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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Hi guys, here's a curiosity if there are any uncovered jewels to be revealed:
I have a Canon PowerShot A720IS. It's great for taking photos, but I'd like to be able to use it to display other images, too.
When you take a photo, it is recorded on the SD card as IMG_XXXX.JPG.
If I try to copy the JPG and save it as IMG_XX+1.JPG, including the original EXIF data, and put the SD card back in the camera, it is unable to read the second image ("Unidentified image").
When I "format" a memory card using the camera, a couple of hundred kilobytes are consumed by I'm guessing some kind of special Canon filesystem. I can't find much information on the net about it, except for mention of ".CTG" files [which appear to be from a different/older series of Canon cameras, and don't appear on my cards].
There don't appear to be any superhidden files on the SD cards, so I'm just wondering where the thumbnails are stored, how I can emulate this format for the purpose of using the camera as a portable picture-viewer. :P
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I guess the thumbnail preview is embedded in the JPEG file itself. It will be a special resolution and format, so the camera can display the thumbnails quickly without scaling.
The camera needs to make a record of what pictures it has taken somewhere, so it knows where to carry on the numbering sequence. If an image is "unidentified" it'll probably be because it's not in this record. (Would you trust an image from the future that you hadn't even taken yet? Did you try taking two existing images and swapping their filenames, to see if the thumbnails follow the files themselves or the names?)
The record is probably in non-volatile memory inside the camera, since the numbering carries on where it left off even if you insert a brand new memory card.
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