Computational modeling and psychophysics in low- and mid-level vision

This thesis addresses a series of topics related to the question of how people find the foreground objects from complex scenes. With both computer vision modeling, as well as psychophysical analyses, we explore the computational principles for low- and mid-level vision.

We first explore the computational methods of generating saliency maps from images and image sequences. We propose an extremely fast algorithm called Image Signature that detects the locations in the image that attract human eye gazes. With a series of experimental validations based on human behavioral data collected from various psychophysical experiments, we conclude that the Image Signature and its spatial-temporal extension, the Phase Discrepancy, are among the most accurate algorithms for saliency detection under various conditions.

In the second part, we bridge the gap between fixation prediction and salient object segmentation with two efforts. First, we propose a new dataset that contains both fixation and object segmentation information. By simultaneously presenting the two types of human data in the same dataset, we are able to analyze their intrinsic connection, as well as understanding the drawbacks of today’s “standard” but inappropriately labeled salient object segmentation dataset. Second, we also propose an algorithm of salient object segmentation. Based on our novel discoveries on the connections of fixation data and salient object segmentation data, our model significantly outperforms all existing models on all 3 datasets with large margins.

In the third part of the thesis, we discuss topics around the human factors of boundary analysis. Closely related to salient object segmentation, boundary analysis focuses on delimiting the local contours of an object. We identify the potential pitfalls of algorithm evaluation for the problem of boundary detection. Our analysis indicates that today’s popular boundary detection datasets contain significant level of noise, which may severely influence the benchmarking results. To give further insights on the labeling process, we propose a model to characterize the principles of the human factors during the labeling process.

The analyses reported in this thesis offer new perspectives to a series of interrelating issues in low- and mid-level vision. It gives warning signs to some of today’s “standard” procedures, while proposing new directions to encourage future research.

New measurements on the absolute cosmic ray ionization from sea level to 1540 kilometers altitude

An air filled ionization chamber has been constructed with a volume of 552 liters and a wall consisting of 12.7 mg/cm² of plastic wrapped over a rigid, lightweight aluminum frame. A calibration in absolute units, independent of previous Caltech ion chamber calibrations, was applied to a sealed Neher electrometer for use in this chamber. The new chamber was flown along with an older, argon filled, balloon type chamber in a C-135 aircraft from 1,000 to 40,000 feet altitude, and other measurements of sea level cosmic ray ionization were made, resulting in the value of 2.60 ± .03 ion pairs/cm³ sec atm) at sea level. The calibrations of the two instruments were found to agree within 1 percent, and the airplane data were consistent with previous balloon measurements in the upper atmosphere. Ionization due to radon gas in the atmosphere was investigated. Absolute ionization data in the lower atmosphere have been compared with results of other observers, and discrepancies have been discussed.

Data from a polar orbiting ion chamber on the OGO-II, IV spacecraft have been analyzed. The problem of radioactivity produced on the spacecraft during passes through high fluxes of trapped protons has been investigated, and some corrections determined. Quiet time ionization averages over the polar regions have been plotted as function of altitude, and an analytical fit is made to the data that gives a value of 10.4 ± 2.3 percent for the fractional part of the ionization at the top of the atmosphere due to splash albedo particles, although this result is shown to depend on an assumed angular distribution for the albedo particles. Comparisons with other albedo measurements are made. The data are shown to be consistent with balloon and interplanetary ionization measurements. The position of the cosmic ray knee is found to exhibit an altitude dependence, a North-South effect, and a small local time variation.