Frustum

Overview

The term Frustum (from Latin for "cone or pyramid stump") originates from 3D computer graphics and describes the pyramid stump-shaped viewing volume of a camera. In film production, the term appears primarily in the context of Virtual Production and ICVFX (In-Camera Visual Effects), i.e., during shoots in front of LED walls ("LED volumes").

On an LED wall, the digital set generated by the render engine (typically Unreal Engine) is divided into two zones: the Inner Frustum and the Outer Frustum. This distinction is crucial for the camera to record a perspective-correct image while simultaneously illuminating the subject coherently with the image content.

Inner Frustum and Outer Frustum

Both zones fulfill different tasks:

• Inner Frustum: The section of the LED wall that corresponds to the current field of view (FOV) of the shooting camera. This area is rendered from the camera's perspective and output in the highest quality ("final pixel"). It moves synchronously with the camera's position, pan, and focal length via camera tracking.
• Outer Frustum: The entire surface of the LED wall outside the camera's field of view. It is displayed at a lower resolution and primarily serves as a light source and reflection surface (image-based lighting). It generates the ambient light emanating from the set and provides reflections on actors, costumes, and shiny surfaces.

For the Inner Frustum to be perspective-correct, the virtual camera in the engine and the real shooting camera must precisely match in film back (sensor size) and focal length. Camera tracking transmits position and lens data to the render engine in real-time.

Significance for On-Set Lighting

From a lighting perspective, the Outer Frustum is particularly relevant: Since the LED wall itself emits light, it directly illuminates the subject with the colors and brightness of the displayed set. This approach does not rely solely on traditional lighting fixtures but uses the wall as a large-format, soft, and color-matched light source. This automatically creates coherent reflections and a consistent lighting impression between the foreground and the digital background.

To ensure that transitions are not noticeable in the image, the Inner Frustum is practically projected slightly wider than the actual captured image area and softly blended into the Outer Frustum at the edges (feathering). The buffer area also compensates for the latency between camera movement and real-time rendering.