Reading JPEG header information : Graphics

I need to be able to interpret the header information in a JPEG image file.

I've been poking through Google hits, but am pretty much lost ... should
I be looking at the EXIF, JFIF or JIF specifications? Or maybe some
other, more global specification document that includes the others?

Can anyone here direct me to a comprehensive specification document with
field and byte layouts for JPEG files?

Right now, I just need to be able to read the dimensions of the image.

Z wrote:

> I need to be able to interpret the header information in a JPEG image file.
>
> I've been poking through Google hits, but am pretty much lost ... should
> I be looking at the EXIF, JFIF or JIF specifications? Or maybe some
> other, more global specification document that includes the others?
>
> Can anyone here direct me to a comprehensive specification document with
> field and byte layouts for JPEG files?
>
> Right now, I just need to be able to read the dimensions of the image.

This pdf document has details on the structure of the header:

http://www.w3.org/Graphics/JPEG/jfif3.pdf

This site contains essentially the same information
with a little different spin:

http://netghost.narod.ru/gff/graphics/summary/jfif.htm

Z wrote:
> I need to be able to interpret the header information in a JPEG image file.
>
> I've been poking through Google hits, but am pretty much lost ... should
> I be looking at the EXIF, JFIF or JIF specifications? Or maybe some
> other, more global specification document that includes the others?
>
> Can anyone here direct me to a comprehensive specification document with
> field and byte layouts for JPEG files?
>
> Right now, I just need to be able to read the dimensions of the image.

When I wanted to reset EXIF timestamps on a batch of images, Google took
me straight to what I needed. Later on I discovered that the
functionality I needed had already been implemented as open source
several times. For the header stuff (not the image itself) check out
libexif. It might save you a lot of time.

Paul Allen

Paul Allen wrote:
>> Right now, I just need to be able to read the dimensions of the image.

> When I wanted to reset EXIF timestamps on a batch of images, Google took
> me straight to what I needed. Later on I discovered that the
> functionality I needed had already been implemented as open source
> several times. For the header stuff (not the image itself) check out
> libexif. It might save you a lot of time.

I found libexif and played a bit, but my target o/s is VMS and because
of the makefiles it's going to take a huge effort to build that code on
any o/s other than Unix/Linux.

I only need a very specific set of data: just the height and width in
pixels (and I'm not sure it's in the EXIF section), so I think my path
of least resistance is to decode the correct header myself.

Jim Townsend wrote:
> Z wrote:
>
>
>>I need to be able to interpret the header information in a JPEG image file.
>>
>>I've been poking through Google hits, but am pretty much lost ... should
>>I be looking at the EXIF, JFIF or JIF specifications? Or maybe some
>>other, more global specification document that includes the others?
>>
>>Can anyone here direct me to a comprehensive specification document with
>>field and byte layouts for JPEG files?
>>
>>Right now, I just need to be able to read the dimensions of the image.
>
>
> This pdf document has details on the structure of the header:
> http://www.w3.org/Graphics/JPEG/jfif3.pdf

Thanks.

In that document I see Xdensity and Ydensity but that can't be height
and width ... I have 2 JPGs, one 200x200 and another 400x400 (a resized
version of the 200x200 image) and both have the same Xdensity and
Ydensity fields within the JFIF header.

200x200:
FF D8 FF E0 10 4A 46 49 46 00
01 01 00 00 01 00 01 00 00 FF E1 1C 86 45 78 69 66 00
( ver 1.1, units = 0, Xdensity = 00 01, Ydensity = 00 01 )

400x400:
FF D8 FF E0 10 4A 46 49 46 00
01 01 00 00 01 00 01 00 00 ff E1 1C 86 45 78 69 66 00
( ver 1.1, units = 0, Xdensity = 00 01, Ydensity = 00 01 )

How do I convert the densities (actually the pixel aspect ratio, since
the units byte is 00) to number-of-pixels?

Z <Z@no.spam> wrote:
>Paul Allen wrote:
>>> Right now, I just need to be able to read the dimensions of the image.
>
>> When I wanted to reset EXIF timestamps on a batch of images, Google took
>> me straight to what I needed. Later on I discovered that the
>> functionality I needed had already been implemented as open source
>> several times. For the header stuff (not the image itself) check out
>> libexif. It might save you a lot of time.
>
>I found libexif and played a bit, but my target o/s is VMS and because
>of the makefiles it's going to take a huge effort to build that code on
>any o/s other than Unix/Linux.
>
>I only need a very specific set of data: just the height and width in
>pixels (and I'm not sure it's in the EXIF section), so I think my path
>of least resistance is to decode the correct header myself.

