Gymnocalycium denudatum rionegro flower buds

This cactus is about to produce 5 flowers, and the buds are impressive! I tried photographing it early this morning against the wall in the conservatory, but I didn't like the texture on the painted wall behind. So I tried again a bit later, against a white-painted wall (rather than the beige colour in the conservatory), with less light, and used fill flash.

Each bud is produced from an auriole, and each auriole on the mature plant has just one spine, as you can see here. The younger heads have up to 3 spines on each auriole.