When I read a pgm file on matlab and read it for some reason it changes the elements, how can i prevent that from happening? I can't attach the image but can someone help me in making sure the elements dont change when reading a pgm file, thank you

myFolder = ''; % this has the path file for where the pgm files are stored
if ~isdir(myFolder)
  errorMessage = sprintf('Error: The following folder does not exist:\n%s', myFolder);
  uiwait(warndlg(errorMessage));
  return;
end
filePattern = fullfile(myFolder, '*.pgm');
pgmFiles = dir(filePattern);
n = 13;
I=zeros(1000,1000,1000);
for k = 1:length(pgmFiles)
  baseFileName = pgmFiles(k).name;
  fullFileName = fullfile(myFolder, baseFileName);
  fprintf(1, 'Now reading %s\n', fullFileName);
  A{k} = imread(fullFileName);
end

[T Vinai Datta Reddy]

I'm facing the same issue too.

For instance, when I try to display the first 15 values of the image in C, I get -

2 2 2 2 2 2 2 2 2 3 3 2 3 3 2

And in Matlab,

img(1:15)

the values are -

265 265 265 265 265 265 265 265 265 397 397 265 397 397 265

Observe that the pattern is same.

Anyway, I'm unable to upload the image file here since .pgm is an unsopperted format, so here you go -

https://drive.google.com/open?id=1QmpKORLX09Rf5qCgcKQf0H0kRu499yGk for the .pgm image

[Image Analyst]

I downloaded it and brought it into Photoshop and it looks like garbage.

It actually looks better in MATLAB!

The image is a 16 bit (2 byte) image. The values MATLAB gives are 100% reasonable and believable. I suspect in C you are assuming it's a uint8 image and reading it a byte at a time, thus giving the incorrect (low) values, instead of reading two bytes (a pair of bytes) into a uint16 variable.

Answer 1

[Image Analyst]

I don't believe it is changing the values. For you to prove it is, you'll have to upload a file that we can read in with MATLAB, and some other program, such as Photoshop, where we can see that the color or graylevel at a certain location is not the same in the two different programs.

[Walter Roberson]

Note: the answer would be quite different for JPEG. For JPEG you should assume that it will change the elements, as the settings to not change elements are seldom used.

[Image Analyst]

For any program doing a round-trip lossy compression will change the values. For a given file (already compressed in a lossy way and saved into a file), I'm not sure if two different programs will decompress in the same way, so I don't know if Photoshop and MATLAB will give the very same pixel values upon reading in a JPG file. I don't know if the PGM format is compressed but it doesn't say it is in the Wikipedia article on it. Still waiting for Syed to upload one of these mysterious files...

[Walter Roberson]

PGM is not compressed; http://netpbm.sourceforge.net/doc/pgm.html

[Image Analyst]

Syed, we can't really do anything more unless you attach the image and say what alternate method (other than MATLAB) you're using to check the values. Just attach an image with non-proprietary content if you can.