Problems with 'uigetfile' and 'imread' in simple MATLAB Web App

Hi All,

I am trying to convert a MATLAB App Designer App to a Web App. After viewing this page on the limitations of a MATLAB Web App, I do not see any incompatibilites. However, I am having a problem with loading images with 'uigetfile' and being able to read them with 'imread'. To investigate this, I created a really simple test app. There is a push button and a UIAxes, and the push button allows the user to upload an image that will be displayed in the UI Axes. Here is the code for said push button.

function imageButtonPushed(app, event)
    [files, path] = uigetfile("*.*", 'Multiselect', 'on');
    imshow(imread(fullfile(path, files)), 'parent', app.UIAxes);
end

But when I upload a test .tif image, I consistently get this error.

When I navigate to the temporary folder where 'uigetfile' stores the image, I can not open them in any image viewing app. They seem to get corrupted in the process. I can not share the image because it is from a lab and I do not want to get in trouble, but I can guarantee that the image is not corrupted and the process works in an offline setting with the exact same image file. Am I missing something very obvious?

Thank you in advance to anyone who lends their help.

[Jan]

uigetfile does not store images but only obtain their names.

How can you guarantee, that the images are not corrupted, when you cannot open them with any software? How is "they" in "They seem to get corrupted in the process"?

If you select multiple files, files is a cell array. Then imshow should fail.

[Christian Rouhana]

I must have been mistaken then. While I was poking around, the behavior I observed in a Web App was that when I uploaded a single image (the 'Multiselect' parameter was from testing something else), a file with the same name and extension (in this case, '1.tif') would be saved in a temporary directory used by the application. 'path' and 'files' would point this temporary directory and the file name respectively. If I navigated to this directory in Windows explorer, a copy of the image with the same name was stored there, but I could not open the image. The default Windows app for viewing photos would give me an error, a behavior that does not happen when I open the original .tif file. That is why I believed the app was storing a corrupted or incomplete copy of the image.

If my goal is to have the user be able to upload one or more image(s) for analysis, is there a correct way to do this? To reiterate, my error is happening with my call to 'imread'

[Walter Roberson]

Is the server running Linux? If it is then libtiff for it is probably too new compared to the matlab version you are using.

[Christian Rouhana]

Sorry Walter I forgot to respond to your answer. I am running the local server on the same pc with Windows 10. Should I investigate this anyways? And could any problems arise from the set up I am using?

[Jan]

So uploading the tiff files to the temp folder does destroy them? Sorry, I did miss this point in my comment. Please compare the size and a hash with the original images.

[Christian Rouhana]

Jan,

Would I compare the hash within MATLAB? I am still rather new to image analysis and processing so I am quite stumped on how to do this. When I navigate to the image stored by the web app, I still cannot read the image with 'imread'. I get the same error in my original post. I can tell you that the original image is 253 KB and the image stored by the web app is 281 KB. Could there be a step missing between 'uigetfile' and 'imread'?

[Jan]

Okay. Delecting the file with uigetfile creates a copy of the file in a temporary folder, which is accessible from the web application. During this copying the contents of the file is changed. If this is the case, uigetfile is not usable for web applications, because a modified file is most likely damaged. You can omit the comparison of the hash values, because the different file sizes prove the modification already.

Please ckeck this with a simple text file. Unfortunately I do not have the compiler to create a test by my own. A test with a simple test TIFF should work also. You could open a copy of the original images, set the set all pixels to white, upload it and post the two files here, without publishing private data.

[Christian Rouhana]

I tried it with a simple .txt file that just contained 'abcd'. Nothing was changed and the file sizes were constant. So by your reccomendation I created a .tif file with all white pixels. I can not attach these images as MATLAB does not allow the .tif file format to be uploaded, but the result was the same. To share my results I also made a test with .png images which gives a different error but the images are still clearly being changed in the process. I attached the respective files. resultpng.png can not be opened in any image viewing application.

The difference in size between these files is only 25 bytes. I haven't been able to find any other replication of my issues online. With this in mind, could it also be an issue with how my development server is set up?

Also, I won't rule out that 'uigetfile' might be unusuable, so are there other ways to upload multiple images to a MATLAB Web App?

[Walter Roberson]

You can zip tif and attach the zip

[Walter Roberson]

It looks to me as if every 0xc2 is missing in the modified png. I have no explanation -- it is just the  character, nothing special.

[Jan]

There are more differences: The "resultpng.png" starts with 0xc2 and some other 0xc2 are instered also. But e.g.

