The mat function pop_back does not seem to exist in Java+OpenCv 3.2.X

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no, it's not your fault, it simply does not exist.

(there's a push_back but no pop_back in java, currently, but if at all, it would be probably: void trainingImages.pop_back(int num_rows);)

then, please do not push 2d images, but "flattened", 1d versions of it, like:

 trainingImages.push_back(img.resize(1,1));

(push_back() actually pushes rows, not images. if you don't flatten them, the context, that they belong to a certain image, will be lost, and you end up adding nrows independant samples)

also don't forget to convert your images to float for both SVM training and testing. (just saying)