Hi James,
Think the main (if not the only) serious cause of those differences here
are the fact that Blender uses memory addresses of its struct as
'in-file uuid' in the .blend. So first thing to do for your usecase
would be to ignore all and every pointers from DNA structs, since those
will change every time.
As for reading a .blend file, besides blender code itself, suggest you
have a look at code in io_blend_utils folder in addons repository, not
yet complete but should give you a good start. You may also want to have
a look to blend2json.py script (in our blender-dev-tools git repository,
https://developer.blender.org/diffusion/BDT/).
Cheers,
Bastien
Le 25/06/2016 à 13:55, James Crowther a écrit :
Hi there,
I have a need to understand why a blend file can be
different on disk after saving it despite nothing actually changing in
the actual scene data, by that I mean D.objects.
From a cursory look at the differences using a file diff viewer like
beyond compare, I can tell that there are some metadata like the file
name that are included in the file's binary, so this would of course
change for some instances where you copy the file for example.
However, the file name is only one part of the vast number of changes
I can see.
To reproduce simply open a blend file, save it. Then copy it. Open the
copy and save it. Do a file diff on the original file and the copy and
you will see many differences throughout the file.
The use case I am working with here is I am distributing a file across
many machines to render parts on each. I need to make sure the file is
the same on each machine so I have some assurance that the file
transfer works (whatever method the transport uses, LAN, USB stick
etc) and that the machines all have the same data.
Originally I was going to do this using something like CRC32 or MD5
depending on speed and security needs, but since the file changes once
it is saved, even if the file is not changed, this presents a problem
since the data of the scene is the same, but the results of a CRC32 or
MD5 will be different due to the effects of saving a file.
Could someone educate me as to how to read the blend file format in
such a way as to remove these differences? I'm more than happy to
write a custom CRC routine so long as I can know which parts of the
file contain the data and which are headers that are altered on a save.
Thanks!
James
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