'   9E 4A F8 FC'             becomes 
'C2 9E 4A C3 B8 C3 BC'

% Or as bits:
    '10011110'     % Original
    '01001010'
    '11111000'
    '11111100'

    '11000010'     % Uploaded
    '10011110'
    '01001010'
    '11000011'
    '10111000'
    '11000011'
    '10111100'

The upload has inserted 25 bytes and modified some existing bytes. It does not look like a confused UTF8 to UTF16 conversion.

Please contact MathWorks to ask for help. I assume that they can debug the problem.

Answer 1

[Christian Rouhana]

Hi all,

Thank you for your help. I contacted MATLAB support and this seems to be a bug found in MATLAB R2021a. I installed MATLAB R2020b and do not have this problem anymore.

[Hitesh Jangir]

Still getting the same error in R2020b? did you get any other solution?

[Image Analyst]

@Hitesh Jangir I don't have that version. Can you upgrade to the latest version? Otherwise can you attach your .mlapp file so that if anyone has that version they can try your code. Attach it in a new question of your own, not here.

Answer 2

[Walter Roberson]

My interpretation would be that libTiff on the server is not the same as libTiff on the online version.

In particular if you are using Linux on the server, then you probably need to install an older version of libTiff for MATLAB to use.

Answer 3

[Image Analyst]

"If my goal is to have the user be able to upload one or more image(s) for analysis, is there a correct way to do this?" <== Waht I would do is to have your app load a listbox full of images they can select from so they can just easily click one or more to analyze, and then click the Analyze or Go button to do the batch processing. I'd also have a button that calls uigetdir() so the user can specify what folder's files get loaded into the listbox.

[Jan]

It is an compiled web app. Then: https://www.mathworks.com/help/compiler/webapps/unsupported-functionality.html:

Opening a folder selection dialog box on the client using uigetdir is not supported.

[Image Analyst]

Wow, that's bad. How are web app users supposed to indicate where on their local computer the data resides? Hand type in the entire path?

[Jan]

You can use uigetfile() in a Web-App to choose files. This uploads the selected files to a local folder, which can be accessed by the web server. The idea is give the web server access only to the selected files. Everything else would be a security problem.

Now the problem of the OP is, that the local copy of the files is damaged by the uigetfile() processing: some bytes are inserted, some bits are modified.

There are more users withthis problem.

Answer 4

[Artur Krukowski]

Hello everybody,

Actually the problem persists in all Matlab versions. I followed most of the threads, including suggestion to use R2020b. All those solutions have failed for me, so I went to look for a workaround solution.

The easiest way to realise what happens is to generate a sample binary file with numbers from 0 to 255. When you try to choose it using uigetfile function from your webapp, you will quickly realise that the uploaded file has a longer size than the original one.

When you then compare the content, you will notice that some bytes have been replaced with a combination of two bytes, whereby the first one is 0xC2 or 0xC3. The second byte is then either a copy of the original one (when the first one is 0xC2) or decreased by 64 (when the first one is 0xC3).

Afterwards the solution is piece easy. The most basic version of a function to unscramble such a file is attached.

In your webapp you can then simply:

1. run uigetfile and make a note of the pathname and filename
2. unscramble the file using the attached function (personally I exclude the path from the output filename)
3. the new file is available for subsequent processing as usual

When processing images, you can use the following example approach:

% Display uigetfile dialog

filterspec = {'*.jpg;*.tif;*.png;*.gif','All Image Files'};

[f, p] = uigetfile(filterspec);

% Make sure user didn't cancel uigetfile dialog

if (ischar(p))

fname = [p f];

try

img = imread(fname);

catch ME

% If face problem reading image, unscramble it and reload

unscramble(fname_in, fname_out);

img = imread(fname_out);

end

end

This should work for all Matlab versions.

NOTE: examples provided do NOT fully check for all fault conditions so you may need to add some extra checks.

[Walter Roberson]

The easiest way to realise what happens is to generate a sample binary file with numbers from 0 to 255.

Exactly how do you do that?

What you are describing is as-if the data has been encoded as UTF-8.

Note: MATLAB does not support a 'b' modifier for fopen().

Answer 5

[Artur Krukowski]

You can use e.g.:

fin=fopen('test.bin','wb');fwrite(fin,0:255);fclose(fin);

Regarding feasibility of "wb" just check example in:

help fwrite

All the best.

Artur

[Walter Roberson]

https://www.mathworks.com/help/matlab/ref/fopen.html#btrnibn-1-permission

There is no 'b' permission.

There is a 'b' for machinefmt, which is an additional parameter.