Comment by chronogram

Your bicubic image looks different to my bicubic images.

Starting with http://www.johncostella.com/magic/small.png when doubling 3 times I get this result in Photoshop CS2 https://i.ibb.co/9tCLPGS/PSCS2triple.png and when doubling 3 times in GIMP’s cubic the result is https://i.ibb.co/p2b0Trq/GIMPtriple.png

This is the result when using GIMP to go straight from 57x43 to 456x344: https://i.ibb.co/r0SXVY2/GIMpstraight.png and Photoshop CS2: https://i.ibb.co/Sn1xt3Y/PSCS2straight.png

Which one is the correct representative of the bicubic filter?