Overlaying

Overlaying is the technique of placing one layer, image, or data set over another to combine, compare, or augment information. Successful overlaying relies on alignment, scale, and often transparency so the top layer integrates with the bottom.

In image processing, overlaying uses alpha blending: a foreground with transparency is composited with a background. Blending modes and the alpha channel control visibility. Uses include watermarking, annotations, and combining photos.

In geographic information systems, overlaying merges spatial layers to analyze relationships. Operations include intersection, union, difference, and symmetric difference. Overlay analysis reveals where features coincide, such as a habitat area overlapping a road network.

In medical imaging, overlays fuse data from different modalities or timepoints to aid interpretation. Co-registration aligns images so features correspond, and overlays help reveal anatomical matches or changes.

In user interfaces, overlays are elements that appear above existing content, such as modal dialogs, tooltips, and popovers. They can focus attention or provide controls, but should remain accessible and dismissible.

In networking, an overlay network is a virtual network built atop a physical network, enabling new topologies and services. Examples include virtual private networks, content overlays, and peer-to-peer systems. Overlays add flexibility but add complexity and potential latency.