Given a cubic Bezier curve defined by points p₁, p₂, p₃, and p₄, a point B on that curve at some t value (where 0 ≤ t ≤ 1), a point A on the line (p₂ — p₃) at distance ratio t from p₂, and a point C that is the intersection of the line (p₁ — p₄) and the line that goes through A and B, the ratio between distance d1 = (A — B) and d2 = (B — C) is a fixed value, regardless of the values for coordinates p₁, p₂, p₃, and p₄
An illustration of this can be found here (I cannot add image links as new user, unfortunately).
I'd like to find the formula that expresses this ratio as a function of t (all interactive graphing experiments suggest that this function is an identity function for cubic Bezier curves, not actually being dependent on the coordinates used for the curve) but I'm having little success coming up with something satisfactory. My math skills are not sufficient...
I initially wrote up a quick data-generator using the "Processing" programming language to see if I could use that data for polynomial regression (based on the fact that the function is symmetrical around t = 0.5, finding the expression for the interval t=0.5 to t=1), but the fact that the ratio is actually asymptotic at t = 0 and t = 1 (towards positive infinity) means that it's not a straight-forward power function.
An illustration of the curve can be found here.
(note: the jsfiddle link doesn't actually log all 5000 step values; normal Processing does)
Would anyone know how to express this ratio function as a proper formula? I don't quite know how to approach this symbolically, as I'm using de Casteljau's algorithm to determine my red and green lines; since I don't know how to symbolically express the values d1 and d2, expressing the ratio d1/d2 as a function is quite hard.
N.B.: Apologies if the tags don't fit the question. I'll take suggestions on using the right ones instead; first question on MathOverflow.