Sublimation Ink
Special Printers and inks are needed to print ink on a special paper that allows the ink to transfer onto polyester in high heat. Though it seems simple, patience and skill is still required as moving too fast can cause ghosting, where the dye might bleed into other areas of the T-shirt if not cooled down enough !
Alongside this, ICC profiles need to be perfect to make sure the transfer onto polyester matches exactly as show on the computer screen, not lighter or darker, so so skill is needed in that department.