How are these PGM files encoded?

Question

I exported a grayscale image as PGM using MATLAB (and OpenCV) and got this file as an output.

    im = imread(src);
    im = rgb2gray(im);
    imwrite(im, dst);

According to the PGM Specification the header contains the "magic number", the width, the height, and the max value of the image.

But below the header, there should be a matrix of grayscale intensity values written in plaintext. But as you can see in the pasted file, I just get some kind of junk out (although it's a completely valid, viewable image)

I want to be able to read in the PGM files and access the individual intensity values as integers using a C/C++ program but I don't know how to interpret this output since it doesn't follow the spec. Perhaps the text encoding is different?

Thanks for any assistance.

_"below the header, there should be a matrix of grayscale intensity values written in plaintext"_ According to whom? — Lightness Races in Orbit, Aug 12 '19 at 00:36
There's a better description here... https://en.m.wikipedia.org/wiki/Netpbm_format — Mark Setchell, Aug 12 '19 at 01:23
Do you mean the example in the "Plain PGM" section that describes _"another version of the PGM format that is fairly rare"_? ;) plz2read the whole document not just select parts of it — Lightness Races in Orbit, Aug 12 '19 at 01:26

score 4 · Accepted Answer · answered Aug 11 '19 at 23:13

You're misreading the spec.

Each gray value is represented in pure binary by either 1 or 2 bytes. If the Maxval is less than 256, it is 1 byte. Otherwise, it is 2 bytes. The most significant byte is first.

So each pixel is either one or two bytes (depending on the Maxval) and in binary, not ASCII.

score 0 · Answer 2 · answered Aug 12 '19 at 09:16

0

I think you're reading the definition of the "plain" format (magic number P2), but you have a "raw" PBM file (magic number P5). You might want to pipe through pnmtopnm -plain to access ASCII-format encoding.

answered Aug 12 '19 at 09:16

Toby Speight

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How are these PGM files encoded?

2 Answers2