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Asked: 2026-05-14T01:23:54+00:00

I have a lot of jpeg files with varying image size. For instance, here

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I have a lot of jpeg files with varying image size. For instance, here is the first 640 bytes as given by hexdump of an image of size 256*384(pixels):

0000000: ffd8 ffe0 0010 4a46 4946 0001 0101 0048  ......JFIF.....H
0000010: 0048 0000 ffdb 0043 0003 0202 0302 0203  .H.....C........
0000020: 0303 0304 0303 0405 0805 0504 0405 0a07  ................
0000030: 0706 080c 0a0c 0c0b 0a0b 0b0d 0e12 100d  ................

I guess the size information mus be within these lines. But am unable to see which bytes give the sizes correctly. Can anyone help me find the fields that contains the size information?

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1 Answer

  1. Editorial Team

    According to the Syntax and structure section of the JPEG page on wikipedia, the width and height of the image don’t seem to be stored in the image itself — or, at least, not in a way that’s quite easy to find.

    Still, quoting from JPEG image compression FAQ, part 1/2 :

    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/.

    (Ergh, that link seems broken…)

    Here’s a portion of C code that could help you, though : Decoding the width and height of a JPEG (JFIF) file

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