Shadow Removal via Flash/Noflash Illumination

Abstract

Shadows bedevil multimedia applications, e.g. seeing into shadow regions in surveillance video. Model-based and non-model based, statistical methods, spatial, temporal, and invariant-based methods have been devised for combatting the shadow problem. Here we take a different approach, by attenuating the shadow by utilizing a second image under another illuminant to remove the effect of shadow-edges from an edge map of each frame. As a precursor step, we examine flash/noflash still image pairs. A flash image provides lessened shadows, but other shadows are produced. We can produce a flash-only (no ambient) image by subtracting the two images, but several artifacts remain. Instead, we have used the pure-flash image to detect the ambient shadows [ICME06]. However that method may fail when there are flash-shadows or specularities in the copy region. Here we manipulate the gradient field using a smoothing step including the directionality of edges near the shadow boundary, with improved results