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High-Precision Photogrammetry for Glaciology
Consumer-grade digital cameras have become ubiquitous tools for documenting short-term variability in the geosciences. However, these devices were not intended for precise timekeeping and surveying, and their use as such requires management of systematic and random errors that inevitably arise.This dissertation presents a suite of methods for registering the place and time of photographs in an absolute reference frame so that they may be analyzed and interpreted alongside other spatial and temporal data. The methods are tested on a 13-year record of 33,000 time-lapse photographs from Alaska's Columbia Glacier. This work provides insights into the capabilities and shortcomings of consumer-grade cameras as scientific instruments, the opportunistic approaches often needed to achieve the best results, and the potential of continuous high-frequency measurements for documenting rapid geomorphic processes.Subsecond-precision image capture times are achieved by measuring the offset to a reference clock display and accounting for the drift, precision, and reporting resolution of the camera clock. Two case studies illustrate the benefit of subsecond precision in contemporary investigations: georeferencing aerial photogrammetric surveys with camera positions time-interpolated from GPS tracklogs, and coupling videos of glacier-calving events to synchronous seismic waveforms. Retroactive dating of photographs, on the order of seconds to hours, is achieved by leveraging phenomena visible in the photographs -- namely, the positions of astronomical objects in the sky or the corresponding variations in solar radiation and sea level.Similarly, retroactive camera calibrations are achieved using surface and topographic features in the photographs -- specifically, point and line features of known absolute position, the motion of static features in images due to camera rotation, and the correspondences between real images and images synthesized from vertical imagery. Camera motion is corrected by computing globally optimal estimates of rotation over arbitrarily-long photographic sequences.Finally, a recently-developed tracking algorithm based on particle filtering theory is refined and applied to estimate Columbia Glacier velocities, their associated uncertainties, and the corresponding strain rate fields at 3-day intervals over a 13-year period, providing an unprecedented look at the seasonal and sub-seasonal variability of tidewater glacier dynamics over long time scales
Photometric Reconstruction from Images: New Scenarios and Approaches for Uncontrolled Input Data
The changes in surface shading caused by varying illumination constitute an important cue to discern fine details and recognize the shape of textureless objects.
Humans perform this task subconsciously, but it is challenging for a computer because several variables are unknown and intermix in the light distribution that actually reaches the eye or camera.
In this work, we study algorithms and techniques to automatically recover the surface orientation and reflectance properties from multiple images of a scene.
Photometric reconstruction techniques have been investigated for decades but are still restricted to industrial applications and research laboratories.
Making these techniques work on more general, uncontrolled input without specialized capture setups has to be the next step but is not yet solved.
We explore the current limits of photometric shape recovery in terms of input data and propose ways to overcome some of its restrictions.
Many approaches, especially for non-Lambertian surfaces, rely on the illumination and the radiometric response function of the camera to be known.
The accuracy such algorithms are able to achieve depends a lot on the quality of an a priori calibration of these parameters.
We propose two techniques to estimate the position of a point light source, experimentally compare their performance with the commonly employed method, and draw conclusions which one to use in practice.
We also discuss how well an absolute radiometric calibration can be performed on uncontrolled consumer images and show the application of a simple radiometric model to re-create night-time impressions from color images.
A focus of this thesis is on Internet images which are an increasingly important source of data for computer vision and graphics applications.
Concerning reconstructions in this setting we present novel approaches that are able to recover surface orientation from Internet webcam images.
We explore two different strategies to overcome the challenges posed by this kind of input data.
One technique exploits orientation consistency and matches appearance profiles on the target with a partial reconstruction of the scene.
This avoids an explicit light calibration and works for any reflectance that is observed on the partial reference geometry.
The other technique employs an outdoor lighting model and reflectance properties represented as parametric basis materials.
It yields a richer scene representation consisting of shape and reflectance.
This is very useful for the simulation of new impressions or editing operations, e.g. relighting.
The proposed approach is the first that achieves such a reconstruction on webcam data.
Both presentations are accompanied by evaluations on synthetic and real-world data showing qualitative and quantitative results.
We also present a reconstruction approach for more controlled data in terms of the target scene.
It relies on a reference object to relax a constraint common to many photometric stereo approaches: the fixed camera assumption.
The proposed technique allows the camera and light source to vary freely in each image.
It again avoids a light calibration step and can be applied to non-Lambertian surfaces.
In summary, this thesis contributes to the calibration and to the reconstruction aspects of photometric techniques.
We overcome challenges in both controlled and uncontrolled settings, with a focus on the latter.
All proposed approaches are shown to operate also on non-Lambertian objects