H18: Realistic Campfire Shader Study

Use Mantra Render and Pyro Shader for Realistic Fire.

I noticed the preset pyro fire simulation is targeting on bigger scale fire. When it comes to smaller scale like campfire, the shader does not match the VFX standard quality. So, in this study is focus on creating a smaller scale realistic fire. Fire simulation and fire movements is not main focus in this study.

Fire Operators in Houdini 18

There are 3 main operators influences the final fire look, they are Pyro simulation, Pyro Shader, and Mantra Render. Each operator can make a significant change of the final fire look. You may use other render engine like Redshift but I will focus on Pyro Shader and Mantra render at this point. In order to get a better approach each time, I recommend start with Pyro Shader.

Real Fire Look

Real fire has very sharp edge even though it constantly moves, and it moves really fast.

When it paused it should have a very solid look and lick fire shape. So, create a very strong edge and solid look is the key focus for the study. In Houdini 18, fire simulation flame field is used to decide the density of the fire. The color determines by temperature field.

Fire is only getting blurry(Motion Blur) on the camera shot with a bigger shuttle angle and bigger aperture.

So, we also need to consider that in our VFX Fire recreate study. In Houdini 18, we can check Allow Motion Blur in Mantra render setting.

Fire Intensity Field

In Pyro Shader, Fire Intensity Field, in default the volume is looking at flame field. Which means the fire shape is determined by flame field. The image on the left, you'll notice that I increased the value to 3 at position 0.01 in the ramp. What does 0 and 1 means?

0 in flame field is where the flame field almost disappear.

1 means the flame field is full of flame.

And what's the value means? The value means how much Fire Intensity at certain flame field. Which means in my case at 0.01 flame field the fire intensity is 3.

Fire Temperature Field

Fire Temperature Volume in default is binding to temperature. I inherit the idea of using temperature to draw the color on flame field. The similar setup like I used in Fire Intensity Field make a high value close to ramp position 0, therefore we will draw a highlight color close to the edge.

If you're not familiar with the temperature look, you can simply go back to pyro object and at Visualization tab check temperature.

Then you'll have a better idea how the temperature move and so on.

Simulation Setup

Even though I don't want to dive into simulation, that is too much to talk about, and there are many different considerations. I do want to mention few fields and parameters are useful to create smaller scale fire.

Viscosity - Gather flame together, which can really help to create a nice sharp edge.
Turbulence - Create the overall shaking movements of the fire. It's the very first step I'd do in any kind of fire simulation.
Shredding - At certain range of flame cut off and create a nice streaks.
Disturbance - Create detailed look of the flame.
Confinement - Strengthen turbulence swirl velocity.

Other Simulations

There are other simulations I did for this study, but the parameters are quite different from the one to another. I think from the references and burning source there's big room to adjust, as an artist I cannot really find a simple method that I could just follow. That's the reason I don't think there is a guide for smaller scale fire simulation.

Last Update: April 7, 2020

Software: Houdini 18

OS: Windows 10

Specs: RTX 2070, AMD 3900x, 64GB RAM

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