images, using exiftool, and then I
discovered that a directory of hundreds of photos shows up much slower
when loaded and displayed in the explorer view. That's because,
lacking the preview image, the XnView tool must extract a
decently-sized 100 kilopixel thumbnail out of a 20MPx image, which,
for a directory containing hundreds of images, it thrashes the HDD, it
hogs the CPU... you get the idea...

Sincerely,
Cosmin

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