question about damaged/recovered .jpg files

Archived from groups: rec.photo.digital (More info?)

trevor_smithson@yahoo.com wrote:

> When I open any of those 5 files in a graphics program, the bottom
> half is blank. The top half is ok, with a bit of noise/garbling
> right in the middle. The odd thing to me is that the picture
> previews like it's intact, both in Windows Explorer and in programs
> like Ulead PhotoExplorer and Adobe Photoshop Elements.
>
> Is the preview data stored seaparately in the file?

Most digital still cameras embed so-called EXIF metadata in each image
file. EXIF data is basically a snapshot of many shoot-time variables
(exposure settings, AF mode, a timestamp from the camera's internal
clock, the make and model of the camera etc.) When viewing pictures
using the built-in viewer in the camera, this information can
(typically) then be displayed superimposed on top of the image.
Additionally, you can also view the same data later on your computer, in
various EXIF-enabled image viewers (such as XnView), which may prove
useful if you want to analyze the shooting conditions or camera settings
on certain pictures.

One part of the EXIF metadata scheme is a thumbnail. As you already
suspected, most digital still cameras store a so-called "EXIF thumbnail"
within the image file. This is a small, separate JPEG image (usually
with the resolution of 160×120 pixels) which the camera displays on its
LCD when you're browsing the pictures.

The EXIF thumbnail did not originally get much use once the image was
transferred on a computer, but this has now changed: modern image
viewers (such as XnView or ACDSee) have increasingly added support for
viewing them. (And why not? Utilizing the embdedded thumbnail makes it a
lot faster to browse image folders in the thumbnail mode - especially on
slower computers.)

Long story short: Yes, you might well be seeing the EXIF thumbnail
instead of the actual image. To check if this is so, download a tool
called "Exifer":

<http://www.friedemann-schmidt.com/software/exifer/>

Exifer will display the actual image and its embedded EXIF thumbnail
(if any) side by side, allowing you to check whether one or the other
has problems.

--
znark