Hello everyone.
I encounter a weird problem.
As you know Gas Project Non Divergent nodes use the Helmholtz-Hodge decomposition to decompose a velocity field into a pressure scalar and a divergent free vector fields.
The gradient of pressure and divergent free vector field should be orthogonal to each other but when I look at the volume trail they are not orthogonal in any way!
No matter how hard I crank up the resolution the result is always the same.
The red lines are divergence-free portions and the green ones are curl-free portions.
I also attached my project in case anyone wants to look at it.
Does anyone have any solution to this problem?
Thank you so much.
Orthogonality in Gas Project Non Divergent Node
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- N-G
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- Soothsayer
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I'm also confused but I have a few thoughts:
- I don't trust the gradient calculation, I never have. Something's not right about it but I don't know what. You can easily get weird results.
- That aside, I don't trust the vector calculations (advect) and the viewport so I did a little test where calculate the dot product. They hover neatly around 0, as they should. The vectors however don't always look right.
- Maybe something with face vs center sampling might interfere but sometimes I can't calculate a gradient at all out of it.
So, I don't know but I have a feeling Houdini isn't telling us the truth.
I attached a file.
- I don't trust the gradient calculation, I never have. Something's not right about it but I don't know what. You can easily get weird results.
- That aside, I don't trust the vector calculations (advect) and the viewport so I did a little test where calculate the dot product. They hover neatly around 0, as they should. The vectors however don't always look right.
- Maybe something with face vs center sampling might interfere but sometimes I can't calculate a gradient at all out of it.
So, I don't know but I have a feeling Houdini isn't telling us the truth.
I attached a file.
--
Jobless
Jobless
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- N-G
- Member
- 209 posts
- Joined: 3月 2018
- Offline
Soothsayer
I'm also confused but I have a few thoughts:
- I don't trust the gradient calculation, I never have. Something's not right about it but I don't know what. You can easily get weird results.
- That aside, I don't trust the vector calculations (advect) and the viewport so I did a little test where calculate the dot product. They hover neatly around 0, as they should. The vectors however don't always look right.
- Maybe something with face vs center sampling might interfere but sometimes I can't calculate a gradient at all out of it.
So, I don't know but I have a feeling Houdini isn't telling us the truth.
I attached a file.
Thanks a lot.
You know the real reason behind the zero dot product is that the lengths for the pressure gradient vectors are too small.
If you normalize both divergence-free and pressure-gradient fields you will see the dot product is not going to be zero anymore!
As you visualized both divergence-free and pressure-gradient fields they are not orthogonal in any way!
I also tested it with VDB project non-divergent node with no luck!
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