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Fireandsmoke

Flickering colours in slow Coloured Smoke

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Hi,

I have a simple smoke sim using the Colored Smoke shelf tool which I have run at timescale 0.1, cached out to VDB, and am using Redshift to render.  Not problems if I render a sim at full speed, but I'm noticing at this slower timescale I'm getting random flickering of my colours, the saturation dropping away to a varying degree every few frames. 

https://giphy.com/gifs/FfpdaTVBqu7CVlcAd8


Any ideas??


Thanks for the advice

Edited by Fireandsmoke

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what happens if you cache out your sim at full speed with substeps and retime it after the simulation?

You should be able to use the new Retime node from Houdini 17 to do this quite easily.

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23 minutes ago, Jesper Rahlff said:

what happens if you cache out your sim at full speed with substeps and retime it after the simulation?

You should be able to use the new Retime node from Houdini 17 to do this quite easily.

Well I would have thought so, but I've been having plenty of issues with the new Retime node..  I posted about it on sidefx forum but nobody has replied, I keep getting very jittery results such as this:  https://giphy.com/gifs/69EhNTJHxxpfE6vHU8
My post is here:  https://www.sidefx.com/forum/topic/59932/

One thing I have not been doing though is adding extra substeps to my initial sim.  Is that a requirement for a velocity advected retime?  And is so is there a relationship between the number of substeps required and the desired retimed speed?  

Any advice would be helpful, I haven't been getting very good results so far!
 

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32 minutes ago, Fireandsmoke said:

Well I would have thought so, but I've been having plenty of issues with the new Retime node..  I posted about it on sidefx forum but nobody has replied, I keep getting very jittery results such as this:  https://giphy.com/gifs/69EhNTJHxxpfE6vHU8
My post is here:  https://www.sidefx.com/forum/topic/59932/

One thing I have not been doing though is adding extra substeps to my initial sim.  Is that a requirement for a velocity advected retime?  And is so is there a relationship between the number of substeps required and the desired retimed speed?  

Any advice would be helpful, I haven't been getting very good results so far!
 

 the way you can retime volumes is to use a Gas Up Res node to basically create subframes from your original simulation and then do a velocity advect on your source volume to gain the exact desired speed. The theory goes that if you know you want to slow down your simulation to 10% you need 10 subframes. meaning frame numbers like 1.0, 1.1, 1.2, 1.3 etc..

By default the retime node will try to create the interpolation on its own, but if you cache out subframes from your original sim, you can feed the retime node those frames and it can do the retime correctly. This way you avoid going through another simulation step. 


The help file if you search Gas Up Res will at the bottom of the page give you an example file of how to retime the volume.

 

 

Edited by Jesper Rahlff

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26 minutes ago, Jesper Rahlff said:

 the way you can retime volumes is to use a Gas Up Res node to basically create subframes from your original simulation and then do a velocity advect on your source volume to gain the exact desired speed. The theory goes that if you know you want to slow down your simulation to 10% you need 10 subframes. meaning frame numbers like 1.0, 1.1, 1.2, 1.3 etc..

By default the retime node will try to create the interpolation on its own, but if you cache out subframes from your original sim, you can feed the retime node those frames and it can do the retime correctly. This way you avoid going through another simulation step. 


The help file if you search Gas Up Res will at the bottom of the page give you an example file of how to retime the volume.

 

 

Ah ok, excellent.  I will have a look at that, thanks.  When you say cache out subframes, does this have to be done as a .sim, or can you cache subframes into .bgeo or .vdb?

My understanding was that a large purpose of the Retime was to avoid having to do all this, relying instead on the method of interpolation (aided by advection), so I'm still curious as to why I'm getting such severe errors (basically its jumping back to some kind of rest state at 5 frame intervals (retiming at a 0.2 speed)).  

Not to hijack my own thread, but whilst a Retime approach may work, I'm still also curious as to why my original problem is occuring with the flickering color, if anybody has a theory about that...? 

