Octave thinks, that image I want to imread() doesn't exist - image-processing

Question

I'm writing a function in Octave to easily add particles on an image, but I have a problem.
function [ out ] = enparticle( mainImg, particleNames, particleData, frames, fpp, sFrame, eFrame )
%particleData format:
% [ p1Xline p1StartHeight p1EndHeight;
% p2Xline p2StartHeight p2EndHeight;
% p3Xline p3StartHeight p3EndHeight;
% ... ]
%particleNames format:
% [ p1Name;
% p2Name;
% p3Name;
% ... ]
pAmount = size(particleData, 1);
for i= 1:pAmount
tmp = particleNames(i,:)
[ pIMG pMAP pALPHA ] = imread( tmp );
end
end
When I run this simple code with
enparticle( "ffield.png", [ "p_dot.png"; "p_star.png"; "p_dot.png" ], [ 100 50 100; 200 50 100; 300 50 100 ], 30, 10, 5, 25 )
I get this written in console
tmp = p_dot.png
error: imread: unable to find file p_dot.png
error: called from
imageIO at line 71 column 7
imread at line 106 column 30
enparticle at line 24 column 23
When I try to imread() file this way, Octave thinks, that there is no file named like this. But it is actually. In the same folder as script file.
The most curious thing is that, when I change
tmp = particleNames(i,:)
to
tmp = particleNames(:,:)
and Octave assigns all names to tmp as array, it magically find all the files with passed names.
But it's not the way I want it to work, because all files will be replaced, or merged, or sth along image processing then.
Why I'm trying to do it that way is corelated with fact, that I want to put every frame (of image and alpha) separately into a cell array later.
I totally don't have any clue, about what I do wrong there and can't google it anywhere also :(

Answer 1

The code:
filenames = [ "p_dot.png"; "p_star.png"; "p_dot.png" ]
does not do what you think it does. This will create a 2 dimensional
array of characters. See
octave> size (filenames)
ans =
3 10
Of interest note is that it lists 10 columns. Take a look at your
filenames and you will notice that their file names are of different
lengths, two have length 9 and one has length 10. But this is just
like a numeric matrix, the only difference is you have ascii
characters. And like a numeric matrix, all rows must have the length.
What is happening, is that the shortest rows get padded with
whitespace. You can confirm this by checking the ascii decimal code
of your filenames:
octave> int8 (filenames)
ans =
112 95 100 111 116 46 112 110 103 32
112 95 115 116 97 114 46 112 110 103
112 95 100 111 116 46 112 110 103 32
Note how the first and third row end in '32'. In ASCII, that's the
space character (see the wikipedia article about
ASCII which has the tables)
So the problem is not that imread does not find a file named
'p_dot.png', the problem is that it does not find a file named
'p_dot.png '.
You should not be using character arrays for this. Instead, use a
cell array. In a cell array of char arrays. Like so:
filenames = {"p_dot.png", "p_start.png", "p_dot.png"}
for i = 1:numel (filenames)
fname = filenames{i};
[pIMG, pMAP, pALPHA] = imread (fname);
## do some stuff
endfor