Oh and...
"Interestingly before Blitz3D existed I made a sprite library that did what (I think) yours does: only alpha the soft edges (and transparent areas) of the sprite."
You are correct. My system determines for all blending modes which pixels are opaque or transparent and blits them fast, and only does blending on those that remain. So an alpha blended sprite which only has edge antialiasing will go much faster than a semi transparent square of cloud.
"Interestingly before Blitz3D existed I made a sprite library that did what (I think) yours does: only alpha the soft edges (and transparent areas) of the sprite."
You are correct. My system determines for all blending modes which pixels are opaque or transparent and blits them fast, and only does blending on those that remain. So an alpha blended sprite which only has edge antialiasing will go much faster than a semi transparent square of cloud.