Output Pnm 🖼️

common pnm file formats

This mini-library is for dealing with common PNM format.

Future (?)

• ✅ add PNM<T>(filename) constructors or a factory method to read files from disk into a PNM<T> object
• ✅ add write_content(ostream&)

Examples: see here

The library was originally written for this project, i decided to create a separate repository to be able to reuse the code more easily.

There is duplicate code and the reason is that originally it was supposed to be only extremely efficient in creating and writing a pbm rgb type file only later it was expanded to include pbm and pgm, the correct approach would have been to avoid any kind of static polymorphism (template) and to use dynamic polymorphism (interfaces, vtable etc) and class inheritance.

The in-memory representation of PNM files is always binary, and include a padding when required by the format, see pbm. The raster data (aka pixmap) of an image w=3 h=2 in pbm format will occupy 2 bytes in memory. Any conversion to ascii occurs later when write_content() is called.

Learning resources

• https://en.wikipedia.org/wiki/Netpbm
• https://netpbm.sourceforge.net/doc/pbm.html
• https://netpbm.sourceforge.net/doc/pgm.html
• https://netpbm.sourceforge.net/doc/ppm.html

// chessboard pattern
PNM<pnm::monochrome_t> chessboard{1920, 1080};
bool color = pnm::monochrome_t::BLACK;

for (int h = 0; h < chessboard.height(); ++h, color = !color)
    for (int w = 0; w < chessboard.width(); ++w, color = !color)
        chessboard(h, w, color);

chessboard.write_file_content("chessboard-bin.pbm");

{ // read the file again from disk but this time save as ascii file
    static const auto &ifstream_open = [](const char *filename) -> std::ifstream {
        using std::literals::string_literals::operator""s, std::ios_base;
        std::ifstream fpnm;
        fpnm.exceptions(ios_base::badbit);
        fpnm.open(filename, ios_base::in|ios_base::binary);
        if (!fpnm) throw std::invalid_argument{"Unable to open file: "s + filename};
        return fpnm;
    };

    bool use_asci_fmt = 1;
    auto is = ifstream_open("chessboard-bin.pbm");

    // you can parse any TPixel (pnm::monochrome_t, pnm::rgb<pnm::BIT_8>, pnm::grayscale<pnm::BIT_8>)
    auto image = PNM<pnm::monochrome_t>::parse(is);
    image.write_file_content("chessboard-ascii.pbm", use_asci_fmt);
}


PNM<pnm::monochrome_t> pbm{3, 2};

pbm(0,0, {255, 0,   0}); // since bits aren't addressable you will use a different syntax
pbm(0,1, {0,   255, 0});
pbm(0,2, {0,   255, 0});

pbm(1,0, {255, 255, 0});
pbm(1,1, {255, 255, 255});
pbm(1,2, {0,   0,   0});

pbm.write_file_content("bin.pbm");
pbm.write_file_content("ascii.pbm", 1);

PNM<pnm::rgb<pnm::BIT_8>> ppm{3, 2};

ppm(0,0) = {255, 0,   0};  // here you can assign since a reference to an addressable pixel (>= 1 byte) is returned
ppm(0,1) = {0,   255, 0};
ppm(0,2) = {0,   0,   255};

ppm(1,0) = {255, 255, 0};
ppm(1,1) = {255, 255, 255};
ppm(1,2) = {0,   0,   0};


ppm.write_file_content("bin.ppm");
ppm.write_file_content("ascii.ppm", 1);

PNM<pnm::grayscale<pnm::BIT_8>> pgm{3, 2};

pgm(0,0) = {255, 0,   0};
pgm(0,1) = {0,   255, 0};
pgm(0,2) = {0,   0,   255};

pgm(1,0) = {255, 255, 0};
pgm(1,1) = {255, 255, 255};
pgm(1,2) = {0,   0,   0};

pgm.write_file_content("bin.pgm");
pgm.write_file_content("ascii.pgm", 1);

Copyright © 2025, Manu-sh, [email protected]. Released under the MIT license.