2019-06-03-reconstruction
Structured light Q&A
- phase shifting
- algorithm for determining correspondence of the different phases
- terminology
- spatial sinusoids
- black stripes have intensity of 1 and white stripes have intensity of 0 and they corresponds to spatial sinusoids
- frequency
- 1 pattern with varying width column stripes
- shift N times
- 1 pattern that is shifted by some N times, each by amount of phase
- phase
- each phase corresponds to a column stripe in a spatial sinusoids
- where did the formula relating intensity and amplitude, offset, phase changes are ?
- https://en.wikipedia.org/wiki/Phase_(waves)#Phase_shift
- phase wrapping/unwrapping from interferometry paper (relation to phase shifting)
- what is light transport coefficient a_{qc}
- describes intensity of reflected light from point q that is received by point c?
- why amplitude is a function of both frequency and amount of defocus?
- why higher mean frequency -> resistance to global illumination
- why frequency measured with respect to pixels
- spatial frequency ?
- w = 16 pixel period means how many 16 pixel in the image
- 1991_automatic_processing_of_fringe_pattern_in_interger_interferometers
- original paper explaining micro-phasing theory
- goal
- determine total phase difference of light waves without interference fringe counting
- 2003_pattern_codification_strategies_in_structured_light_system
- coded structured light
- for recovering surface of objects
- no location constraint
- idea
- project a light pattern
- view the illuminated scene from one or more points of view
- decoded points can be triangulated and 3D information obtained
- patterns
- the projected images are called patterns
- stereo vision
- view scene from two or more points of view and then finding correspondence between different images to triangulate the 3D position
- coding strategies
- patterns
- designed to have codewords assigned to a set of pixels
- f: codewords -> coordinates of pixel in pattern
- codewords are numbers, i,e. grey levels, colors,
- based on what to classify
- what kind of codeword is used
- if codeword encodes a single axis or two spatial axis
- time-multiplexing strategy
- patterns successively projected onto measuring surface
- codeword for a pixel: sequence of illumination values
- the good
- accurate because
- codeword basis tends to be small, so distinguishable
- since pattern is successively projected, …
- subclasses
- binary codes
- 0-1-1-0-1- …for 1 pattern axis
- a sequence of m patterns,
- 2^m stripes, i.e. codewords, using a binary code
- codeword is a sequence of 0s and 1s from m patterns, first pattern is one contains most significant bit, 0-black, 1-white
- n-ary
- reduces number of patterns by means of increasing the number of intensity levels used to encode the stripes
- multilevel Gray code
- Gray code with alphabet of n symbols -> reduce number of patterns
- i.e. n^m instead of 2^m stripes
- gray code combined with phase shifting
- same pattern projected several times, shifting it in certain direction in order to increase resolution
- Gray code methods unambiguous
- hamming distance of 1 -> good against noise
- phase shift methods
- hybrid
- 2012_micro_phase_shifting
- http://www.cs.columbia.edu/CAVE/projects/micro_phase_shifting/
- goal
- shape recovery addressing global allumination + illumination defocus
- problems & solutions
- global illumination
- project sinusoidal patterns with frequencies limited to a narrow frequency band, over which global illumination remain cosntant
- defocus effects of projecter (limited FOV)
- frequency in a narrow band,
- so amplitudes for all frequencies are approximated the same, … single unknown
- reduce number of input images for Micro PS
- resolving depth ambiguities
- fact
- high freq sinusoids -> high resolution info, but in small depth range
- use low freq sinusoids to disambiguate phase info over larger range
- solution
- emulate a low frequency sinusoid with a period equal to product of periods of several high-frequency sinusoids
- phase shifting
- phase corresponds to correspondence of projector pixels -> depth ..