Sweeper.Next() doesn't increment sw.i #2

I think you want:

// Next returns the next image and its label in the data set.
// If the end is reached, present is set to false.
func (sw *Sweeper) Next() (image RawImage, label Label, present bool) {
if sw.i >= len(sw.set.Images) {
return nil, 0, false
}
i, l, p := sw.set.Images[sw.i], sw.set.Labels[sw.i], true
sw.i++
return i, l, p
}

Otherwise it just keeps returning the first image.