Rigid Alembic Caches: Velocity, UVs and POINT NUMBER!

[ben.fox@framestore.com]

We are testing Caronte to see if it is something that we want to use in production and we have two issues that we are running into.
The first question is about importing the stitched Alembic cache into Houdini. Is there any way to get a velocity attribute? The only way we have found to get true motion blur is to load the Alembic cache as an Alembic Archive in Houdini, which is not the best way to work. The more manageable Alembic Cache node imports the data as Houdini geometry which needs point velocity to do motion blur. Of course we can calculate it, but it isn't the pure Realflow velocity that we love. Just checking if anyone has any tips. Would importing is an SD work? That is what we are looking into now.

Most important though is UV's!
We are losing our UV's in the cached Alembic files. They have UVs in Realflow...but when we load the caches in Maya or Houdini they have no UVs.
What are we doing wrong here.

### EDIT MOST IMPORTANT QUESTION ####
Importing a preshattered Alembic file as a multibody changes the point numbering!!!
When we import the stitched Alembic cache after simulating in Realflow the point numbering has changed.
This could be a deal breaker for us...please help!

Any info would be a big help
Thanks so much
~Ben

[gus]

Hi Ben,

Which version of RealFlow are you running for this tests?
We will try and replicate here tomorrow morning and give you an answer asap.

Cheers,

g-)

[ben.fox@framestore.com]

We are running: Realflow 2013 7.1.3.0152 on Linux

[enrique]

Hi Ben,
Thanks for reporting this. We have been looking into your problem and we might have found something:

1 About the velocity question: We export an xform for each object in the alembic file. With the xform you can get the velocity information of each point of the object. What do you mean by "true motion blur"? The information is the same you would get if we bake the velocity for each vertex. Is that what you would like?

2 About the UV's question: We have found that we are not exporting UV's when the object is a multibody. This bug will be solved in the next version. However there are some workarounds: you could use independent objects instead of a multibody containing all of those objects, the UV's are correctly exported then. After the simulation, you can stitch those independent alembic files into one big alembic with all the information (and UV's as well). If that is not possible or manageable, you can always come back to using SD.

3 About the vertex numbering: We have not being able to reproduce it, we tried to shatter one object in another platform, save it and compare it with the one that was loaded, simulated and exported in RealFlow. The vertex numbering is the same. Could you provide with an example file or the exact steps necessary to reproduce this?

Enrique.

[ben.fox@framestore.com]

Enrique,
Thanks so much for looking into these questions, it is a huge help!
The transform export works great in Maya but in Houdini you have the option of loading an Alembic File as a much more manageable Alembic Cache file which uses the transform the data to create Houdini Geometry as a single mesh with grouping. Once loaded as Houdini geometry, there is no velocity information...which is TOTALLY understandable. But it might be cool if there were an option to add the transform velocity to the points. In most cases this would not be needed, but really spinning twisting elements would not motion blur correctly if I needed to computer the point velocity in Houdini. Again, this is just when loading as the SOP level Alembic Cache node...which in most cases is preferable with things like shattered geo with 1000s of pieces.

As for the UV issue, that is great to know that the issue will be fixed...thanks for taking care of that!

And the big question: Point Order!
THIS IS NOT A REALFLOW BUG! Yay...but please read on because we need a way to import preshattered geo into Realflow and sim as a Multibody.
We were testing how to load pre-shattered geometry from Houdini into Realflow as a multibody for simming. When we tried exporting our pre-shattered mesh from Houdini as an SD file it doesn't simulate a shattered object (We may be doing something wrong at this stage...please let me know about this.)
Because we couldnt get this to work with an SD file out of Houdini we tried an exporting our shattered mesh as an Alembic which works as a Multibody when exported from Houdini with Partitioning by a name attribute. The issue with this is a bug in the Houdini Alembic Exporter. When using the Partition functionality on the Houdini Alembic Exporter while this doesn't change the point count, it changes the point order. We confirmed this was a Houdini only bug by comparing the source geo and the partitioned geo, which should be the same at the point level...but are not.
So where does this leave us. Realflow is the best and has no issues with changing the point numbers! But we would love to know how to imported pre-shattered geo and simulate as a multibody.

Thanks so much for all of your help!
~Ben

[enrique]

Hi Ben, I am glad it helped

About the velocity thing: you are right, this is an option we considered this morning. It could be a problem with rotating objects, so I will talk about this issue next week with my colleagues.

The UV's in alembic with multibodies problem has just been fixed, it will work in the next beta of 2014.

About the shattered geometry: it should work well with an SD file, what do you mean with "it doesn't simulate"? can you check that the geometry is properly shattered? I mean, no holes or weird things. We are talking about simulating it with caronte, right? It could help us if you could provide us with an example of such geometry to track down this issue.

Thanks again for your feedback Ben.

Enrique.

[ben.fox@framestore.com]

Thanks for looking into those issues.
I think the velocity thing could be useful for Houdini users. Not a huge deal, but it tripped up the guy who was testing Caronte for usability.

As for the SD as Multibody issue...what I mean by not simulating in Caronte is that is behaves like a single object, not shattered pieces.
Once loaded as a Multibody, the File > Summary Info only lists it as a single object. So I guess it is not reading it correctly as separate chunks.
I have attached a shattered cube that I can't get to work as a multibody for the life of me. Just a basic Houdini voronoi shattered cube exported as an SD.
Cube+PointsFromVolume+VoronoiFracture and export as SD.
I have exaggerated the chunk separation to try to illustrate the issue.
Not sure if the issue is on the Houdini side or Realflow side...so please let me know your thoughts, or what steps I can take to get something like this to work.
Thanks so much for your help on this.
Houdini Alembic's implementation issues makes the SD route seem like the only option.

[ben.fox@framestore.com]

As an additional note, when I use the Houdini SD importer to reimport the SD file I write out of Houdini to check the file, it has multiple copies of the same mesh...though they aren't there when I write them out. FYI there is no animation or anything on it.

[gus]

Hi Ben,

Got that fracture file and when importing it in RF I see what you say, a single node inside the MultiBody in the Summary info, so...I took it into Maya and did a Mesh>Separate. To my surprise that gives me the individual faces, instead of what I was expecting, which was the individual pieces also.
Could it be that the operation that you have done to shatter it in houdini isn´t giving objects but components as output?

Unfortunately we are not houdini savvy here

g-)

[gus]

As an additional note, when I use the Houdini SD importer to reimport the SD file I write out of Houdini to check the file, it has multiple copies of the same mesh...though they aren't there when I write them out. FYI there is no animation or anything on it.

Ok. Will have to check this with the plugin developer to see if it is a bug we were not aware off.
Thanks Ben.

g-)