Assessment of Normalized Difference Skin Index Robustness in Aquatic Environments.

摘要

The Normalized Difference Skin Index (NDSI) is a numeric value generated from two wavelengths of the electromagnetic spectrum, a feature that can be utilized for a dismount detection system. Skin is a major anatomical trait of human beings, furthermore, the most exposed trait to the elements. An aspect of a person's skin is typically uncovered, such as their face or hands. If skin pixels are detected in a spectral image, we can confidently conclude a person is present in the scene. There has been successful research on the use of spectral imagery for NDSI skin detection in ideal conditions. However, in e orts to achieve an operational dismount detection system, the robustness of NDSI has to be explored for more austere conditions. The focus of this thesis is on aquatic environments. Water in aquatic environments poses a challenging problem in the spectral domain because of its absorptive characteristic in several regions of the electromagnetic spectrum, known as water absorption bands. In these particular bands, spectral information becomes occluded from the 'dominance' of water. Unfortunately, the two NDSI wavelengths exist in close proximity with these bands. Experiments were devised to emulate scenarios that may arise between skin and water in a pixel of a hyperspectral image. With a mixed pixel of skin with water in the background, the spread of NDSI values were within a range defined by previous research until 64% or less of the pixel constituted skin. With a mixed pixel of skin with water droplets, the evaluated amount of water had negligible impact on NDSI. A mixed pixel of skin under a thin layer of water rendered NDSI useless with a shallow depth of 5 mm; water layers prove to be extremely detrimental to NDSI. Also, the temporal factor of water absorption by skin was assessed. Within the evaluated durations, up to 6300 s, the results showed negligible impact on NDSI.