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{{Short description|Data structure used in image rendering}}
In [[computer science]] a [[level set]] [[data structure]] is designed to represent discretely [[Sampling (statistics)|sampled]] dynamic level sets functions.▼
{{Use British English|date=June 2025}}
▲In [[computer science]], a
A common use of this form of data structure is in efficient image [[Rendering (computer graphics)|rendering]]. The underlying method constructs a [[distance transform|signed distance field]] that extends from the boundary, and can be used to solve the motion of the boundary in this field.
==Chronological developments==
The powerful [[level
formulations". ''Journal of Computation Physics'' 79:12–49.</ref> However, the straightforward implementation via a dense d-dimensional [[array data structure|array]] of values, results in both time and storage complexity of <math>O(n^d)</math>, where <math>n</math> is the cross sectional resolution of the spatial extents of the ___domain and <math>d</math> is the number of spatial dimensions of the ___domain.
===Narrow band===
The narrow band level set method, introduced in 1995 by Adalsteinsson and Sethian,<ref name="Adalsteinsson">Adalsteinsson, D. & Sethian, J. A. 1995. "A fast level set method for propagating interfaces." ''[[Journal of Computational Physics]]''. 118(2)269–277.</ref> restricted most computations to a thin band of active [[voxel]]s immediately surrounding the interface, thus reducing the time complexity in three dimensions to <math>O(n^2)</math> for most operations. Periodic updates of the narrowband structure, to rebuild the list of active voxels, were required which entailed an <math>O(n^3)</math> operation in which voxels over the entire volume were accessed. The storage complexity for this narrowband scheme was still <math>O(n^3).</math> Differential constructions over the narrow band ___domain edge require careful interpolation and ___domain alteration schemes to stabilise the solution.<ref>{{cite journal|title=A fast level set Method for Propagating Interfaces|
===Sparse field===
This <math>O(n^3)</math> time complexity was eliminated in the approximate "sparse field" level set method introduced by Whitaker in 1998.<ref name=Whitaker>Whitaker, R. T. 1998. "A level-set approach to 3d reconstruction from range data." ''[[International Journal of Computer Vision]].'' 29(3)203–231.</ref> The sparse field level set method employs a set of linked lists to track the active voxels around the interface. This allows incremental extension of the active region as needed without incurring any significant overhead. While consistently <math>O(n^2)</math> efficient in time, <math>O(n^3)</math> storage space is still required by the sparse field level set method. See
===Sparse block grid===
The sparse block grid method, introduced by Bridson in 2003,<ref name=Bridson>Bridson, R. 2003. "Computational aspects of dynamic surfaces (dissertation)." [[Stanford University]], Stanford, California.</ref> divides the entire [[bounding volume]] of size <math>n^3</math> into small cubic blocks of <math>m^3</math> voxels each. A coarse grid of size <math>(n/m
)^3</math> then stores pointers only to those blocks that intersect the narrow band of the level set. Block allocation and deallocation occur as the surface propagates to accommodate to the deformations. This method has a suboptimal storage complexity of <math>O\left((nm)3 + m^3n^2\right)</math>, but retains the constant time access inherent to dense grids.
===Octree===
The [[octree]] level set method, introduced by Strain in 1999
===Run-length encoded===
The [[run-length encoding]] (RLE) level set method, introduced in 2004,<ref name=Houston2004>Houston, B., Nielsen, M., Batty, C., Nilsson, O. & K. Museth. 2006. "Hierarchical RLE Level Set: A Compact and Versatile Deformable Surface Representation." ''[[ACM Transactions on Graphics]]''. 25(1).</ref> applies the RLE scheme to compress regions away from the narrow band to just their sign representation while storing with full precision the narrow band. The sequential traversal of the narrow band is optimal and storage efficiency is further improved over the octree level set. The addition of an acceleration lookup table allows for fast <math>O(\log r)</math> random access, where r is the number of runs per cross section. Additional efficiency is gained by applying the RLE scheme in a dimensional recursive fashion, a technique introduced by Nielsen & Museth's similar DT-Grid.<ref name="Nielsen">Nielsen, M. B. & Museth K. 2006. "Dynamic Tubular Grid: An efficient data structure and algorithms for high resolution level sets." ''[[Journal of Scientific Computing]]''. 26(1) 1–39.</ref>
===Hash Table Local Level Set===
The Hash Table Local Level Set method
# to obtain accurate results, a rather large band is required close to the interface, which counterbalances the absence of grid nodes far from the interface;
# the performances are deteriorated by extrapolation procedures on the outer edges of the local grid and
# the width of the band restricts the time step and slows down the method.
===Point-based===
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