Shadow attenuation with high dynamic range images. Creating RGB images that allow feature classification in areas otherwise obscured by shadow or oversaturation.
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; and 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 can provide more accurate measurements of bare soil cover for ecosystem monitoring and assessment.
Cox
Samuel E
SE
Agricultural Science Technician and Rangeland Scientist, USDA Agricultural Research Service, High Plains Grasslands Research Station, 8408 Hildreth Rd, Cheyenne, WY 82009, USA. Samuel.Cox@ars.usda.gov
Booth
D Terrance
DT
D004784:Environmental Monitoring / Q000379:methods*; D007089:Image Enhancement / Q000379:methods; D010781:Photography / Q000379:methods*