Relative scale/size of Phoenix/Max objects

[TL4500]

Hi.

I am fairly new to Phoenix and although I have done a few examples I am looking for clarification regarding scale/size.

I am working on a project at the moment where I have a Max animation created in 3DS Max Design 2014. My system units and display units in Max are both set to mm. In previous tests/examples I found that because my scene objects tend to be quite large (say 10000mm diameter or 10000 cube for example) when I placed a Phoenix grid the maximum of 9999 was far too small for the scene but so big that it was too large to create effects. For this reason, in my current project, I scaled all my objects down to a very small size so that I could insert a Phoenix grid about 250x150x150 thinking this would then enable me to create a suitable flame in the grid. I tested the flame briefly before continuing with my Max animation set up.

I did all my Max modelling/lighting/animation and everything is completed except the flame. When I came to create the flame I had difficulty getting it to look how I wished. After much testing, I went back to basics and worked with an example kindly supplied to me by Chaos support. This file had system and display units both set to cm. I created a suitable flame based on these settings and a few tweaks. When I created a new flame using the same settings in a new file with system and display units set to mm the resulting flame was no good. I realise that this is something to do with the physical properties and the "real world/unrealistic" size of the objects.

I think for future reference I will create a flame first and then scale my objects around the Phoenix objects.

My question is:

In this instance, as I have already set up my material scales and animation, is there any way I can use the suitable flame that I have created within my existing Max file?

I have logged this as a call with Chaos support but any other input in helping me understand the scaling/size issue will be most welcome. I also need to sort this A.S.A.P. as I have a deadline looming!

Many thanks,

Terry

[TSR_trix]

If you need display units to be mm, I would use it only in display units and set system units to be cm in simulations.

[Svetlin.Nikolov]

Hello,

Yes, if you change the system scale, you will generally not get the same results.
In the sample scenes you provided, if you need to reduce the scale 10 times from 1cm to 1mm and still keep the grid the same relative size, you may try to:
- change the cell size to 10mm to match 1cm (up 10 times).
- return the X/Y/Z sizes to the same as before, because touching the cell size will change them.
- scale the grid down to 10.0% in the viewport to get the same size as before.
- increase the discharge of the source 10 times.
- reduce the gravity 10 times
- the cooling is also affected, but the scale there is exponential, so you may need to adjust by feeling (larger grid scale means the fluid will cool down more slowly, so you need to increase to adjust). In your case, you need a really small correction.

I hope this helps

[Ivaylo Katev]

Well, your conclusion about Phoenix scale sensitivity is correct, the scale is important.
If you find Svetlin's solution too complicated, i would recommend to change the gravity in the dynamics panel. if your simulation is good when you change the mm system units to cm, then you will have good result with gravity 0.1 and mm units.

[TL4500]

Thanks.

I get Svetlin's explanation OK.

I sent some updated questions to Chaos support based on my "improved" version. I found that four flames added to my model increased frame render time from 5 minutes to over four hours! Not sure if it is VRay settings that are too "good" (e.g. Indirect Illumination/Light cache settings are "high") or whether I need to "simpilfy" the flames.

Many thanks,

Terry

[Ivaylo Katev]

i suppose your settings are too hard for fire rendering
i would recommend to render the fire and smoke without GI, because the atmosphere effects may produce significant slow down when GI is used, because each GI ray causes multiple atmosphere calculations.
better render the scene twice - the geometry part with GI and the fire without GI
if you need the light of the fire, use the "emit if not renderable" option of phoenix (colors and transparency, fire panel)

[TL4500]

Thanks for that.

Sorry Ivaylo, I am a bit new to Phoenix. When you say render twice, once with for geometry and once without GI for fire, how do I then combine the two?

Thanks,

Terry

[Ivaylo Katev]

with a compositing software, it's widely used and i assumed you have one.

[TL4500]

Thanks.

I have After Effects, although I have not done any compositing before. Any tips on getting started? I am limited trying to find tutorials as YouTube is blocked by our company. I looked on Adobe TV and the basic compositing tutorial deals with compositing "objects" on layesr as opposed to render frames.

How do I go about rendering so that just the flames are saved in one render and the other render has no flames?

Thanks,

Terry

[Ivaylo Katev]

How do I go about rendering so that just the flames are saved in one render and the other render has no flames?

use the "renderable" option in the object properties, right click when the simulator is selected

[TL4500]

Thanks Ivaylo.

So, just to clarify, does this workflow make sense:

1. I render the 300 frames with the flames turned off and with GI turned on.
2. I render the 300 frames with GI turned off. I have the flames "Renderable". I have visible in the scene any objects that interact/overlap with the flames BUT I have "Renderable" unticked in their object properties. I also use somerthing like Targa and use the Alpha channel so that the background is transparent.
3. I then composite these by putting the flames on a layer above the original render in After Effects.

Many thanks,

Terry

[Ivaylo Katev]

yes, in general correct
i have only one additional note - no need to disable the renderable option of all the objects (if they are big in count this may be hard to manage), just add the atmosphere render element and use its result, it will contain only the fire without the objects.

[TL4500]

Thanks Ivaylo.

"just add the atmosphere render element and use its result".

By this do you mean:

Go to the Render elements tab of VRay render set up.
Click on Add, and VRayAtmosphere.
Output to TGA.

I did this but when I started to render other objects in the scene also rendered?

I ticked "Enable" and got the message that the Render element will be shown as off but still work OK.

Is there something else I need to set so that I just get the flames rendering?


Thanks,

Terry

[Ivaylo Katev]

when you render with render elements enabled, you obtain the whole result and as additional sequence images of the desired render elements.
you can see them changing the combo placed left-up at vray frame buffer

[TL4500]

Thanks Ivaylo.

I'm not sure what you mean by any of this?

"when you render with render elements enabled, you obtain the whole result and as additional sequence images of the desired render elements"

We are trying to obtain just the flames, so if we get the whole result as well will this not take twice as long (or more)?

"you can see them changing the combo placed left-up at vray frame buffer"

I am not sure at all what you mean by this?

Thanks,

Terry