[Nottingham] Tweaking the orientation flag in a jpeg

Martin martin at ml1.co.uk
Thu Sep 2 23:13:07 UTC 2010

On 03/09/10 00:03, Martin wrote:
> On 01/09/10 14:25, Martin wrote:
>> On 01/09/10 12:44, Graham Dicker wrote:
>>> Hi everyone
>>> Does anyone know of a utility that can modify just one byte at a given offset 
>>> in a file? I want to change the orientation flag in a set of jpegs for which 
>>> lossless rotate is not an option. The flag is at the same offset in every file 
>>> and just needs to be changed from a 1 to a 6.
>> Clever use of "dd";
> Roger gets the prize for a fast solution. The only minor improvement to
> be made there is perhaps to use mmap to tweak the file in situ.
> The dd trick is:
> dd bs=Offset-1 count=1 if=File of=FileNew
> echo 6 >>FileNew
> dd bs=Offset skip=1 if=File >>FileNew
> with no test whether the target byte is a "1" or not.
> That really would need to be wrapped up as a bash script...

Instead, you could also do:

echo 6 | dd bs=1 count=1 seek=Offset of=File

to write the '6' in situ...


Martin Lomas
martin at ml1.co.uk

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