Abstract: A method of obtaining continuous tone reproductions in a raster scanned digital image having a given number of pixels for each halftone cell of the digital image and using four modulating levels for each pixel of the output scanner including a black and a white pixel level, the method comprising the steps of: choosing two reflectance gray levels intermediate the black and white levels to combine with the black and white pixel levels for providing quadruples, four numbers specifying the number of each kind of pixel in the halftone cell, for modulating the output scanner, determining a total number of average halftone cell gray levels to be available for the digital image, creating a list of quadruples that describe all possible average halftone cell grey levels available with the four modulating levels, and selecting a set of quadruplets from said list equal to a desired number of gray levels, said set of quadruplets selected to maximize the occurrence of black and white pixel levels in each halftone cell.

Abstract: To inhibit marine growth on a submerged framework, one or more collars are placed loosely around each member of the framework, the collars having outwardly projecting vanes so that ambient water movement causes movement around or along the member. The vanes (5; 25; 45; 55; 65) are preferably hinged to the collar for limited pivotation and may be twisted or provided with extension portions to ensure axial movement.