RE: Retrieving JPG and GIF file data in an AS/400 program

["Bob Crothers" <bob@xxxxxxxxxxxxxx>]

And don't forget to pay attention to Little vs Big endian issues (Byte
Ordering).  The AS/400 is Big endian,  Intel is little.  Unix is both
depending on platform.

Bob

-----Original Message-----
From: owner-midrange-l@midrange.com [mailto:owner-midrange-l@midrange.com]On
Behalf Of John Taylor
Sent: Sunday, August 27, 2000 11:15 AM
To: MIDRANGE-L@midrange.com
Subject: RE: Retrieving JPG and GIF file data in an AS/400 program

Rob,

The following excerpt is from the JPEG-FAQ:



Subject: [22] How can my program extract image dimensions from a JPEG file?

The header of a JPEG file consists of a series of blocks, called "markers".
The image height and width are stored in a marker of type SOFn (Start Of
Frame, type N).  To find the SOFn you must skip over the preceding markers;
you don't have to know what's in the other types of markers, just use their
length words to skip over them.  The minimum logic needed is perhaps a page
of C code.  (Some people have recommended just searching for the byte pair
representing SOFn, without paying attention to the marker block structure.
This is unsafe because a prior marker might contain the SOFn pattern, either
by chance or because it contains a JPEG-compressed thumbnail image.  If you
don't follow the marker structure you will retrieve the thumbnail's size
instead of the main image size.)  A profusely commented example in C can be
found in rdjpgcom.c in the IJG distribution (see part 2, item 15).  Perl
code can be found in wwwis, from http://www.tardis.ed.ac.uk/~ark/wwwis/.



The entire faq (2 parts) can be retrieved from the following FTP location:


ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/news/answers/jpeg-faq/



Have fun,


John Taylor
Canada

> -----Original Message-----
> From: owner-midrange-l@midrange.com
> [mailto:owner-midrange-l@midrange.com]On Behalf Of Rob Dixon
> Sent: Sunday, August 27, 2000 4:50 AM
> To: MIDRANGE-L@midrange.com
> Subject: Retrieving JPG and GIF file data in an AS/400 program
>
>
> I have been asked to load about 25,000 images into an AS/400 web site
> with the prospect of rather larger quantities to come.  I store them in
> the IFS and the transfer is not a problem.  My product ERROS was used to
> build the web site.  This generates HTML on the fly and automatically
> changes the image size dependent upon the number of images displayed on
> the screen at any one time.  To do this, and to make sure that I do not
> display an image in a size greater than its size as supplied,  I need to
> store the image size data for each image in my Neural Database.  So far
> I have only put up a few hundred images and I have put this information
> in manually, but, with this quantity of images, I need to automate the
> process.
>
> Does anyone know how to extract the image size data from image files in
> the IFS using an AS/400 program or even where it is stored in a JPG or
> GIF file?  The files will have been created in Adobe Photoshop.
>
> Many thanks
>
> Rob Dixon
> ________________________________________________________
>
> Erros plc
>
> 44 (0) 1844 239 339
>
> http://www.erros.co.uk - The AS/400 Neural Database for the Internet
>
> _________________________________________________________
>
>
> +---
> | This is the Midrange System Mailing List!
> | To submit a new message, send your mail to MIDRANGE-L@midrange.com.
> | To subscribe to this list send email to MIDRANGE-L-SUB@midrange.com.
> | To unsubscribe from this list send email to
> MIDRANGE-L-UNSUB@midrange.com.
> | Questions should be directed to the list owner/operator:
> david@midrange.com
> +---
>

+---
| This is the Midrange System Mailing List!
| To submit a new message, send your mail to MIDRANGE-L@midrange.com.
| To subscribe to this list send email to MIDRANGE-L-SUB@midrange.com.
| To unsubscribe from this list send email to MIDRANGE-L-UNSUB@midrange.com.
| Questions should be directed to the list owner/operator:
david@midrange.com
+---

+---
| This is the Midrange System Mailing List!
| To submit a new message, send your mail to MIDRANGE-L@midrange.com.
| To subscribe to this list send email to MIDRANGE-L-SUB@midrange.com.
| To unsubscribe from this list send email to MIDRANGE-L-UNSUB@midrange.com.
| Questions should be directed to the list owner/operator: david@midrange.com
+---