What if an image has no previews? [OSX]
Replies (5)
RE: What if an image has no previews? [OSX] - Added by Robin Mills over 3 years ago
Patrick
At the moment, exiv2 can only read previews in images. You cannot modify, delete, or add previews to images. There is a project being discussed to add this capability, however it's still in early stages of discussion by private email.
The storage of thumbnails is complex. It's not a single tag and there can be multiple thumbnails at different resolutions. Additionally there can be thumbnails embedded in the MakerNote. And for even more complexity, different formats (TIFF, JPEG, PNG etc) can store thumbnails in format specific ways which are not part of the Exif standard.
You can download my favourite test file from http://clanmills.com/Stonehenge.jpg
There are a couple of tags explained on page 24 (physically p29) of the Exif standard: http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
We can manually extract the thumbnail from the image as follows:
$ exiv2 -pa --grep JPEG ~/Stonehenge.jpg
Exif.Thumbnail.JPEGInterchangeFormat Long 1 4442 <--- location of thumbnail
Exif.Thumbnail.JPEGInterchangeFormatLength Long 1 10837 <--- length of thumbnail
$ exiv2 -pS ~/Stonehenge.jpg # examine Structure of image file
STRUCTURE OF JPEG FILE: /Users/rmills/Stonehenge.jpg
address | marker | length | data
0 | 0xffd8 SOI
2 | 0xffe1 APP1 | 15288 | Exif..II*...................... <--- location of the exif data
15292 | 0xffe1 APP1 | 2604 | http://ns.adobe.com/xap/1.0/.<?x
17898 | 0xffed APP13 | 96 | Photoshop 3.0.8BIM.......'.....
17996 | 0xffe2 APP2 | 4094 | MPF.II*...............0100.....
22092 | 0xffdb DQT | 132
22226 | 0xffc0 SOF0 | 17
22245 | 0xffc4 DHT | 418
22665 | 0xffda SOS
$ We extract the exif data. The segment is 2 bytes into the file and its marker is 4 bytes (2 byte marker and 2 byte length). There are 6 leading bytes ("Exif\0\0") in the segment.$ dd bs=1 skip=$((2+4+6)) count=15288 if=~/Stonehenge.jpg > Stonehenge.exif 15288+0 records in 15288+0 records out 15288 bytes transferred in 0.164319 secs (93039 bytes/sec) $ dd bs=1 skip=4442 count=10837 if=~/Stonehenge.exif | exiv2 - # extract thumbnail and pipe through exiv2 10837+0 records in 10837+0 records out 10837 bytes transferred in 0.039951 secs (271257 bytes/sec) File name : - MIME type : image/jpeg Image size : 160 x 120 -: No Exif data found in the file $As you can see, this isn't trivial. The reverse process (to write the tags/binary thumbnail) is formidable. It's unlikely that you can add/del/mod exif thumbnail until we implement thumbnail editing in Exiv2.
Incidentally the exif data is an embedded tiff:
$ exiv2 Stonehenge.exif # exif data is a TIFF ! File name : Stonehenge.exif MIME type : image/tiff <--- It's a TIFF! ... Thumbnail : image/jpeg, 10837 Bytes <--- Thumbnail in embedded tiff! ... $
I suspect there is a simpler way to write a thumbnail directly into a segment in a JPEG (not embedded in the Exif data), however I have to do more research to understand this.