Convert a raw sequence of N x M values into a proper image format with minimal effort

Question

I have a file with a sequence of NxM unsigned integral values of fixed width - let's even assume they're single bytes - and I would like to "wrap" in some kind of common image file format. Preferably, something usable with popular image viewers; and otherwise with an image editor like GIMP.

What image format would require the minimum amount of conversion work, i.e. be as close as possible to just slapping some small header onto the raw data?

Notes:

• This is a grayscale/intensity image - there are no color channels / interleaved values etc.
• I don't care if the image format is row-major or column-major, i.e. if the image appears transposed relative to the order I wrote the data originally.

Answer 1

I've just noticed the Portable Pixmap Formats, which include the PGM format. The closest I've seen to a spec is on this page.

PGM files are supported by apps such as: Eye of Gnome (on Linux), IrfanView (on Windows), GIMP and others.

Create the image file programmatically

If I understand correctly, the following C function should convert the raw data OP has into a PGM file:

void write_pgm(FILE* file, size_t width, size_t height, uint8_t* data)
{
    const char magic = "P5"; // for binary graymap
    fprintf(file, "%2s %zu %zu 255\n", magic, width, height);
    fwrite(data, 1, width * height, file);
}

This is a variation on a similar function for a PPM, here.

You can easily adapt this to whatever programming language you like.

Converting a file

So, suppose you've put your output in a file on disk. Apparently, there's a utility for making a PGM of it: rawtopgm. Here's a invocation for your case:

rawtopgm -bpp 1 -maxval 255 N M my_data_file > out.pgm

or, exploiting defaults:

rawtopgm N M my_data_file > out.pgm

pretty simple.