how to crop images within a DataStore with labeled bounding boxes

I have a set of labeled images created with imageLabeler (Bounding boxes, not pixel-based segmentation). The images are too large to use as-is for training and the target objects are too smal to simply resize the images so I want to crop the images and create a training set with these cropped images and the corresponding bounding boxes they contain. As reported in a previous post (https://www.mathworks.com/matlabcentral/answers/525484-augmentedimagedatastore-center-crop-does-not-return-datastore-with-labels) using augmentedImageDataStore returns only the cropped images without the bounding boxes. Using pixelLabelImageDataStore as suggested in that post does not provide a solution since I am interested in bounding boxes and not segmentic segmentation. Any suggestion?

Answer 1

[Aylin]

Does BoxLabelDatastore (R2019b and newer) work for you? See: https://www.mathworks.com/help/vision/ref/boxlabeldatastore.html

[Raphael]

Rylan, Can you elaborate a little? This is what I have a the moment:

data = load('FrontLabels.mat'); % create by exporting the labels from imageLabeler Session

GT_data=data.gTruth;

GT_files=GT_data.DataSource.Source;

GT_labels=GT_data.LabelData;

GT_files_table=cell2table(GT_files);

FrontDataset=[GT_files_table GT_labels];

rng(0);

shuffledIndices = randperm(height(FrontDataset));

idx = floor(0.6 * length(shuffledIndices) );

trainingIdx = 1:idx;

trainingDataTbl = FrontDataset(shuffledIndices(trainingIdx),:);

imdsTrain = imageDatastore(trainingDataTbl{:,'GT_files'});

bldsTrain = boxLabelDatastore(trainingDataTbl(:,2:end));

trainingData = combine(imdsTrain,bldsTrain);

As you see I am already using boxLabelDatastore but I don't see how this could somehow help me create a DataStore that contains only some region of each images and the corresponding labeled bounding bxoes

[Aylin]

To clarify, are you trying to crop each image in the ImageDatastore using the bldsTrain data? I think you can use datastore transform to apply imcrop to each image:

function data = myTransformFcn(image, rect)
    data = imcrop(image, rect); % Note, this might need to be modified slightly if image and rect are cells.
    data = {data rect}; % Add the bounding boxes back to the result if you want it.
end

trainingData = transform(imdsTrain, bldsTrain, @myTransformFcn);

Note that you might need to change the myTransformFcn function depending on the box label syntax you're using.

If you're trying to generate new files out of this, you can use the writeall function for datastores: https://www.mathworks.com/help/matlab/ref/matlab.io.datastore.writeall.html?s_tid=doc_ta

I hope this helps!

[Raphael]

Thanks