title: Resolving Measurement Errors Inherent with Time-of-Flight Range Imaging Cameras

authors: Whyte, Refael Zabdi

subjects: Time-of-Flight Range Camera

abstract: Range imaging cameras measure the distance to objects in the field-of-view (FoV) of the camera, these cameras  enable new machine vision applications in robotics, manufacturing, and human computer interaction. Time-of-flight (ToF) range cameras operate by illuminating the scene with amplitude modulated continuous wave (AMCW) light and measuring the phase difference between the emitted and reflected modulation envelope. Currently ToF range cameras suffer from measurement errors that are highly scene dependent, and these errors limit the accuracy of the depth measurement.  The major cause of measurement errors is multiple propagation paths from the light source to pixel, known as multi path interference. Multi-path interference typically arises from: inter reflections, lens flare, subsurface scattering, volumetric scattering, and translucent objects. 

This thesis contributes three novel methods for resolving multi-path interference: coding in time, coding in frequency, and coding in space.  Time coding is implemented by replacing the single frequency amplitude modulation with a binary sequence. Fundamental to ToF range cameras is the cross-correlation between the reflected light and a reference signal. The measured cross-correlation depends on the selection of the binary sequence. With selection of an appropriate binary sequence and using sparse deconvolution on the measured cross-correlation the multiple return path lengths and their amplitudes can be recovered. However, the minimal resolvable path length is dependent on the highest frequency in the binary sequence.  

Frequency coding is implemented by taking multiple measurements at different modulation frequencies. A subset of frequency coding is operating the camera in a mode analogous to stepped frequency continuous wave (SFCW). Frequency coding uses techniques from radar to resolve multiple propagation paths. The minimal resolvable path length is dependent on the camera's modulation bandwidth and the spectrum estimation technique used to recover distance, and it is shown that SFCW can be used to measure depth of objects behind a translucent sheet, while AMCW measurements can not. Path lengths below quarter a wavelength of the highest modulation frequency are difficult to resolve. 

The use of spatial coding is used to resolve diffuse multi-path interference. The original technique comes from direct and global separation in computer graphics, and it is modified to operate on the complex data produced by a ToF range camera. By illuminating the scene with a pattern the illuminated areas contain the direct return and the scattering (global return). The non-illuminated regions contain the scattering return, assuming the global component is spatially smooth. The direct and global separation with sinusoidal patterns is combining with the sinusoidal modulation signal of ToF range cameras for a closed form solution to multi-path interference in nine frames. With nine raw frames it is possible to implement direct and global separation at video frame rates. The RMSE of a corner is reduced from 0.0952 m to 0.0112 m.  Direct and global separation correctly measures the depth of a diffuse corner, and resolves subsurface scattering however fails to resolve specular reflections. Finally the direct and global separation is combined with replacing the illumination and reference signals with a binary sequence. The combination allows for resolving diffuse multi-path interference present in a corner, with the sparse multi-path interference caused mixed pixels between the foreground and background. The corner is correctly measured and the number of mixed pixels is reduced by 90%.    

With the development of new methods to resolve multi-path interference ToF range cameras can measure scenes with more confidence. ToF range cameras can be built into small form factors as they require a small number of parts: a pixel array, a light source and a lens. The small form factor coupled with accurate range measurements allows ToF range cameras to be embedded in cellphones and consumer electronic devices, enabling wider adoption and advantages over competing range imaging technologies.

text: Resolving Measurement Errors Inherent with Time-of-Flight Range Imaging Cameras Range imaging cameras measure the distance to objects in the field-of-view (FoV) of the camera, these cameras  enable new machine vision applications in robotics, manufacturing, and human computer interaction. Time-of-flight (ToF) range cameras operate by illuminating the scene with amplitude modulated continuous wave (AMCW) light and measuring the phase difference between the emitted and reflected modulation envelope. Currently ToF range cameras suffer from measurement errors that are highly scene dependent, and these errors limit the accuracy of the depth measurement.  The major cause of measurement errors is multiple propagation paths from the light source to pixel, known as multi path interference. Multi-path interference typically arises from: inter reflections, lens flare, subsurface scattering, volumetric scattering, and translucent objects. 