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14 minutes ago, Fireandsmoke said:

Ah ok, excellent.  I will have a look at that, thanks.  When you say cache out subframes, does this have to be done as a .sim, or can you cache subframes into .bgeo or .vdb?

you can cache subframes into whatever format you want.

 

coming back to the original question, where is the smoke getting the color from?

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3 minutes ago, Jesper Rahlff said:

you can cache subframes into whatever format you want.

 

coming back to the original question, where is the smoke getting the color from?

Its at home so I can't double check but 95% sure it was temp

 

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17 minutes ago, Fireandsmoke said:

My understanding was that a large purpose of the Retime was to avoid having to do all this, relying instead on the method of interpolation (aided by advection), so I'm still curious as to why I'm getting such severe errors (basically its jumping back to some kind of rest state at 5 frame intervals (retiming at a 0.2 speed)).

that is correct. however if you dive inside the retime node and look at how it is build, you will see there is no advection going on. its a sequence blend, trying to interpolate v@P over time. This calculation is fairly easy to do on points, but not on voxels.

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3 minutes ago, Fireandsmoke said:

Its at home so I can't double check but 95% sure it was temp

 

can you share the file when you get home?

 

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8 minutes ago, Jesper Rahlff said:

can you share the file when you get home?

 

Will do!  It currently imports a reasonably heavy vdb and requires redshift, but the sim is still in there so you can see the settings I used.   Thanks Jesper, much appreciated.  Ill start looking into Gas Upres and substeps tonight.    

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14 hours ago, Jesper Rahlff said:

can you share the file when you get home?

 

Well Ive worked out where the problem was coming from, but damned if I know why.. maybe its a bug.  For Redshift to render coloured smoke, you have to convert the volume to a VDB (among other things).  And basically I'm getting the error if I cache out to VDB and then load the VDB back in and render.  If I cache to Bgeo, and then do a the conversion to VDB at render time, no problems.   

Iv'e been playing all even with the GasUpRes...  I LOVE it!   It's the first tool I've used in Houdini that makes timescaling a sim do what I want, I wish I'd looked into it ages ago.    One question I have, which maybe by chance you can answer...  I'm trying to use Gas UpRes with the Colored Smoke shelf tool. but the color is not transferring to the upressed volume.  The volumes are created but the Cd values are black.  I found a hacky way to use Volume Mix to mix back in a frame with colour from the low res smoke, but I'm wondering if there's a better fix...?  

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4 hours ago, Fireandsmoke said:

Iv'e been playing all even with the GasUpRes...  I LOVE it! 

Thats awesome. Yeah it makes retiming mostly pain free for sure.

4 hours ago, Fireandsmoke said:

The volumes are created but the Cd values are black.  I found a hacky way to use Volume Mix to mix back in a frame with colour from the low res smoke, but I'm wondering if there's a better fix...?  

so I am not sure this is the reasons why, but if you go to advanced < bindings on the up res node, at the bottom you can specify extra fields that it needs to take into consideration from the low res volumes. Try to input the name of your color volume there.

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6 hours ago, Jesper Rahlff said:

Thats awesome. Yeah it makes retiming mostly pain free for sure.

so I am not sure this is the reasons why, but if you go to advanced < bindings on the up res node, at the bottom you can specify extra fields that it needs to take into consideration from the low res volumes. Try to input the name of your color volume there.

Thanks Jesper. Nice try but no cigar unfortunately!

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8 minutes ago, Fireandsmoke said:

Thanks Jesper. Nice try but no cigar unfortunately!

I will have to look at a scene in order to be able to trouble shoot properly. Right now I am just firing from the (dot)hip

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Ha..!  :)
I will have more of a look tonight as well. 
Very easy to replicate the problem, just lay down a coloured smoke (via shelf), and gas upres it (also via shelf).  Youll see straight away that the upres smoke is black.  
-J

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hm yeah I see the problem. That is a good question. I am at work right now, but I will try to dive deeper into it when I am off.

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