You are correct. As Harley Acheson noted, 128-bit UUID can be considered
globally unique. Even in the scenario I was describing in my previous
message, when there are copies of the same .blend files with an object
with the same UUID, it is easy to make it globally unique for any
practical purpose - just regenerate UUID in the copy.
I recently worked on implementation with 32-bit hashes - unlike 128-bit
UUIDs, 32-bit can only be considered unique within one file (this was
sufficient for my purposes), since chances of random collision are
pretty high even within my own collection of .blend files, if I would
have generated 32-bit UIDs for all objects within them.
128-bit UUID is much better in this regard, chance of random collision
is practically 0, unless you make a copy of it, but then it is not
random and it is expected, even useful for some purposes - if you see an
object with the same UUID, it is practically guaranteed to be either a
copy or derivative of the original object with the same UUID (unless its
UUID was regenerated to make it globally unique).
If Blender had UUID support, it would saved me a lot of time and effort,
since the only reason I ended up implementing 32-bit UIDs (instead of
128-bit like UUID supposed to have) is that my addon already had 32-bit
hash function for generating Cryptomatte IDs, and I just reused it for
generating random UIDs, this way I did not have to add a function for
128-bit hash. Even 32-bit hash function implemented in pure Python does
not have good performance, if there is a need to process large number of
objects. But loops in Python are often slow in general, so this is
expected. Because of this I had many difficulties in my implementation,
and it took a lot of effort to achieve acceptable performance when there
thousands or even tens of thousands of objects in the scene.
My point is, even though implementing UUID support in Blender addon is
technically possible, it is inefficient and complicated. If there was
native UUID support, I would not need to write all this additional code
and the performance would be much better too. And there are many other
use cases for UUIDs so many addons could benefit from native UUID
support in Blender if it will be implemented in the future.
On 23/11/20 11:25 am, Toni Alatalo via Bf-committers wrote:
Just a bit of echo,
AFAIK UUIDs are a good solution to deal with integrating Blender and many
kinds of other systems with which we want good integration, via exports or
even two-way export-import cycles.
Back about 10 years ago, we made a pretty cool integration from Blender to
a Ogre3D based virtual worlds system, realxtend.org, in rex2blender Blender
add-on which was developed by Caedes (Pablo Martin, Crystal Space and
Blender add-on dev at the time).
It also used UUIDs to know which objects from Blender, maybe continuously
reimported to the game engine side, were the same objects as in a previous
export. Worked well, I don't recall any problems, though the design is
certainly not trivial when you think of how to deal with the issues
mentioned here, duplicates, copies of the same project file etc. But having
real UUIDs there always helps, whatever you decide to do, and it never does
any harm (except mem use if you go really low level, but we only considered
it on object level).
In my understanding, they really should be globally unique, not within a
blend. That's the first U in UUID: *Universally* unique.
We work with BIMs a lot now, and they also have UUIDs in the format spec,
but I recently learned that many BIM creation tool vendors have been
stupid, and just put whatever IDs there, sometimes even overlapping IDs
within the same project, and with zero quarantees of those IDs being unique
cross projects, or different creation tools. So it's just sad and useless,
all the folks who write import tools need to create new sensible IDs.
Learned that on twitter here:
https://twitter.com/xeolabs/status/1320671615441686529
So please, real UUIDs, afaik the math for it works well.
2cently yours,
-Toni
On Sun, Nov 22, 2020 at 9:03 PM Lissanro Rayen via Bf-committers <
bf-committers@blender.org> wrote:
In theory, you are right. But in practice, chance of collision in many
real-world scenarios is 100%. As an example, consider the following
scenario:
I have a .blend with some objects ("A"), all objects in it already have
generated UUID. I open the file, edit something, save as different file
("B"). Then again I open the original file, and save it under different
name ("C"). Then I create new empty file and append objects from "B" and
"C". If I append the same object from both files, there will be UUID
collision. But this is not an issue as long as there is a check to make
sure that each appended or duplicated object has unique UUID in the
scope of the current .blend file, and if not, just reset its UUID (it
can be generated later on demand if necessary). I think this is
preferable to blindly regenerating (or dropping) UUIDs, since if there
is no collisions, I may have a reason to preserve UUIDs as is when
appending my objects. Besides, Blender already does collision detection
for object names to automatically rename them if it happens. But for
UUID, it is safe to just invalidate it if it conflicts with another UUID
of already existing data block, and generate a new one later when
necessary.
On 21/11/20 6:33 pm, Harley Acheson via Bf-committers wrote:
It would be impossible task to make them truly unique across all .blend
files, there are always a possibility of collision.
UUIDs have limitations...will not change unless there is a collision).
That does not sound right.
If 128-bit UUID are implemented properly according to standard, the
chances
of collision is *zero* *for all practical purposes*. The chances of
collision are so low that implementations can ignore it. To get to a 50%
probability of at least one collision you need to create 2.71
quintillion -
so generating a billion of them per second for 85 years. Just to get a
one-in-a-billion chance of a duplicate requires making 103 trillion.
Cheers, Harley
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