Harvey W. Yurow @ Unblinking Eye said:
One of the last relatively unexplored areas in analog photography is permanganate bleach re-reversal (PBR), which can result in striking, surrealistic photographic diapositives, having both negative and positive portions on a single frame. These can consist of low density shadow areas of a grainy appearance, contrasting with relatively high density middle-tones and clear highlights. In addition, very thin, dark contour lines occur at the borders between shadows and middle-tones, and shadows and highlights.