s = Blur[imag, 4];
g = GradientFilter[imag, 2] // ImageAdjust;
ds = UpperTriangularize[#, 120] & /@ ImageData[s, Interleaving -> False]; dg = LowerTriangularize[#, -120] & /@ ImageData[g, Interleaving -> False]; dh = UpperTriangularize[#, -119] & /@ (LowerTriangularize[#, 119] & /@ ImageData[imag, Interleaving -> False]);
Image[ds + dg + dh, Interleaving -> False]