Sequential image writing in a loop

Hi,

I'm trying to develop a code simply reads frames from a video, processes the frames and then writes them back to an output video. My code in general works but the output video have some issues that I detected and couldn't figure out.

The problem occurs at the step of creating a video from processed frames. If my frames are named like 001.png, 002.png, etc. the following code calls all them in correct sequence, i.e. first 001.png, second 002.png, etc. The code is:

clc;

close all;

clear all;

workingDir1 = 'C:\Users\John\MATLAB\bin\Delete\';

workingDir2 = 'C:\Users\John\MATLAB\bin\Test_Grow2_Frames\';

imageNames1 = dir(fullfile(workingDir1,'*.png'));

imageNames1 = {imageNames1.name}';

imageNames2 = dir(fullfile(workingDir2,'*.png'));

imageNames2 = {imageNames2.name}';

for k = 1:length(imageNames1)

toread1=fullfile(workingDir1,imageNames1{k});

toread2=fullfile(workingDir2,imageNames2{k});

img1 = imread(toread1);

img2 = imread(toread2);

imshow([img1 img2]);

end

END OF CODE

In the code, 'C:\Users\John\MATLAB\bin\Delete\' folder contains images named as 1.png, 2.png, 3.png up to 329.png and 'C:\Users\John\MATLAB\bin\Test_Grow2_Frames\' folder contains images named as 001.png, 002.png, etc. up to 329.png.

The interesting part is here: When the for loop iterates

• k=1, toread1 has the value 1.png and toread2 has the value 001.png which is OK,
• k=2, toread1 has the value 10.png and toread2 has the value 002.png which is not OK for toread1 since it supposed to be 2.png,
• k=3, toread1 --> 100.png, toread2 --> 003.png,
• k=4, toread1 --> 101.png, toread2 --> 004.png.

So there is a something going on here, can anybody help on that?

BTW, I generate 1.png, 2.png type names with the code:

imwrite(RGB,['C:\Users\John\MATLAB\bin\Delete\', num2str(curr_frm_num), '.png']);

END OF CODE

If someone has an idea to generate 001.png type names, it is also appreciated.

Thanks,

Answer 1

[Image Analyst]

I do this in my demo, which I've posted many times. I attach it here.

[JohnDylon]

I've been following your posts and demos for a while, they are really great and helpful. The point I want to figure out now is why the same code picks in the order of 1.png, 10.png, 100.png. 101.png, 102.png from one set of images but picks in the order of 001.png, 002.png, 003.png from another set of images?

Thanks in advance,

[Image Analyst]

It could be the way the operating system returns the files to dir() - maybe the sorting is not alphanumeric, and possibly not consistent from one folder to the next. Perhaps you should sort them with sort() so that the sorting is consistent.

Or use Stephen's File Exchange function: http://blogs.mathworks.com/pick/2014/12/05/natural-order-sorting/

[JohnDylon]

It seems the problem occurs the way the computer read orders. 001 has a three digit name and so all names with three digits are subject to "compare the first digits of all samples, pick lowest; compare the second digits of all samples, pick lowest; compare the third digits of all samples pick the lowest" which eventually dictates to pick 001, 002, and so on. The same rule generates 1, 10, 100, 101, 102, and so on. 1, 10, 100, 101, 102 all have 1 in terms of first digit and 1 is superior to 10 and 10 is superior to 100.

Thus, one should be careful about how the computer interprets orders vs how a human does.

Thank you for all contribution.

[Image Analyst]

If it sorts alphanumerically then that's right. This is a well known problem. See the FAQ: http://matlab.wikia.com/wiki/FAQ#How_do_I_sort_file_names_numerically.3F

Answer 2

[JohnDylon]

For the naming as 001.png part I've already had the solution as:

imwrite(RGB,['C:\Users\John\MATLAB\bin\', sprintf('%3.3d.png', 1)]);

which is working.