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djeison canuto avatar image
djeison canuto asked ·

Xparticle OVDB ISSUE

Hi,

I recently got X-particles 4 and I started to use their new eXplosiaFX feature. I export an OVDB but when I import the OVDB into the same scene it's gigantic. I contact Insydium and they told me that redshift and octane open them up fine. So, the issue may be on Arnolds end. Please let me know if you can help.

Best,

arnoldscenex-particlesscenes
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Peter Horvath avatar image
Peter Horvath answered ·

I'm not sure what's going on, I'll take a look when I have a chance. Meanwhile you can try to scale down the Arnold Volume object (maybe 0.01?) as a workaround.

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djeison canuto avatar image
djeison canuto answered ·

Thanks, @Peter Horvath. I have tried that but its like it loses resolution and detail when I do. The bigger issue is that I have a moving object that it needs match but it doesn't.

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Peter Horvath avatar image
Peter Horvath answered ·

Sorry for the late reply, I finally get a chance to test xp4.

The principle C4DtoA follows regarding scaling is that translating a default primitive (e.g. Sphere) from any DCC should be the same size. (In other words, export a primitive from C4D and import it in other DCC should match the default primitive of the DCC.) Therefore 100 cm in C4D is translated to 1 unit in Arnold.

The rule X-Particles uses while exporting a VDB file is to translate 1 cm to 1 unit. As I see the scale parameter in the xpCache applies in object space instead of world space, which results in a wrong result when the solver is not in the origo. So that won't solve the problem.

The only solution I see at the moment is to scale down the Arnold Volume object to 0.01. Since the standard_volume shader depends on the distance, you also have to scale the density and emission up to 100.

I'm trying to figure out something more user friendly in a future release.


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Thank you for the tip. I'll try your (hopefully temporary) solution, because I did run into the same issue when trying on XP4 OVDB ...

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Scott Frizzle avatar image
Scott Frizzle answered ·

Just to add to the above: In addition to the scaling issue, some of the Open VDB channels generated by X Particles/ Explosia don't seem to translate correctly to the Standard Volume shader. Density seems to translate exactly as expected, but Heat and Fuel don't seem to produce predictable results, even taking into account your suggestions above. I'm sure it would be a considerable amount of work, but full native support for the Explosia object would be ideal; with all available channels translating into the Standard Volume shader, and with the ability to render tests without caching VDB files, as is currently the case with Cycles 4D.

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Peter Horvath avatar image
Peter Horvath answered ·

We might need a specific shader with the knowledge how to handle these channels. Also currently the XP SDK does not allow us to read the channels of the Explosia object.

So the point is, we need a collaboration with Insydium to properly support the new Explosia solver. Hopefully it will happen in the (near) future.

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Chadi Abo avatar image
Chadi Abo answered ·

Hello dear friends, the 0,01 scaling down trick don't appear to work in my case too, somehow the center of the volume object shift the whole object to unpredictable directions, I am applying the VDB from the ExplosiaFX object to a moving object, and I need to stick that object to the VDB voxels I created from. Anyhelp or trick will be appreciated.

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Peter Horvath avatar image
Peter Horvath answered ·

There's a difference in the coordinate system used by C4D and Arnold. It seems xp exports with the C4D coordinates, so you have to invert the Z. Basically just use a -1 z scale in the Arnold Volume object:

If that does not help, just send me a scene please.


xp-ovdb-invz.png (22.0 KiB)
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