MULTIVIEW ANALYSIS OF MIXED PIXELS IN THE FRACTION AND REFLECTANCE DOMAINS FOR UNDERSTANDING SUB-PIXEL TOPOGRAPHIC STRUCTURE

Abstract. The spectral mixture analysis (SMA) plays a vital role in spectral data analysis and extraction of subpixel information. However, this technique provides only quantitative information regarding the materials’ abundance fractions within the pixel. On the other hand, the Bidirectional Reflectance Distribution Function (BRDF) indicates that sub-pixel topography affects the surface’s directional reflection to a large extent. Unfortunately, despite the high importance of the BRDF effect and the SMA in remote sensing, only very few research works addressed their mutual influence. Thus, in this work, we propose a study that addresses this mutual influence and suggests an approach for extracting sub-pixel topographic information from mixed pixels. For this purpose, we conducted two multiview imaging experiments under controlled conditions using artificial mixed surfaces. Each surface type is made of two materials and has a varying structural pattern. Then we measured the BiConical Reflectance Factor (BCRF) of each surface from various viewing zenith angles. Next, we applied spectral unmixing to estimate the abundance fraction of three endmembers (EMs) in each surface’s pattern. Finally, we tested the relationship between the sup-pixel topography and the fraction variation vs. the multiple imaging directions. The first experiment results showed that multiview spectral measurements allow the separability between surfaces combining the same materials’ composition but with different sub-pixel structural arrangements.

Moreover, such separability is more accurate in the fraction space than in reflectance space. Besides, and most importantly, the second experiment revealed exciting outcomes regarding the relationship between the sub-pixel topographic feature and the variation of the EM fraction vs. the imaging viewing direction. Specifically, we showed a high correlation between the EMs’ fractions and the height of a repetitive element within the sub-pixel topography with a determination coefficient that reaches 0.89.