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POpython geometry help
 

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  1. POpython geometry help
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    <p>I have the following problem, I am trying to find the following distances (F1 and F2): <a href="http://i50.tinypic.com/15xac5d.png" rel="nofollow noreferrer">circle http://i50.tinypic.com/15xac5d.png</a></p> <p>This is what I have as of now:</p> <pre><code>def FindArrow(self, X1, Y1, X2, Y2, X3, Y3): self.X1 = float(X1) self.Y1 = float(Y1) self.X2 = float(X2) self.Y2 = float(Y2) self.X3 = float(X3) self.Y3 = float(Y3) #center coords of the circle self.Xc = None self.Yc = None #radius self.R = None #F1 and F2 self.FAB = None self.FBC = None #check if the coordinates are collinear invalide = self.X1 * (self.Y2 - self.Y3) + self.X2 * (self.Y3 - self.Y1) + self.X3 * (self.Y1 - self.Y2) if (invalide == 0): return #get the coords of the circle's center s = (0.5 * ((self.X2 - self.X3)*(self.X1 - self.X3) - (self.Y2 - self.Y3) * (self.Y3 - self.Y1))) / invalide self.Xc = 0.5 * (self.X1 + self.X2) + s * (self.Y2 - self.Y1) self.Yc = 0.5 * (self.Y1 + self.Y2) + s * (self.X1 - self.X2) #get the radius self.R = math.sqrt(math.pow(self.Xc - self.X1, 2) + math.pow(self.Yc - self.Y1, 2)) </code></pre> <p>Until here everything seems to work, now what would be the next steps to get F1 and F2 ?</p> <p><strong>EDIT:</strong> Both of the answers (Tomer and inerjay) seem to work for the first image, but I have tried the following image and it does not work anymore :</p> <p><a href="http://i45.tinypic.com/20b232b.png" rel="nofollow noreferrer">circle2 http://i45.tinypic.com/20b232b.png</a></p>
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    1. COThe problem is that the arcs have "direction". The answer is F2 or (2*R - F2) depending on the direction.
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      1. USDr. belisarius
    2. COhow can I find the direction?
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    3. COYou don't "find" it. You've to specify it. If you specify P1->P2 or P2->P1, one of them subtends an angle greater than 180 degrees. That one uses the (2 R - F2) solution
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      1. USDr. belisarius
 

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