Grabbing any given bits of information from the EXIF headers is
fairly easy to do (like everything else, once you figure out
how), and can be done with Standard C that will compile on
virtually any platform.

A while back I was fooling around with a short program to read
EXIF headers from Nikon NEF formatted files, and more or less on
a whim also added the ability to read JPEG files. I was only
really interested in NEF files though, and while I researched
that to the bottom of the stack, I only looked into JPEG EXIF
headers enough to pull out a few things. I don't recall having
found any really good information on JPEG headers, and mostly
just reverse engineered the format from examples generated by
the particular (Nikon D1) camera that I was interested in at the
time. I've also tested it on JPEGs produced by a Canon 5D,
where the only information that appears to be correct is the
camera, the date, and the image size. :-)

The program is strictly ANSI, and uses stdio.h, stdlib.h,
and strings.h only. No system/platform specific headers at all.
And nothing other than libc is linked.

My code obviously has 100 times more clutter than you need, but
if it would be any help, point me at a URL that has an example
JPEG file you want information from, and I'll post the shortest
most basic example code I can come up with to extract the size
in pixels from it. It might save you a little thrashing...

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com

Z wrote:
> Paul Allen wrote:
>
>>> Right now, I just need to be able to read the dimensions of the image.

>> When I wanted to reset EXIF timestamps on a batch of images, Google took
>> me straight to what I needed. Later on I discovered that the
>> functionality I needed had already been implemented as open source
>> several times. For the header stuff (not the image itself) check out
>> libexif. It might save you a lot of time.

> I found libexif and played a bit, but my target o/s is VMS and because
> of the makefiles it's going to take a huge effort to build that code on
> any o/s other than Unix/Linux.
>
> I only need a very specific set of data: just the height and width in
> pixels (and I'm not sure it's in the EXIF section), so I think my path
> of least resistance is to decode the correct header myself.

I did that myself before I stumbled over libexif and the various things
on top of it. I've got some code that parses the internal structures of
a JPEG file. I built it to increment the datestamps on a whole bunch of
images, but that logic is up in the main program. The underlying exif.c
logic knows all about things like the tags containing the image
dimensions. It does have a Makefile, but that's not really needed.
It definitely does not use autoconf and does not have a configure
script.

If you're interested, I can put it up on my web site. It's been a
really long time (20 years, in fact) since I touched VMS. Can you
handle a gzipped tarfile? A plain tar archive? Plain newline-delimited
ASCII text files? I once warped the GNU C compiler on an Ultrix VAX
into a cross-compiler for Minix running on a 386, so I guess VAXen have
the same byte-sex as PC's.

Paul Allen

Thank you all for the help ... I've found what I need here:

http://dev.w3.org/cvsweb/Amaya/libjpeg/ (rdjpgcom.c)

Small, compact and easy to build on VMS.

On Fri, 11 Nov 2005 21:46:45 -0800, Z <Z@no.spam> wrote:

>Paul Allen wrote:
>>> Right now, I just need to be able to read the dimensions of the image.
>
>> When I wanted to reset EXIF timestamps on a batch of images, Google took
>> me straight to what I needed. Later on I discovered that the
>> functionality I needed had already been implemented as open source
>> several times. For the header stuff (not the image itself) check out
>> libexif. It might save you a lot of time.
>
>I found libexif and played a bit, but my target o/s is VMS and because
>of the makefiles it's going to take a huge effort to build that code on
>any o/s other than Unix/Linux.

VMS as in DEC? An 11/780 in your home office?

Actually, I remember someone telling me that (around 1985) about one
quarter of the VAXen in use were running some version of UNIX instead
of VMS.

Z <Z@no.spam> writes:

> Paul Allen wrote:
>>> Right now, I just need to be able to read the dimensions of the image.
>
>> When I wanted to reset EXIF timestamps on a batch of images, Google took
>> me straight to what I needed. Later on I discovered that the
>> functionality I needed had already been implemented as open source
>> several times. For the header stuff (not the image itself) check out
>> libexif. It might save you a lot of time.
>
> I found libexif and played a bit, but my target o/s is VMS and because
> of the makefiles it's going to take a huge effort to build that code
> on any o/s other than Unix/Linux.

Try the Perl package Image::ExifTool. It's pure Perl, and Perl runs
everywhere.

> I only need a very specific set of data: just the height and width in
> pixels (and I'm not sure it's in the EXIF section), so I think my path
> of least resistance is to decode the correct header myself.

Decoding the width and height from the JPEG header is rather easy.
ITU T81 is the document you want for reference.

--
Måns Rullgård
mru@inprovide.com