This thesis contributes three novel methods for resolving multi-path interference: coding in time, coding in frequency, and coding in space.  Time coding is implemented by replacing the single frequency amplitude modulation with a binary sequence. Fundamental to ToF range cameras is the cross-correlation between the reflected light and a reference signal. The measured cross-correlation depends on the selection of the binary sequence. With selection of an appropriate binary sequence and using sparse deconvolution on the measured cross-correlation the multiple return path lengths and their amplitudes can be recovered. However, the minimal resolvable path length is dependent on the highest frequency in the binary sequence.  

Frequency coding is implemented by taking multiple measurements at different modulation frequencies. A subset of frequency coding is operating the camera in a mode analogous to stepped frequency continuous wave (SFCW). Frequency coding uses techniques from radar to resolve multiple propagation paths. The minimal resolvable path length is dependent on the camera's modulation bandwidth and the spectrum estimation technique used to recover distance, and it is shown that SFCW can be used to measure depth of objects behind a translucent sheet, while AMCW measurements can not. Path lengths below quarter a wavelength of the highest modulation frequency are difficult to resolve. 

The use of spatial coding is used to resolve diffuse multi-path interference. The original technique comes from direct and global separation in computer graphics, and it is modified to operate on the complex data produced by a ToF range camera. By illuminating the scene with a pattern the illuminated areas contain the direct return and the scattering (global return). The non-illuminated regions contain the scattering return, assuming the global component is spatially smooth. The direct and global separation with sinusoidal patterns is combining with the sinusoidal modulation signal of ToF range cameras for a closed form solution to multi-path interference in nine frames. With nine raw frames it is possible to implement direct and global separation at video frame rates. The RMSE of a corner is reduced from 0.0952 m to 0.0112 m.  Direct and global separation correctly measures the depth of a diffuse corner, and resolves subsurface scattering however fails to resolve specular reflections. Finally the direct and global separation is combined with replacing the illumination and reference signals with a binary sequence. The combination allows for resolving diffuse multi-path interference present in a corner, with the sparse multi-path interference caused mixed pixels between the foreground and background. The corner is correctly measured and the number of mixed pixels is reduced by 90%.    

With the development of new methods to resolve multi-path interference ToF range cameras can measure scenes with more confidence. ToF range cameras can be built into small form factors as they require a small number of parts: a pixel array, a light source and a lens. The small form factor coupled with accurate range measurements allows ToF range cameras to be embedded in cellphones and consumer electronic devices, enabling wider adoption and advantages over competing range imaging technologies.

year: 2015

title: Physiological constraints on the latitudinal distribution of the mangrove Avicennia marina (Forsk.) Vierh. subsp. australasica (Walp.) J. Everett in New Zealand

authors: Beard, Catherine

subjects: Avicennia marina

abstract: ﻿New Zealand's only mangrove species, Avicennia marina (Forsk.) Vierh. subsp. australasica (Walp) Everett (Avicenniaceae), has a clearly defined southern natural limit around latitude 38° S. Historically, this has been attributed to the lethal effects of winter frosts (Chapman & Ronaldson, 1958) and, more recently to poor species dispersal and lack of suitable habitat south of the present species boundary ( de Lange & de Lange, 1994). 
A series of investigations of frost tolerance, leaf gas exchange, stomatal conductance, chlorophyll a fluorescence, water relations and stable carbon isotope analyses of New Zealand Avicennia were undertaken in the laboratory and field in order to characterise frost tolerance limits and photosynthetic performance under normal and stressed conditions. By so doing, the relative importance of winter frosts and low air temperatures in defining the range of Avicennia in New Zealand could then be determined. 
It was found that photosynthetic performance in Avicennia was increasingly depressed near its southern limits. Photosynthetic responses were consistent with those typical for cold-sensitive species after exposure to low temperatures. Large decreases in photosynthetic production, photoinhibition and chlorophyll loss were evident following exposure to night temperatures lower than 4°C, suggesting that critical damage occurred even at temperatures well above freezing. 
Frosts probably still play an important, additional and episodic role in defining the range of mangrove in New Zealand. The physiological stress effects of low temperatures were compounded by sub-zero temperatures and productivity in frostprone areas was significantly reduced by physical damage to leaves, branches and reproductive tissue. Freeze-injury of this form also reduced the likelihood of successful seedling establishment and long-term survival of saplings. 
These findings indicate that latitude 38° S acts as a physiological barrier for mangrove in New Zealand. Avicennia might persist further south than its present natural range if dispersal to suitable microclimates occurred. However, the probability of this occurring naturally is small, given the species poor dispersal abilities, and the reduced likelihood of survival in the face of damage and physiological stresses brought about by cold winter temperatures.

