A hierarchical image representation similar to pyramids <202>, they have nodes that correspond to the cells of a pyramid and each non-terminal node has four children, but unlike a pyramid, a quad tree may be pruned so as to be unbalanced. For example, when all nodes in a subtree have the same gray value, the subtree may be represented by its root without loss of information, so that significant storage savings can be obtained for many images. More significantly, quad trees allow some operations to be performed efficiently by recursive procedures.
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