pbmreduce(1) pbmreduce(1) NNAAMMEE pbmreduce - read a portable bitmap and reduce it N times SSYYNNOOPPSSIISS ppbbmmrreedduuccee [--ffllooyydd|--ffss|--tthhrreesshhoolldd ] [--vvaalluuee _v_a_l] _N [_p_b_m_- _f_i_l_e] DDEESSCCRRIIPPTTIIOONN Reads a portable bitmap as input. Reduces it by a factor of _N, and produces a portable bitmap as output. _p_b_m_r_e_d_u_c_e duplicates a lot of the functionality of _p_g_m_- _t_o_p_b_m_; you could do something like ppnnmmssccaallee || ppggmmttooppbbmm,, but _p_b_m_r_e_d_u_c_e is a lot faster. _p_b_m_r_e_d_u_c_e can be used to "re-halftone" an image. Let's say you have a scanner that only produces black&white, not grayscale, and it does a terrible job of halftoning (most b&w scanners fit this description). One way to fix the halftoning is to scan at the highest possible resolution, say 300 dpi, and then reduce by a factor of three or so using _p_b_m_r_e_d_u_c_e. You can even correct the brightness of an image, by using the --vvaalluuee flag. OOPPTTIIOONNSS By default, the halftoning after the reduction is done via boustrophedonic Floyd-Steinberg error diffusion; however, the --tthhrreesshhoolldd flag can be used to specify simple thresh- olding. This gives better results when reducing line drawings. The --vvaalluuee flag alters the thresholding value for all quantizations. It should be a real number between 0 and 1. Above 0.5 means darker images; below 0.5 means lighter. All flags can be abbreviated to their shortest unique pre- fix. SSEEEE AALLSSOO pnmenlarge(1), pnmscale(1), pgmtopbm(1), pbm(5) AAUUTTHHOORR Copyright (C) 1988 by Jef Poskanzer. 02 August 1989 1