[Bf-python] Understanding blend file binary data

[Bastien Montagne]

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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