Ages ago had a mathematician shard image processing across cores. Started with a pair of loops for each image, foreach col, foreach row. Problem: C++ 2d array

Ages ago had a mathematician shard image processing across cores. Started with a pair of loops for each image, foreach col, foreach row. Problem: C++ 2d arrays are row-col not col-row. Halfway through multi-threaded performance was much worse than single threaded. Eventually we switched to row-col processing, single-threaded was fast enough, back to two loops per image.