# Calculate miter points of stroked vectors in Cartesian plane

I have two vectors CA and CB which I 'stroked' with lines of width a and b. I need to calculate D and E points to draw miter joint between two stroked vectors. What I know is:

• A point coordinates
• B point coordinates
• C point coordinates
• β angle
• a length
• b length

What I'm looking are coordinates of points D and E. I need to find universal formula that lets me to calculate those points at any β. (please see pic.1 below)

I can calculate those points if use stroke with same length for both vectors (a == b). I'm doing it by reflecting point C over vector AC with distance 0.5*a. Then I have right angle triangle, on which γ angle at point C equals 90° - (0.5 * β) angle. Therefore I have all three angles for the triangle and length of CF (half of a) which lets my to calculate coordinates of point D. (please see pic.2 below). I use triangle CGE to calculate E coordinates in the same way as above.

My problems start when I need to use different width for the vector's stroke (a != b) (please see pic.3 below). In that case when I draw CFD triangle I cannot calculate γ angle as it is not 90° - (0.5 * β) anymore and I have no idea how to calculate D and E coordinates. Can someone point me in the right direction how to find γ angle or if there is any other (better) way to calculate coordinates of D and E?

## 1 Answer

Draw two vectors $\vec u$ and $\vec v$ as in the picture below. They form a parallelogram having $a/2$ and $b/2$ as altitudes. It follows that $$u={b\over 2\sin\beta},\quad v={a\over 2\sin\beta}.$$ It is then easy to compute $\vec u$ and $\vec v$: $$\vec u={A-B\over AB}{b\over 2\sin\beta},\quad \vec v={C-B\over BC}{a\over 2\sin\beta}$$ and finally: $$D=B+\vec u+\vec v,\quad E=B-\vec u-\vec v.$$

• Amazing! It looks much cleaner and simpler from what I was trying to do. I know it may be pretty basic, but I'm not sure how to do computations on points (A-B/AB), therefore I'm not sure how to compute u⃗ and v⃗. Are A, B and C row matrices in your equation? Would you mind to show me an example calculation? – Guferos Jul 6 '16 at 10:16
• $A-B$ and $C-B$ are displacement vectors. Example: $A=(1,3)$, $B=(5,1)$, $A-B=(-4,2)$. While $AB$ is the distance between $A$ and $B$. For the example at hand, $AB=\sqrt{20}$ so that $${A-B\over AB}=(-{4\over\sqrt{20}},{2\over\sqrt{20}}).$$ – Intelligenti pauca Jul 6 '16 at 13:43