Bella Render GUI v21.2.0.0


Bella is a spectral render engine which models the complexities of optical physics in ways that are both as correct as possible, and which are yet made intuitive and accessible.

Simplicity is a key focus of Bella. The Bella solvers have no obscure non-physical parameters, such that there is very little to learn, and very little trial and error involved in rendering any particular scene.

Spectral Rendering

Bella works in spectral space, allowing effects such as BSDF wavelength dependency, diffraction, or atmosphere to be modeled far more accurately than in color space.

Complex Caustics

Bella is capable of solving caustics through specular surfaces, for accurate rendering of swimming-pool caustics, caustics seen through windows, emitters enclosed in glass, etc.

Customer-first Licensing

Bella’s licensing is uncommonly — and deliberately — customer-centric. Both licenses (seat and node) are floating, permanent, and designed to be trouble-free.

Layered Materials

Bella’s materials provide real layers with inner subsurface scattering. Light interacts physically with the substrate, the layer, and thin films, in a very efficient way.

Thin Film

The Thin Film node, which may be applied to a material layer or substrate, simulates a nanometer-scale film, providing physically-accurate iridescence.

Nested Dielectrics

The nested priority feature in Bella materials allows correct and convenient rendering of intersecting dielectric (i.e. glass, liquid) volumes.

Complex IOR

The Complex Material provides a simple way of using measured spectral refractive index data. Use of external IOR files or data is optional.

PBR & Smart Materials

Bella’s Smart Materials provide a quick and easy way to create materials with just a few simple parameters. Its PBR, Principled, and Uber materials allow use of PBR textures which ranges from a single-click, to full customization.


Like everything else, textures in Bella are node-based, allowing an output from one texture to be used as an input to another.

Pervasive Instancing

Bella provides extensive support for instancing — indeed, everything that can be seen in a Bella scene is inherently an instance.

Physical Sky & Sun

Bella’s physical sun and sky model may be controlled by date/time and location, or by specifying direction explicitly.

Procedural Lights

Bella provides several types of built-in procedural lights, including point, spot, area, and directional.

Thin & Thick Lens

In addition to its standard thin lens, Bella also provides a unique thick lens, which simulates complex effects like chromatic aberration.

Sensor Bloom

The Bella camera supports simulation of sensor bloom: when a pixel is saturated beyond its capacity, it bleeds energy into adjacent pixels.


When light passes through the aperture of a diaphragm or grating filter, it diffracts, producing characteristic light patterns in the sensor.

Cooperative & Resume

Cooperative rendering allows you to render in several computers and merge result. This also allows resuming of a render after it has been stopped.

Intel Denoiser

Bella incorporates Intel’s excellentOpen Image Denoise library, for both IPR and production rendering.

Industry Standards

Bella makes use of several industry-standard libraries, for seamless compatibility with a wide range of formats and standards.

Render Passes

In addition to production beauty pass, Bella also provides passes such as alpha, shadow, material & object ID, normal, etc.

Geometry Import

The Bella GUI supports import of several common formats including OBJ, FBX, DAE, 3DS, and STL.

Scene Transport

Through its .BSZ format, Bella provides for easy single-file transport of scenes and their dependencies.

Customer-first Licensing

We believe that customers should not be made to pay the price for securing our product from unauthorized use.

We are against draconian licensing systems, which can be so difficult to configure and administer, and which so often fail at the worst possible moment — and usually in favor of the vendor, rather than the customer, who is considered guilty until proven innocent. We’ve been bitten by these types of systems enough times, ourselves.

We also dislike subscriptions, which in addition to having those deficiencies already mentioned above, constantly occupy mental real estate to no good end, and generally require constant or periodic connection to validation servers, and so forth.

The fact is: most software vendors are not expert at designing licensing systems — we certainly don’t pretend to be. And when that is the case, they may either naively implement something from scratch, or they may pay to license a system from one of the big names in that sector.

In the former case, we say: good luck. But even in the latter, there is still much room for error, as the integration of a 3rd-party system can be quite error-prone, and as the system, itself being quite complex, will still be subject to various modes of failure, whether due to bugs, or just to the vagaries of dealing with networks on customer sites, and the like.

