1. A computer processor-based method for contrast enhancement for digital images, comprising: filtering, by a computer processor controlled electronic device, an original image, denoted ORIG, having original color values ORIG(i,j), to generate a first filtered image, denoted F 1 , and a second filtered image, denoted F 2 , wherein the first and second filtered images have smooth local color variation relative to the original image; deriving, by the electronic device, local highlight scale factors, denoted LH(i,j), by applying a variable highlight response curve, denoted HRC, to the first filtered image, LH(i,j)=HRC(F 1 ( i,j )); deriving, by the electronic device, local shadow scale factors, denoted LS(i,j), by applying a variable shadow response curve, denoted SRC, to the second filtered image, LS(i,j)=SRC(F 2 ( i,j )); deriving, by the electronic device, local offset values, denoted OFST(i,j), by applying an offset curve, denoted OC, to the first filtered image, OFST(i,j)=OC(F 1 ( i,j )); and processing, by the electronic device, the original image ORIG, comprising: subtracting the local offset values from the original color values to generate shifted color values; scaling the shifted color values by the local highlight scale factors; and further scaling the shifted color values by the local shadow scale factors, thereby generating a contrast-enhanced image from the original image, ENHNCD=LS*LH*(ORIG−OFST).