A couple of months ago I was working with a bunch of pictures I had taken at home. I had about 40 of them, and I needed to crop and resize them all the same way. Naturally, I wrote a MATLAB script to do it.
This experience reminded me that customers sometimes ask "How do I use the Image Processing Toolbox to do batch processing of my images?" Really, though, this isn't a toolbox question; it's a MATLAB question. In other words, the basic MATLAB techniques for batch processing apply to any domain, not just image processing.
Contents
Step 1: Get a list of filenames
If you use the dir function with an output argument, you get back a structure array containing the filenames, as well as other information about each file.
Let's say I want to process all files ending in .jpg:
files = dir('*.jpg')files =
42x1 struct array with fields:
name
date
bytes
isdir
The files struct array has 42 elements, indicating that there are 42 files matching "*.jpg" in the current directory. Let's look at the details for a couple of these files.
files(1)
ans =
name: 'IMG_0175.jpg'
date: '12-Feb-2006 10:49:30'
bytes: 962477
isdir: 0
files(end)
ans =
name: 'IMG_0216.jpg'
date: '12-Feb-2006 11:09:10'
bytes: 1004842
isdir: 0
Step 2: Determine the processing steps to follow for each file
There are four basic steps to follow for each file:
1. Read in the data from the file.
2. Process the data.
3. Construct the output filename.
4. Write out the processed data.
Here's what my read and processing steps looked like:
rgb = imread('IMG_0175.jpg'); % or rgb = imread(files(1).name); rgb = rgb(1:1800, 520:2000, :); rgb = imresize(rgb, 0.2, 'bicubic');
You have many options to consider for constructing the output filename. In my case, I wanted to use the same name but in a subfolder:
output_name = ['cropped\' files(1).name] % Use fullfile instead if you want % multiplatform portability
output_name = cropped\IMG_0175.jpg
Here's another example of output name construction. You might use something like this if you want to change image formats.
input_name = files(1).name
input_name = IMG_0175.jpg
[path, name, extension] = fileparts(input_name)
path =
''
name =
IMG_0175
extension =
.jpg
output_name = fullfile(path, [name '.tif'])output_name = IMG_0175.tif
Step 3: Put everything together in a for loop
Here's my complete batch processing loop:
files = dir('*.JPG'); for k = 1:numel(files) rgb = imread(files(k).name); rgb = rgb(1:1800, 520:2000, :); rgb = imresize(rgb, 0.2, 'bicubic'); imwrite(rgb, ['cropped\' files(k).name]); end
Get
the MATLAB code
Published with MATLAB® 7.2
hope you would kindly consider my problem and give me a solution for it.
thank you
above program is not workin
zahid - Unless you provide a lot more detail about what problem you are having, I can’t possibly help you.
its very useful to develop my self in image processing .
thanking you sir.
with warm regards
R.Raja
Hi Steve
I am trying to run part of your for loop for batch processing a folder of 52 images. The only part I am interested in is the
reading a batch of images in. That is I want to extract area of an object as opposed to cropping. So I started with the beginning of your code
files = dir(’F:\EPD Validation Study\Lab Processing from Field Trip 221107\Boags A\greater than 2\4\Boags 4 Lab JPGS Cropped\*.jpg’)
for k = 1:numel(files)
rgb = imread(files(k).name)
end
but I receive an error message “??? Error using ==> imread at 315
File “R0010864.JPG” does not exist.”
R0010864.jpg is the first image file in the folder “Boags 4 Lab JPGS Cropped”.
I am not sure what I am doing wrong. These images are rgb and are large in size (2000 x 2000 pix)and are being stored on an external hard drive.
I would appreciate any helpful suggestions/solutions you might have.
Thanks
Lucy
Lucy—If you look at files(k).name, you’ll see that it doesn’t include the full path. You either need to modify files(k).name to prepend the full path, or add the image directory to the MATLAB path, or make that directory your working directory.
Thanks for your help Steve. I shifted my images into the current (working) directory and used the code listed below:
files = dir(’*.JPG’);
for k = 1:(numel(files));
rgb = imread(files(k).name)
end
and I received the following error message:
??? Error using ==> imagesci\private\readjpg
Too many input arguments.
Error in ==> imread at 389
[X, map] = feval(fmt_s.read, filename, extraArgs{:});
This appears to be a syntax error that may be associated with the wrong file format? (my image is class unit 8 it is not an indexed color map). But I also initially though that this loop would read all images into the workspace (at once) but I have since realized that this cannot be the case as there is only a single variable rgb. Therefore each image must be read in one at a time and once the algorithm is applied to the image (or any other functions in the for loop) it will move onto the next image file in the directory. Am I on the right page or have I misunderstood what this code is doing?
Lucy—I don’t see any problem with your code, except that you left off the semicolon on the imread line. That will cause the pixel values (all of them!) to display in the command window. I recommend that you use the debugger to help with your debugging problem. Single-step through the code, and in particular step into imread. Examine the input arguments to imread to make sure they are what you expect. And yes, your loop (once debugged) will read all the images into the same variable, one at a time. If that’s not what you want, consider using a cell array instead.
Thanks Steve. Problem Solved!
Hi Steve,
can I import a big number of MODIS images in HDF-EOS format whithout using HDF-toolbox?
Thank in advance, Manu
Manu—MATLAB supports HDF-EOS import and export via the functions hdfgd, hdfpt, hdfsw. I don’t know what you mean by “HDF Toolbox.” There’s no MathWorks product with that name.
Thanks so much Steve.You are simply the best.I knew the problem was simple but just did not know where to look for.I googled “matlab code to read all images in a folder” and got the best result.its quite surprising to see that the crux of this problem is just one line of code, namely, “files = dir(’*.JPG’);”.
Lakshman—I’m glad you found this information to be useful.