Volumetric rendering stands at the forefront of visual simulation technology. It intricately models how light interacts with myriad tiny particles to produce stunningly realistic visual effects such as smoke, fog, fire, and other atmospheric phenomena. This technique diverges significantly from traditional rendering methods that predominantly utilize geometric shapes (such as polygons in 3D models). Instead, volumetric rendering approaches these phenomena as if they are composed of an immense number of particles. Each particle within this cloud-like structure has the capability to absorb, scatter, and emit light, contributing to the overall visual realism of the scene.
This is not solely useful for generating lifelike visual effects in movies and video games; it also serves an essential function in various scientific domains. Volumetric rendering enables the visualization of intricate three-dimensional data crucial for applications such as medical imaging, where it helps in the detailed analysis of body scans, and in fluid dynamics simulations, where it assists in studying the behavior of gases and liquids in motion. This technology, thus, bridges the gap between digital imagery and realistic visual representation, enhancing both our understanding and our ability to depict complex phenomena in a more intuitive and visually engaging manner.
How does this work?
Let’s start by talking about direct volume rendering. Instead of trying to create a surface for every object, this technique directly translates data (like a 3D array of samples, representing our volumetric space) into images. Each point in the volume, or voxel , contains data that dictates how it should appear based on how it interacts with light.
For example, when visualizing a CT scan, certain data points might represent bone, while others might signify soft tissue. By applying a transfer function—a kind of filter—different values are mapped to specific colors and opacities. This way, bones might be made to appear white and opaque, while softer tissues might be semi-transparent.
The real trick lies in the sampling process. The renderer calculates how light accumulates along lines of sight through the volume, adding up the contributions of each voxel along the way. It’s a complex ballet of light and matter, with the final image emerging from the cumulative effect of thousands, if not millions, of tiny interactions.
Let us make this a bit more concrete. We first have transfer functions, a transfer function maps raw data values to visual properties like color and opacity. Let us represent the color assigned to some voxel as C(v) and the opacity as α(v). For each pixel in the final image, a ray is cast through the data volume from the viewer’s perspective. For this we have a ray equation:
Where P(t) is a point along the ray at parameter 𝑡, P0 is the ray’s origin, and 
is the normalized direction vector of the ray. As the ray passes through the volume, the renderer calculates the accumulated color and opacity along the ray. This is often done using compositing, where the color and opacity from each sampled voxel are accumulated to form the final pixel color.
You probably used Volumetric Rendering
Volumetric rendering transforms CT and MRI scans into detailed 3D models, enabling doctors to examine the anatomy and functions of organs in a non-invasive manner. A specific application includes most of the modern CT viewers. Volumetric rendering is key in creating realistic simulations and environments. In most AR applications, it is used under the hood to overlay interactive, three-dimensional images on the user’s view of the real world, such as in educational tools that project anatomical models for medical students.
Now for the fun part (see the rules here), using volume rendering in a sentence by the end of the day:
Serious: The breakthrough in volumetric rendering technology has enabled scientists to create highly detailed 3D models of the human brain.
Less Serious: I tried to use volumetric rendering to visualize my Netflix binge-watching habits, but all I got was a 3D model of a couch with a never-ending stream of pizza and snacks orbiting around it.
…I’ll see you in the blogosphere.