[Question] Does JPEG specification define thumbnail image generation in JPEG images

Questions
1. Does JPEG specification define thumbnail image generation for JPEG
images?
    if so any references on web.

2. How thumbnails are generated for high resolution JPEG images and
placed as part of the JPEG file?

Any relevant information will be appreciated.
George Wilson

"George Wilson" <george_wilson_mit@yahoo.com> wrote in message 
news:1157458455.224314.73330@e3g2000cwe.googlegroups.com...
> Questions
> 1. Does JPEG specification define thumbnail image generation for JPEG
> images?
>    if so any references on web.

Not really. Check out Exif:
http://www.exif.org/specifications.html

Also, the JFIF spec has a JFXX option for thumbnail:
http://www.w3.org/Graphics/JPEG/jfif.txt

>
> 2. How thumbnails are generated for high resolution JPEG images and
> placed as part of the JPEG file?

Typically the thumbnail is 160 x 120 JPEG.

Some cameras also have an intermediate "screennail" image,
usually 640 x 480:
http://search.cpan.org/src/EXIFTOOL/Image-ExifTool-6.29/html/TagNames/FlashPix.html

Other cameras have proprietary smaller images tacked on in
a variety of ways.

Hope this helps. 

George Wilson wrote:
> 
> Questions
> 1. Does JPEG specification define thumbnail image generation for JPEG
> images?
>     if so any references on web.
> 
> 2. How thumbnails are generated for high resolution JPEG images and
> placed as part of the JPEG file?
> 
> Any relevant information will be appreciated.


It is possible to easily and quickly extract thumbnails that are 1/2,
1/4 or 1/8 the size of the original. This is due to the structure of the
data. It isn't rally part of the jpeg specification as far as I know.
I's just that the low frequency content is already computed and stored
for those particular resolutions.

-jim

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