The idea is to find the position of each viewport pixel center formula_9 which allows us to find the line going from eye formula_10 through that pixel and finally get the ray described by point formula_10 and vector formula_12 (or its normalisation formula_13). First we need to find the coordinates of the bottom left viewport pixel formula_14 and find the next pixel by making a shift along directions parallel to viewport (vectors formula_15 i formula_16) multiplied by the size of the pixel. Below we introduce formulas which include distance formula_17 between the eye and the viewport. However, this value will be reduced during ray normalization formula_13 (so you might as well accept that formula_19 and remove it from calculations).