For these reasons, the Bella license is implemented as a simple text file. The text for the file is available to you at any time, without needing to contact support — you can access it whenever you like simply by logging into your account and looking up your order. Copy the license text, paste it into a dialog in the Bella GUI (or put it directly in a text file, if for instance, you wish to distribute it across a farm of machines), and you are up and running.

A consequence of this is that every Bella license is floating, permanent, and requires no secondary registration or validation — exactly the kind of license that we, ourselves, prefer.


In order to more accurately simulate the equations governing the optical properties of materials, it is essential to render using many spectral energy bands instead of the usual three primary colors.

In Bella we perform all light transport computations in spectral space, from beginning to end, which allows to compute with greater precision a multitude of optical phenomena such as light dispersion, Fresnel reflectance, iridescence, BSDFs with wavelength dependence, atmospheric simulation, aperture diffraction, and so forth.

It is only at the very end of the process that we transform the computed spectral energies (irradiance) to the chosen color space.


The Bella render architecture is based on having one or more Solvers render the scene. Our current beauty-pass solvers (which to make it clear, are all currently CPU-based) are:


An advanced unbiased path tracer, Atlas is the default solver for production beauty pass, as it is highly optimized for solving complex lighting scenarios, including difficult caustics that cannot be solved feasibly by some other render engines.


Ares is a sophisticated unbiased (but non-bidirectional) path tracer, which may be used for production beauty pass, potentially rendering faster than Atlas in scenes which have less complex lighting.


An unbiased path tracer, the IPR (Interactive Preview Render) solver has been created & tuned for giving quick initial feedback on changes in the scene. This solver is used to implement any realtime interactive rendering windows in the Bella’s GUI and plugins.

Difficult swimming-pool caustics from sun light.

It has long been a well-known and difficult problem to render correctly, without tricks or artifacts, the caustic illumination that we perceive through reflection or specular refraction. And yet, this is a situation that arises frequently in life, and therefore in professional rendering work.

One of the most infamous cases is that of sunlight falling on a swimming pool and producing its characteristic caustic patterns on the bottom of the pool, which must then make their way back through the water surface to reach the camera.

Emitter enclosed in glass, resulting in caustics-only illumination.

Light sources are often contained within a glass envelope such that when modeled accurately, they will contribute only caustic illumination. This can make it quite difficult to render objects such as flashlights, auto headlight assemblies, and the like.

Bella is capable of rendering such difficult scenarios using its advanced Atlas solver, without any special knowledge or settings. It is possible to artificially limit bounces and computation of caustics, but this is fully optional, and up to the artist.

LAYERED materials
Gold complex IOR, and with layer+scattering.

At its core, Bella makes use of two fundamental material types: conductors and dielectrics. Conductors are opaque surfaces and dielectrics are transparent surfaces. Both are referred to as substrates, and are allowed to have a layer stacked above them. A layer is a dielectric surface, with a medium where optional scattering may be defined. The layer interacts with the substrate below it, through multiple bounces, combining to produce a very realistic material.

For example, in the image at right, both statues use a conductor (gold), but the one on the right is augmented with a layer that makes use of scattering.

Like conductors, dielectrics may also make use of a layer, generating a result by way of the interaction of light between the dielectric substrate and the layer. This is not just a blending operation, is a true multi-bounce interaction depending on angle, transmittance, Fresnel, roughness, anisotropy, and so forth.

Dielectric, sheet, and dielectric+layer.

The decanter at left uses just a dielectric, the one in the middle uses just a layer (well in fact, a sheet,), and the one at right uses the combination of dielectric + layer.

Sheet material.

The Sheet material is like a layer with virtual thickness (controlled by its transmittance), and no substrate. This is useful for simulating translucent thin surfaces, with scattering.


Yet another type that may be used in the layered system is the Thin Film, which allows adding iridescence to a surface. Changing the film’s thickness (by scalar value or by texture map) or index of refraction will produce corresponding interference patterns.

The thin film can be applied to the surface of any substrate, layer, or sheet.

Thin-film added to dielectric, metal, and conductor+layer.
Thin-film on sheet.

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