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# Bezier Clipping: control points

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Hello, I am implementing the bezier clipping algorithm atm and I have a little problem to understand the distribution of the control points of the transformed bezier curve. Lets say the degree is n=3 so you get 4 control points The line equation is ax+by+c = d(p) Now I plug this line equation into the bezier function an get d(B(t)) = d(sum {i=0->n} c_i*B_i^n(t)) => d(t) = sum {i=0->n} d(c_i)*B_i^n(t) => d(t) = sum {i=0->n} (a*c_i.x+b*c_i.y+c)*B_i^n(t) => d(t) = sum {i=0->n} d_i*B_i^n(t) Now the Nishita paper states that the coordinates of the control points of the distance function are c_i(i/n,d_i) My question is, how can I explain the first coordinate (i/n) ?? There must be a derivation, unfortunately all the papers I can find don t explain it at all thx in advance

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The curve that you want to represent in Bézier form is (t,d(t)) and t = sum_{i=0}^n (i/n)B_i^n(t) when 0<=t<=1. I don’t really like to work with series and I will try to give a proof using the de Casteljau algorithm.

I want to prove that if the coefficients of the Bézier curve are i/n then the coefficients in the de Casteljau algorithm are defined by the following formula:

B_i^k(s) = (i + k*s)/n

If this is the case then the point B_0^n on the curve is

B_0^n(s) = (0 + n*s)/n = s

I will prove it by induction. The basic case is B_i^0(s) = i/n = (i + 0*s)/n. I now suppose the formula is correct for k and prove that it works for k+1:

B_i^(k+1)(s) = (1 - s)*(i + k*s)/n + s*(i + 1 + k*s)/n = (i + k*s - i*s - k*s2 + i*s + s + k*s2)/n = (i + (k+1)*s)/n Q.E.D.

EDIT: t and sum_{i=0}^n (i/n)B_i^n(t) are equals for each t and not only in [0,1]. I have previously inserted this restriction because this is usually the only interval considered when working with Bézier curve.

There is a better way to prove that sum_{i=0}^n (i/n)B_i^n(t) = t. Two C^1 functions differ by a constant iff their derivatives are equals.

d/dt t = 1
d/dt sum_{i=0}^n (i/n)B_i^n(t) = sum_{i=0}^{n-1} n((i+1)/n - i/n)B_i^{n-1}(t) = sum_{i=0}^{n-1} B_i^{n-1}(t) = 1

To calculate the constant I evaluate the two functions in 0:

c = 0 - sum_{i=0}^n (i/n)B_i^n(0) = 0

The two functions are equals. Q.E.D.

[Edited by - apatriarca on July 14, 2008 4:05:56 PM]

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Thx, that really helped me a lot to understand it.

Just a note, the B_i^n(t) in the derivative should be B_i^{n-1}(t)

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Quote:
 Original post by BasirorJust a note, the B_i^n(t) in the derivative should be B_i^{n-1}(t)

Yes, it was B_i^{n-1}(t). I have changed it in the original post.

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I did some further research.

The curve type is called " exlicit / non parametric bezier curve " (uniformly distributed x coordinate)

Just for the case that someone is looking for this.

thx

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