Perceptually Optimal Color Representation of Fully Polarimetric SAR Imagery.

Georgia Koukiou

doi:10.3390/jimaging8030067

Abstract

The four bands of fully polarimetric SAR data convey scattering characteristics of the Earth’s background, but perceptually are not very easy for an observer to use. In this work, the four different channels of fully polarimetric SAR images, namely HH, HV, VH, and VV, are combined so that a color image of the Earth’s background is derived that is perceptually excellent for the human eye and at the same time provides accurate information regarding the scattering mechanisms in each pixel. Most of the elementary scattering mechanisms are related to specific color and land cover types. The innovative nature of the proposed approach is due to the two different consecutive coloring procedures. The first one is a fusion procedure that moves all the information contained in the four polarimetric channels into three derived RGB bands. This is achieved by means of Cholesky decomposition and brings to the RGB output the correlation properties of a natural color image. The second procedure moves the color information of the RGB image to the CIELab color space, which is perceptually uniform. The color information is then evenly distributed by means of color equalization in the CIELab color space. After that, the inverse procedure to obtain the final RGB image is performed. These two procedures bring the PolSAR information regarding the scattering mechanisms on the Earth’s surface onto a meaningful color image, the appearance of which is close to Google Earth maps. Simultaneously, they give better color correspondence to various land cover types compared with existing SAR color representation methods.

Highlights

Nowadays, radar systems are capable of providing very-high-resolution images of theEarth’s surface by means of Synthetic Aperture Radar (SAR) [1,2]
These two procedures bring the polarimetric SAR image (PolSAR) information regarding the scattering mechanisms on the Earth’s surface onto a meaningful color image, the appearance of which is close to Google Earth maps
The proposed PolSAR coloring technique is novel as far as its completeness in terms of the elements it takes into account for the coloring is concerned, such as the correlation between the R, G, and B components that resembles that of the natural color images and simultaneously covers all perceptible color shades by means of the CIELab color space

Summary

Introduction

Radar systems are capable of providing very-high-resolution images of the. Earth’s surface by means of Synthetic Aperture Radar (SAR) [1,2]. The purpose is to obtain a color PolSAR image that is pleasant to the eye of the operator and has the appearance of physical images of the Earth’s surface as they appear from a satellite with an optical sensor (Google Earth maps). This is achieved by means of the Cholesky decomposition method [24] which provides the capability of regulating the correlation between the RGB components of the obtained color image so that it inherits the properties of a natural color image.

Polarimetry and SAR Imaging

Fusion of the Four PolSAR Images into a Natural RGB Representation

Equalization of Color Information in the CIELab Color Space

Experimental Approach and Color Relevance to Scattering Mechanisms

Discussion and Comparisons

Conclusions