12:14 it is not a linear increase: at an angle of 45 degrees (halfway in between) the dot product is not 0.5. The change is not linear but sinusoidal, it's proportional to the cosine of the angle between the vectors.
@@levuhoangkhai8816 Bui Tuong Phong, a Vietnamese studying at University of Utah, published in his 1973 dissertation "Illumination for Computer Generated Pictures". If you search for the title you can find the whole original document ( a008786.pdf ) and a shorter review article he wrote for ACM in 1975. It is always great to read the original material :) I don't provide links here, as RU-vid seems to dislike that lately (causing the comment to end up in review, often without notifying the channel owner.)
