A structured-light 3D scanner is a device for measuring the three-dimensional shape of an object using projected light patterns and a camera system.

This paper presents a comprehensive survey on coded structured light techniques.

The patterns are specially designed so that codewords are assigned to a set of pixels. Every coded pixel has its own codeword, so there is a direct mapping from the codewords to the corresponding coordinates of the pixel in the pattern.The codewords are simply numbers, which are mapped in the pattern by using grey levels, color or geometrical representations.

Shows pattern projection techniques classified in three groups according to their coding strategy:

- time-multiplexing - generate the codewords by projecting a sequence of patterns along time, so the structure of every pattern can be very simple

- neighborhood codification - represents the codewords in a unique pattern

- direct codification - define a codeword for every pixel, which is equal to its grey level or color

Time-multiplexing strategy

A set of patterns are projected onto the measuring surface, the codeword for a given pixel is formed by the sequence of illuminance values for that pixel across the projected patterns. The codification is called temporal because the bits of the codewords are multiplexed in time.

This kind of pattern can achieve high accuracy in the measurements. This is due to two factors:

1- since multiple patterns are projected, the codeword basis tends to be small (usually binary) and therefore a small set of primitives is used, being easily distinguishable among each other;

2- a coarse-to-fine paradigm is followed, since the position of a pixel is being encoded more precisely while the patterns are successively projected.

They clasify the different techniques based on time-multiplexing as:

a) techniques based on binary codes: a sequence of binary patterns is used in order to generate binary codewords

b) techniques based on n-ary codes: a basis of n primitives is used to generate the codewords

c) Gray code combined with phase shifting: the same pattern is projected several times, shifting it in a certain direction in order to increase resolution

d) hybrid techniques: combination of time-multiplexing and neighborhood strategies

Spatial neighborhood

Tend to concentrate all the coding scheme in a unique pattern

clasification of techniques:

a) strategies based on non-formal codification: the neighborhoods are generated intuitively

b) strategies based on De Bruijn sequences: the neighborhoods are defined using pseudorandom sequences

c) strategies based on M-arrays: extension of the pseudorandom theory to the 2-D case

Direct codification

The entire codeword for a given point is contained in a unique pixel. In order to achieve this, it is necessary to use either a large range of color values or introduce periodicity.In theory, a high resolution of 3D information can be obtained.The sensitivity to noise is very high because the "distance" between "codewords", i.e. the colors used, is nearly zero.Moreover, the imaged colors depend not only on the projected colors, but also on the intrinsic color of the measuring surface.

They discuss of two groups of methods:

a) codification based on grey levels: a spectrum of grey levels is used to encode the points of the pattern

b) codification based on color: these techniques take advantage of a large spectrum of colors.

After clasification they implement some methods, compare them and present their resuls.

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