[Image-SIG] Determine byte offset of JPEG SOI marker

[Ned Batchelder]

In the files I'm looking at, the SOI marker (ffd8) is at offset 0.  But 
assuming you have some where it is not, why not just open the file, and 
look for the marker?:

    head = open(jpg_filename, "rb").read(4000)
    soi_offset = head.find('\xff\xd8')

You're going to have to open and read the file to compute the checksum 
anyway...

--Ned.
http://nedbatchelder.com

Franz Buchinger wrote:
> Hi,
>
> I want to implement some sort of "post header checksum" for JPEG
> images, i.e. the checksum should only change if the actual image data
> was altered, EXIF/IPTC metadata modifications should have no effect.
> My plan to do this is to scan for the JPEG SOI marker, read n bytes
> after this marker and calcultate an MD5/SHA checksum for this.
>
> Can PIL tell me the byte offset of this marker?
>
> Greetings,
>
> Franz
> _______________________________________________
> Image-SIG maillist  -  Image-SIG at python.org
> http://mail.python.org/mailman/listinfo/image-sig
>
>
>
>   

-- 
Ned Batchelder, http://nedbatchelder.com

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