[lug] 2.6M Lossless jpeg rotations?

[Bear Giles]

Jeffrey Veiss wrote:
> Hi all,
> 
> I've read that jpegtran from the v6b JPEG library is supposed to
> perform "Lossless JPEG transformations" and I'm a little confused
> about what that actually means.
> 
> I have a jpeg from my camera that, according to gqview, has the
> following properties:
> 
>    file size:   867,475
>    dimensions:  1280x960
>    compression: 17.6%
>    image size:  4.7MB
> 
> After rotating it with "jpegtran -rotate 270", gqview reports the
> properties to now be:
> 
>    file size:   840,678
>    dimensions:  960x1280
>    compression: 30.4%
>    image size:  2.6MB

Try stripping the cruft from the original file.  You might find 
the original image file was only about 2.6MB, with 2.1MB of 
various cruft.

In general, you would be surprised how much faster a website can 
be made by stripping the cruft from images, then resaving them as 
interlaced images.  The former eliminates the need to download 
stuff that the client never needs to see, the latter actually 
bumps the image size slightly but it's offset by the perception 
that the download proceeds faster since the HTML rendering engine 
can quickly stablize the results and things then just get sharper.

I believe that the "lossless rotation" promised by jpegtrans is 
exactly that - it uses byte manipulation of the data blocks to 
perform the rotation, not a decompression/swap/compression cycle.

> However, I loaded the original and jpegtran-rotated image into Adobe
> Photoshop and took a look at the properties of each:
> 
> Original:
> 	Document size:  8.889" x 6.667"
>    Resolution:     144 pixels/inch
> 
> after jpegtran:
> 	Document size:  13.333" x 17.778"
>    Resolution:     72 pixels/inch

Photoshop puts a lot of cruft into image files, just so you know. 
   I don't know why jpegtran changed the image resolution, but I 
believe you can force it via the command line.  Or you could 
easily change it by hand... JFIF files (which contain JPEG images, 
long story....) have a fairly simple format.  It's dealing with 
the compressed image data that makes you want to run into the night.

 > I think it's interesting that Photoshop reports that the
> DPI was cut in half but the Document size was doubled.

Not really.  It's computing the dimensions from the number if 
pixels and the stated DPI.

Bear