Inner Frustum

Overview

The Inner Frustum (conceptually "inner field of view") is a central term in virtual production with LED walls or LED volumes (In-Camera VFX, or ICVFX). It denotes the area of the LED wall that corresponds to the actual field of view of the production camera, and within which the digital background image is rendered with correct perspective for that specific camera position.

The Inner Frustum moves in real-time with the camera across the wall: if the camera pans or moves, the high-quality rendered section shifts accordingly. This creates correct spatial depth (parallax) between foreground and background – the essential difference compared to a classic greenscreen, where perspective is only generated in post-production.

Relationship to the Outer Frustum

The Inner Frustum is always seen in conjunction with the Outer Frustum, the surrounding area of the LED wall outside the camera's view:

• Inner Frustum: covers the camera's field of view, is rendered in the highest quality and with matching perspective; only this area is actually captured.
• Outer Frustum: surrounds the Inner Frustum, is typically displayed with less detail, and primarily serves as an interactive light source for reflections, highlights, and colored spill on actors, sets, and props.

This division optimizes processing load: the render engine's computing power is concentrated on the area that ends up in the image, while the rest of the wall primarily handles the realistic illumination of the scene.

Functionality and Camera Tracking

A prerequisite for the Inner Frustum is precise camera tracking. The position, orientation, and lens data (including focal length/field of view and focus) of the physical camera are captured in real-time and passed to a render engine like Unreal Engine. From this data, the engine calculates the perspective the camera should see from the virtual scene and positions the Inner Frustum accordingly on the LED wall.

To maintain the illusion's stability, the Inner Frustum must always be slightly larger than the pure image crop to buffer latencies and rapid camera movements. With multiple cameras, each camera can have its own Inner Frustum, increasing the demands on computing power and synchronization.

Use on Set

The Inner Frustum is the core of any LED volume production. To ensure the captured section remains clean, set teams pay attention to, among other things:

• Correct calibration of tracking and lens data so that perspective and parallax are accurate.
• Sufficient size of the Inner Frustum so that it doesn't "run out" of the frame during fast movements.
• Avoiding visible edges between the Inner and Outer Frustum within the image crop.
• Coordination with the lighting department, as the Outer Frustum acts as part of the set lighting.