Rendering Semi-Transparent Fire Volumes In Blender Cycles

Setting Up the Fire Simulation

To create a realistic fire simulation in Blender, we need to use the built-in smoke and fire simulators. This allows hot gases and flames to rise in a physically plausible way based on temperature differences and turbulence. Here are the key steps to set up a basic fire simulation:

1. Add a Smoke Domain object to contain the simulation. This defines the boundaries and resolution. Increase the division values to get more detail.
2. Add a Smoke Flow object inside the domain. This will emit hot smoke and flames. Enable temperature and set it above 0 to emit fire.
3. In the Physics tab, increase the Smoke High Resolution value to add more definition.
4. Bake the simulation for the required frames to cache the calculations.
5. For rendering, add a Principled Volume shader to the domain to visualize the flames and smoke.

The smoke domain acts as the container for the simulation, while the flow object injects heat and turbulence over time to create realistic rising flames. It’s important to bake an adequately long simulation to achieve a continuous looping effect.

Optimizing Fire Simulations

There are several ways to optimize fire simulations in Blender to achieve a good balance of speed and quality:

• Start with a small domain bounds and low divisions to test the simulation quickly.
• Reuse baked smoke caches to iterate quickly without re-simulating.
• Add turbulence fields for increased complexity without needing more divisions.
• Use adaptive domain divisions to focus computing in key areas.
• Utilize viewport caching for fast feedback while tweaking.
• Lower the emitter’s temperature gradually for thinner wispy flames.
• Bake high resolution only for the required frames to save memory.

Finding the optimal point between quality and speed for the shot requirements takes some trial and error. Frequently saving caches and reusing them helps to refine the sim faster.

Creating a Fire Material with Principled Volume

The key to making realistic fire in Blender is setting up an advanced volume material that can scatter and emit light properly. We’ll use the Principled Volume shader which simulates absorption and emission inside a 3D volume using ray marching.

Volume Absorption and Scattering

The Principled Volume shader has several parameters to customize the way light passes through:

• Density affects the overall transparency and thickness.
• Anisotropy adds directional scattering effects.
• Absorption tints light passing through a volume.
• Emission Colors and Strength contributes new light.

For fire, we want a moderately dense core that lets some light penetrate through the outer flames. Setting anisotropy to a small positive value adds banding effects seen in real fires. Absorption can give flames a darker sooty appearance towards the base.

Adding Blackbody Emission

Real flames burn at around 1200–2400 °C, emitting light according to Planck’s law of blackbody radiation. We can simulate this by enabling Temperature and setting an appropriate value in Kelvin degrees.

This tints the Volume’s emission colors towards the intense reds, oranges and yellows seen in real fire. Cooler values around 1000-1500K give more naturalistic results. Higher temperatures emulate gas burner flames.

Blending this principled emission color with a regular shader creates a multi-layered volume with a hot core and cooler outer flames.

Rendering Considerations for Fire Simulations

There are several aspects to consider when setting up rendering for fire simulations to balance realism with fast iteration times:

Choosing a Render Engine

Blender offers various render engines for integrating smoke and fire. Cycles and Eevee can both create realistic results with different approaches:

• Cycles uses path tracing for accurate lighting and volumetrics.
• Eevee utilizes real-time approximations requiring less memory.

In most cases Cycles will achieve more photorealism due to unbiased light transport. But Eevee can provide quicker previews to speed up workflows.

Render Sampling Settings

Cycles requires enough samples per pixel to properly resolve complex volume details and noise patterns:

• Use at least 100 Render Samples to clear noise inside flames.
• Enable Adaptive Sampling to focus effort only where needed.
• Increase Max Samples on the Principled Volume to improve quality.

Denoisers can help recover lost details from sparse sampling for quicker draft renders. Optix AI Denoiser works well with Cycles volumetrics.

Memory Management

Heavy simulations require large bake caches, which quickly eat up GPU memory during rendering. Some tips include:

• Use an atmosphere volume instead of simulating smoke away from flames.
• Delete unused sections of the domain object to optimize cache size.
• Lower the domain’s overall divisions value where possible.
• Add a Subdivision Surface modifier to increase flame detail.

Strategic optimizations let you reclaim memory for extended renders at higher sampling settings for the final frames.

Compositing Fire Over Live Footage

Compositing combines rendered CG elements with real footage to create integrated scenes. This lets us mix fire sims into shots to achieve more dramatic visual effects sequences.

Tracking Motion

The first step is to track the camera movement in the footage so that the perspective and position of elements matches precisely:

1. Import image sequence into camera tracked scene.
2. Add tracking markers on high-contrast features.
3. Solve the camera motion over the clip.
4. Composite fire sim into reconstructed 3D space.

Good tracking is essential for the combined visual elements to move realistically together in 3D space.

Lighting Integration

It’s important that lighting color, direction and intensity on the CG elements match the real footage:

• Add image texture emission planes for ambient lighting.
• Use masking objects and texture coordinates for projection.
• Match directional real-world sun and fill lights.

Consistent scene lighting helps composite elements look like a cohesive environment.

Compositing Nodes

Blending modes available in Blender’s compositing nodes help integrate fire and smoke simulations:

• Add for a brighter overall look.
• Screen to composite flames without shadows.
• Overlay to intensify colors and contrasts.

Composting nodes give extensive flexibility when blending rendered layers with live footage.

Example Fire Material Node Setup

Here is one approach to constructing a multi-layered fire shader using Principled Volumes:

1. Add several Volume Output nodes mixed with Transparent shaders.
2. Enable Emission and Absorption on each at different strengths.
3. Set the innermost to a high Temperature for hot colors.
4. Add Noise nodes to increase variation over time.

Chaining volume shaders together builds up depth and complexity in the way light scatters through flames towards the camera.

The final render emerges from the combined interactions of all participating media, creating a heterogeneous fire texture not achievable from surface materials alone.