text: Physiological constraints on the latitudinal distribution of the mangrove Avicennia marina (Forsk.) Vierh. subsp. australasica (Walp.) J. Everett in New Zealand ﻿New Zealand's only mangrove species, Avicennia marina (Forsk.) Vierh. subsp. australasica (Walp) Everett (Avicenniaceae), has a clearly defined southern natural limit around latitude 38° S. Historically, this has been attributed to the lethal effects of winter frosts (Chapman & Ronaldson, 1958) and, more recently to poor species dispersal and lack of suitable habitat south of the present species boundary ( de Lange & de Lange, 1994). 
A series of investigations of frost tolerance, leaf gas exchange, stomatal conductance, chlorophyll a fluorescence, water relations and stable carbon isotope analyses of New Zealand Avicennia were undertaken in the laboratory and field in order to characterise frost tolerance limits and photosynthetic performance under normal and stressed conditions. By so doing, the relative importance of winter frosts and low air temperatures in defining the range of Avicennia in New Zealand could then be determined. 
It was found that photosynthetic performance in Avicennia was increasingly depressed near its southern limits. Photosynthetic responses were consistent with those typical for cold-sensitive species after exposure to low temperatures. Large decreases in photosynthetic production, photoinhibition and chlorophyll loss were evident following exposure to night temperatures lower than 4°C, suggesting that critical damage occurred even at temperatures well above freezing. 
Frosts probably still play an important, additional and episodic role in defining the range of mangrove in New Zealand. The physiological stress effects of low temperatures were compounded by sub-zero temperatures and productivity in frostprone areas was significantly reduced by physical damage to leaves, branches and reproductive tissue. Freeze-injury of this form also reduced the likelihood of successful seedling establishment and long-term survival of saplings. 
These findings indicate that latitude 38° S acts as a physiological barrier for mangrove in New Zealand. Avicennia might persist further south than its present natural range if dispersal to suitable microclimates occurred. However, the probability of this occurring naturally is small, given the species poor dispersal abilities, and the reduced likelihood of survival in the face of damage and physiological stresses brought about by cold winter temperatures.

year: 2006

title: The discovery of gold at Te Aroha and its consequences: January to October 1880

authors: Hart, Philip

abstract: By mid-1880, when there were expectations that gold would be found at Te Aroha, at last Ngati Rahiri permitted prospecting. A government-subsidized prospecting party under Hone Werahiko set out in early August and announced an apparently valuable find one month later. As the discovery was on Ngati Rahiri land, a goldfield could not be declared open immediately, but despite this increasing numbers of prospectors arrived. Newspapers were cautious, not wanting to encourage a rush based on nothing but rumours, some of them extravagant, notably one of finding ore that would produce 200oz to the ton. 
Despite being doubtful about the prospects, officials prepared to proclaim a new goldfield. In late October, good specimens were shown in Thames, encouraging more to leave both there and the Waikato to explore. After the warden inspected the ground where Werahiko had found gold, which was off limits to everyone except the original prospectors, he arranged to have the only test made before the field was opened. Although this gave an uncertain indication of whether the ore was payable, and little real work was being done outside the prospectors’ claim, regulations were devised permitting the proclamation of the new field.

text: The discovery of gold at Te Aroha and its consequences: January to October 1880 By mid-1880, when there were expectations that gold would be found at Te Aroha, at last Ngati Rahiri permitted prospecting. A government-subsidized prospecting party under Hone Werahiko set out in early August and announced an apparently valuable find one month later. As the discovery was on Ngati Rahiri land, a goldfield could not be declared open immediately, but despite this increasing numbers of prospectors arrived. Newspapers were cautious, not wanting to encourage a rush based on nothing but rumours, some of them extravagant, notably one of finding ore that would produce 200oz to the ton. 
Despite being doubtful about the prospects, officials prepared to proclaim a new goldfield. In late October, good specimens were shown in Thames, encouraging more to leave both there and the Waikato to explore. After the warden inspected the ground where Werahiko had found gold, which was off limits to everyone except the original prospectors, he arranged to have the only test made before the field was opened. Although this gave an uncertain indication of whether the ore was payable, and little real work was being done outside the prospectors’ claim, regulations were devised permitting the proclamation of the new field.

year: 2016