ppmtobmp

Updated: 20 December 2018
Table Of Contents

NAME

ppmtobmp - convert a PPM image into a BMP file

SYNOPSIS

ppmtobmp [-windows] [-os2] [-bpp=bits_per_pixel] [-mapfile=filename] [ppmfile]

Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen to denote options. You may use white space in place of the equals sign to separate an option name from its value.

DESCRIPTION

This program is part of Netpbm.

ppmtobmp reads a PPM image as input and produces a Microsoft Windows or OS/2 BMP file as output.

OPTIONS

In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options), ppmtobmp recognizes the following command line options:

-windows

Tells the program to produce a Microsoft Windows BMP file. (This is the default.)

-os2

Tells the program to produce an OS/2 BMP file. (Before August 2000, this was the default).

-bpp

This tells how many bits per pixel you want the BMP file to contain. Only 1, 4, 8, and 24 are possible. By default, ppmtobmp chooses the smallest number with which it can represent all the colors in the input image. If you specify a number too small to represent all the colors in the input image, ppmtobmp tells you and terminates. You can use pnmquant or ppmdither to reduce the number of colors in the image.

Before Netpbm 10.85 (December 2018), ppmtobmp ignores this option if the input is PBM and produces a BMP with 1 bit per pixel. With these versions, if you want more than that, use pbmtopgm to convert the PBM to PGM first.

-mapfile=filename

This identifies a file to use as the BMP palette (aka "colormap"). In one BMP subformat, the BMP stream contains a palette of up to 256 colors, and represents the image raster as indices into that palette. Normally, ppmtobmp takes care of computing a suitable palette, but if you are going to dissect the BMP output in some way, you may want certain values for the palette indices. E.g. you might want red to be 13, where ppmtobmp would (arbitrarily) choose 39. In that case, you can construct the palette yourself and use this option to tell ppmtobmp to use your palette.

This option does not control what colors are in the output. The colors in the output are exactly those in the input, and the palette you supply must contain at least all the colors that are in the input. You can use pnmremap to adjust your input image so that it contains only colors from your palette.

The palette file is a Netpbm format file with one pixel per palette entry. Each pixel must have a distinct color (no repeats). The order of the BMP palette ppmtobmp generates is the order of the pixels in the palette file, going from top to bottom, left to right.

A BMP palette may have at most 256 colors, so the palette file must have at most 256 pixels.

You may find pnmcolormap useful in generating the palette file. pamseq too.

In the case of grayscale image, if you are processing the BMP image, it may be convenient for you to have the actual gray values in the raster part of the image rather than arbitrary indices into a palette. There is no BMP format specifically for that, but you can achieve it by using a palette in which each index is equal to the indexed gray value, and then ignoring the palette when you process the BMP image.

Here is an example of doing that:

    
    $ pamseq 1 255 > mapfile.pgm

    $ ppmtobmp -mapfile=mapfile.pgm input.pgm > output.bmp

This option was new in Netpbm 10.45 (December 2008).

NOTES

To get a faithful reproduction of the input image, the maxval of the input image must be 255. If it is something else, the colors in the BMP file may be slightly different from the colors in the input.

Windows icons are not BMP files. Use ppmtowinicon to create those.