Shading the Winter Explosion

By Hammer Chen, Kristin Ivanova

We recently introduced the Artillery Explosion tutorial available in Phoenix FD’s documentation. Now, we want to show you a variation of this scene setup - a winter explosion with its proper shading.

This scene is essentially the same as the Artillery Explosion one, except for the smoke color and HDRI lighting. The smoke color is set to light gray in order to get a snowy look. To focus on the shading we also limit the number of bombs to just one.

Since there is a hot explosion at the core, it could make the rendering over-exposed or lose detail in the white smoke volumetric shading.

The good news is Phoenix FD provides a cohort of parameters allowing you to fine-tune the shading. Here we compare some of the useful settings that deal with this issue.

Overall Setup
The scene we use here is modified from the Artillery Explosion scene, except we add one Phoenix FD Turbulence helper and change the color of the smoke. For a step-by-step tutorial of how the Fire / Smoke sources and thinkingParticles are set up, please check out the Artillery Explosion tutorial.

See the screenshot, the color of V_dust_A and V_dust_B are set to light gray. RGB values are (203, 177, 159) and (252, 249, 247) respectively.

When light from the red hot explosion pierces the white smoke, it is very hard to avoid washout with the default settings (shown above). Even if we adjust the exposure in the VFB we still wouldn’t get  balanced exposure in every area of an image.

To overcome this problem, intuitively the first thing we do is to lower the Fire Multiplier in the Volumetric Render Settings of the Simulator. It does alleviate the problem as the parameter is the general multiplier for the fire color's intensity.

Alternatively, while keeping the Fire Multiplier to 1, we can lower down both the Light Power on Self and Light Power on Scene to 0.1. We get similar results.

- Light Power On Self controls the light intensity of the simulator's smoke.
- Light Power On Scene controls the light intensity over all scene objects except for the Phoenix FD Simulator itself.

But if you look more closely: the second option (adjusting the Light Power) gives you more details in the shading. So, we favor lowering the Light Power over the Fire Multiplier.

While keeping both Light Power on Self and Light Power on Scene to 0.1, what else can we do to increase details of our smoke shading? The answer is by adjusting the Physically Based option.This is a realistic model that multiplies the fire intensity by the Black Body Radiation model, which gives strong brightness to the hot parts of the fire. It transitions between an artistic look of the fire,when set to 0 and a realistic physically-based Intensity, when set to 1. In this case, we gave it a value of 0.2.

To further improve the appearance of our smoke, we can apply LUT to our final image. In the V-Ray Frame Buffer you can remap the image colors based on an IRIDAS .cube LUT (Look-Up Table) file. It transforms color input values to output values based on the specific LUT file.

Many LUTs emulate the film process. Film emulation gives you mid-tune control in an image. In this case, I use a LUT file called - 8650 Log from IWLTBAP. You don't have to use this specific LUT file, try any other LUT that works for you. Alternatively, you can achieve similar results by adjusting Levels & Curve in V-Ray Frame Buffer. You might need to sharpen your skill but you will get there.

Be sure to tick the "Save in image" if you want your rendering baked with the LUT. You can also tune down the LUT effect by reducing the weight in the VFB.

As you can see, after applying the LUT color correction, we see more middle tune and more details in the smoke shading.

Here are what we do to improve the smoke shading:
- Lower the value of "Light Power on Self and Light Power on Scene"
- Lower the value of Physically Base
- Apply a LUT file to the final image

Final animation

Here is the download link to the 3ds Max scene file. Note, you need Phoenix FD 4 and cebas thinkingParticles 6 to run the scene. The download pack does not include the HDR file (Winter Lake 01) nor the LUT file. However, for the HDRI, you can download it from HDRI Haven.

Enjoy your simulation project with Phoenix FD!