Location: Rangeland Resources Research
Title: Shadow Attenuation With High Dynamic Range Images Authors
Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 22, 2008
Publication Date: November 7, 2008
Citation: Cox, S.E., Booth, D.T. 2008. Shadow Attenuation With High Dynamic Range Images. Environmental Monitoring and Assessment 158:23-33. Interpretive Summary: Remote sensing efforts have historically been hindered by shadows in imagery so that feature measurements from the images are inaccurate or incomplete. We used composite images created using multiple, varied-exposures (high-dynamic range or HDR images) to remove most of the shadow. We then compared HDR and conventional images of the same grassland plots for measurements of ground cover and various measurements of image technical qualities. Because of reduced shadow HDR images showed a 26% increase in the area of the images that could be studied and measurements taken. This study is the first to demonstrate the successful application of HDR imagery in remote sensing, and has application potential for image analysis in any field.
Technical Abstract: Shadow often interferes with accurate image analysis. To mitigate shadow effects in near-earth imagery (2 m above ground level), we created high dynamic range (HDR) nadir images and used them to measure grassland ground cover. HDR composites were created by merging three differentially-exposed images spanning a wide exposure range, and resulted in lightened shadows. HDR images showed more detail; reduced the numbers of pure black, pure white, and pixels visually indistinguishable from black and white; reapportioned skewed luma values towards a normal distribution; increased the Euclidean distance between litter and bare ground RGB values—allowing increased feature separation; all of which facilitated an increase in real feature classification through manual image analysis. Drawbacks to the method included decreased image sharpness due to minor misalignment of images or moving vegetation, time required to create HDR images, and difficulty with acquiring primary images from a moving platform. We conclude that HDR imagery is a potential solution to the shadow problem.