pamstretch

scale up a Netpbm image by integer factors with interpolation

TLDR

Stretch an image 2x in both dimensions

$ pamstretch [2] [input.pam] > [output.pam]

Stretch 3x and drop the extra edge pixel for cleaner dimensions

$ pamstretch -dropedge [3] [input.pam] > [output.pam]

Stretch horizontally only (4x wide, same height)

$ pamstretch -xscale=[4] -yscale=[1] [input.pam] > [output.pam]

Stretch vertically only (same width, 2x tall)

$ pamstretch -xscale=[1] -yscale=[2] [input.pam] > [output.pam]

Interpolate to black at the right/bottom edges

$ pamstretch -blackedge [2] [input.pam] > [output.pam]

Stretch from standard input in a Netpbm pipeline

$ cat [input.pnm] | pamstretch [2] > [output.pnm]

SYNOPSIS

pamstretch [-xscale=X] [-yscale=Y] [-blackedge|-dropedge] N [file]

pamstretch scales up a PAM, PNM, PPM, PGM, or PBM image by integer factors, interpolating between neighboring source pixels rather than simply replicating them. This yields smoother results than plain pixel duplication for photographs and other continuous-tone images.The tool supports independent horizontal and vertical factors through -xscale and -yscale, or a single uniform factor given as the final positional argument. Edge behaviour is configurable: by default extra pixels are synthesised, -blackedge fades to black, and -dropedge trims the trailing row and column.

PARAMETERS

Uniform integer stretch factor applied to both axes when -xscale/-yscale are not given.

-xscale=X

Integer horizontal scale factor (default 1 if only -yscale is given).

-yscale=Y

Integer vertical scale factor (default 1 if only -xscale is given).

-blackedge

Interpolate toward black at the right and bottom edges, preserving output dimensions.

-dropedge

Drop one source pixel at the right and bottom edges. Produces a cleaner result but yields non-standard output dimensions (width * N - N + 1).

file

Input PAM/PNM file. Reads from standard input if omitted.

CAVEATS

Only integer scale factors are supported; for non-integer scaling use pamstretch-gen or pamscale. Option names accept minimum unambiguous abbreviations and may use single or double hyphens.

HISTORY

pamstretch is part of the Netpbm toolkit, a set of graphics file manipulation utilities originating from Jef Poskanzer's pbmplus package (1988). It was introduced as an interpolating alternative to pnmenlarge, which only replicates pixels.