Black Offset

The primary objective of a multi-projection system is to produce a homogeneously aligned and geometrically accurate image across the entire projection surface. In configurations where adjacent projector images overlap, a seamless transition between these images is crucial. This is achieved through edge-blending techniques, which compensate for luminance accumulation in overlap zones by gradually fading out each projector's output.

Limitations of Standard Edge Blending

Standard soft edge blending techniques are designed to gradually reduce the intensity of input images in overlapping areas, typically to zero, to ensure a smooth transition. However, these techniques cannot compensate for the additional light produced by projector black offset. This excess light in the overlap zone cannot be "reduced" to a level below the projector's inherent black output.

Understanding Black Offset

For inherent technical reasons, most projectors are unable to reproduce a perfect black. The residual light emitted by a projector when the input is absolute black (RGB 0,0,0) is referred to as the black offset. This phenomenon becomes particularly noticeable in the overlapping regions, where accumulating black offsets can create perceptible brighter areas, especially when the displayed content is relatively dark.

Consider the following illustration:

• Both projectors projecting black (RGB 0,0,0) with visible black offset in the overlap.
• Both projectors projecting a dark scene with a visible black offset effect in the overlap.

It is important to note that the effects of black offset are generally imperceptible when projecting lighter scenes or images.

Projector black offset is an inherent characteristic of lower-quality or commodity projectors, where light is emitted even when the input image is entirely black (RGB 0,0,0). When these projectors are used in multi-projection systems with image overlap and soft edge blending, this black offset can lead to visible seams or brighter artifacts in the overlapping regions, compromising image homogeneity.

Recommended Solutions for Black Offset Compensation

1. High-Quality Projectors and Low-Gain Screens

   The most effective approaches for mitigating black offset involve hardware-based solutions: The optimal solution involves utilizing high-quality, high-contrast projectors that exhibit minimal inherent black offset. Complementing these projectors with low-gain projection screen coatings (with gains typically in the range of 0.6 – 0.7) can further help in compensating for minimal black offset by the projection surface itself.

2. Physical masks or Dynamic Black Offset Correction

   Traditionally, physical masks have been employed to mechanically block the projector's light path during the projection of dark scenes.

   An alternative, albeit less ideal, approach involves dynamic black offset correction by increasing the intensity of non-overlapping areas on the projection screen.