1. A system, comprising: at least one data processor; and at least one memory storing instructions which, when executed by the at least one data processor, result in operations comprising: generating a concatenated feature map set by at least combining a first feature map set and a second feature map set, the first feature map set and the second feature map set each indicating one or more occurrences of a feature within an image, the second feature map set generated by at least applying a pooling function to the first feature map set, the application of the pooling function removes, from the first feature map set, at least one feature map such that the second feature map has a different scale than the first feature map set, the combining of the first feature map set and the second feature map set restores the at least one feature map removed from the first feature map set, and the concatenated feature map set including the at least one feature map restored by the combining of the first feature map and the second feature map; and classifying, based at least on the concatenated feature map set, the image.