How do I solve this error: Error using matlab.io.​datastore.​ImageDatas​tore/readi​mage (line 32)?

I am trying to use feature extraction to recognise faces, however when I run the code these errors come up:

Error using matlab.io.datastore.ImageDatastore/readimage (line 32)
Expected input number 2, INDEX, to be a scalar with value <= 8.

Error in Alexnet_Feature_Test (line 31)
    I = readimage(imdsTest,idx(i));   

Below is my code, I have tried using vgg19 and Alexnet and get the same errors for both

%Load zipped images
unzip('ATTDatabase.zip');
imds = imageDatastore('ATTDatabase','IncludeSubfolders',true,'LabelSource','foldernames');

[imdsTrain,imdsTest] = splitEachLabel(imds,0.7,'randomized');

%Display sample images from zip file
numTrainImages = numel(imdsTrain.Labels);
idx = randperm(numTrainImages,10);
figure
for i = 1:10
   subplot(2,5,i)
   I = readimage(imdsTrain,idx(i));
   imshow(I)
end

%Load pretrained network (AlexNet)
net = vgg19();
net.Layers
inputSize = net.Layers(1).InputSize;

%Extracting image features
augimdsTrain = augmentedImageDatastore(inputSize(1:2),imdsTrain);
augimdsTest = augmentedImageDatastore(inputSize(1:2),imdsTest);

layer = 'fc7';
featuresTrain = activations(net,augimdsTrain,layer,'OutputAs','rows');
featuresTest = activations(net,augimdsTest,layer,'OutputAs','rows');

%Extracting class labels
YTrain = imdsTrain.Labels;
YTest = imdsTest.Labels;

%Image classifier
classifier = fitcecoc(featuresTrain, YTrain);

%Classification from test
YPred = predict(classifier,featuresTest);

%Showing 4 images with labels
idx = [1 5 10 15];
figure
for i = 1:numel(idx)
    subplot(2,2,i)
    I = readimage(imdsTest,idx(i));
    label = YPred(idx(i));
    imshow(I)
    title(char(label))
end

%Accuracy of model
accuracy = mean(YPred == YTest)

The majority of the code can be found here: https://uk.mathworks.com/help/deeplearning/examples/feature-extraction-using-alexnet.html

I am not sure why this is coming up, please help!

[Mark Sherstan]

To confirm, lines32 is YTest = imdsTest.Labels correct? Could you post a subset of your data for testing please?

[Walter Roberson]

you assume that there are 15 images in the database which is not necessarily true . The code appears to be taken from an example with more images.

[Sujit Mistry]

Hi Mark,

No line 32 is in the read image function,

function [data, info] = readimage(imds, index)
%READIMAGE Read a specified image from the ImageDatastore.
%   IMG = READIMAGE(IMDS,I) reads the I-th image from IMDS.
%   By default, IMG is an
%      [MxN] Integer   - For grayscale images
%      [MxNx3] Integer - For color images
%      [MxNx4] Integer - For CMYK images
%
%   [IMG,INFO] = READIMAGE(IMDS,I) also returns a structure with
%   additional information about IMG. The fields of INFO are:
%      Filename - Name of the file from which the image was read
%      FileSize - Size of the file in bytes
%      Label    - Label for the file
%
%   Example:
%   --------
%      folders = fullfile(matlabroot,'toolbox','matlab',{'demos','imagesci'});
%      exts = {'.jpg','.png','.tif'};
%      imds = imageDatastore(folders,'FileExtensions',exts);
%
%      img2 = readimage(imds,2);       % Read 2nd image
%
%   See also imageDatastore, read, readall, hasdata, preview, reset.

%   Copyright 2015 The MathWorks, Inc.
try
    validateattributes(index, {'numeric'}, ...
        {'scalar', 'positive', 'integer', '<=', imds.NumFiles}, ...
        'ImageDatastore', 'INDEX', 2);
    [data, info] = readUsingSplitReader(imds, index);
catch ME
    throw(ME);
end
end

Line 32 in here is: throw(ME);

[Sujit Mistry]

Hi Walter,

I currently have 28 images in the database, I will increase it later but I'm just trying to get it working first

[Walter Roberson]

You only post 10 images, but

idx = [1 5 10 15];
...
    I = readimage(imdsTest,idx(i));

so you are reading up to image idx(4) = 15.

[Sujit Mistry]

I am only allowed to post 10 images per 24 hours, i will post some tomorrow

[Sujit Mistry]

Here are a few more images

[Walter Roberson]

you can zip files together .

[Walter Roberson]

Did you obtain the AT&T face database from http://www.cl.cam.ac.uk/research/dtg/attarchive/facedatabase.html ?

[Sujit Mistry]

Sorry I initially tried using the AT&T database but as they are black and white, they won't work. Here is the link for the images i used: https://fei.edu.br/~cet/facedatabase.html

Answer 1

[Sean de Wolski]

It looks like the datastore (probably imdsTest) only sees 8 images so you can't read the 10th or 15th one.

[Sujit Mistry]

Yes, it only reads up to the 8th image and won't work after that, do you know how to solve this issue?

Thanks

[Sean de Wolski]

[imdsTrain,imdsTest] = splitEachLabel(imds,0.7,'randomized');

Why do you care about image 10, 15? The validation step is randomly grabbing 8 images (30%) from the original datastore so the indices 10,15 don't mean much anyway. Why not validate against the 8 it picked?

idx = [1 2 5 7]; % or whatever