It’s just the technique of encoding more than just the RGBA value in a pixel.
In traditional compositing technique the artist has to deal with oversampling, motion blur i.e. artifacts in semi transparent areas.
Deep images deals with this by storing multiple samples per pixel at various depths or we can say, a deep image records the 3d renderer’s list of all points in depth that contribute to the pixel. Every surface or volume sample making up the final pixel is stored with its appearance information about color, opacity, and depth.
Deep images are used in several rendering and shading approaches for shadowing and occlusion calculation.
Deep shadow maps is the technique of creating filtered shadow buffers with color and transparency information It produces fast, high-quality shadows for primitives such as hair, fur, and smoke. Unlike traditional shadow maps, which store a single depth at each pixel, deep shadow maps store a representation of the fractional visibility through a pixel at all possible depths. Deep shadow maps have several advantages. First, they are prefiltered, which allows faster shadow lookups and much smaller memory footprints than regular shadow maps of similar quality. Second, they support shadows from partially transparent surfaces and volumetric objects such as fog. Third, they handle important cases of motion blur at no extra cost.
Deep images are used in several rendering and shading approaches for shadowing and occlusion calculation. Most prominent is Pixar’s RenderMan deep shadow technique in which deep textures are rendered through a light camera and evalu- ated at points visible to the render camera.