im2col (lowering)
Beginner
Rearranging an image into a big table, copying overlapping patches, so a picture-filtering job becomes an ordinary grid multiplication.
Novice
“Image to column”: copying every input patch a filter touches into one row (or column) of a matrix, so a convolution becomes a single matrix multiplication with the filters.
Expert
Produces a data matrix in which each input value appears up to times. Materializing it costs memory and traffic, so libraries and accelerators (Gemmini, for example) generate it on the fly at the array edge.