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From How to draw a line passing through a point and perpendicular to another? I learned to use shorten to extend lines. But the extended part doesn't seem to "really exist". For example, I would like to connect two predefined dots and extend the line segment by 1cm and then define the new end as "C" for later use. But \draw[shorten >=-1cm] (A) -- (B) coordinate (C) will only give me a C which is actually still B.

What I really need to do is to find the intersection of two extended lines, but I think the tough part is to make the extended lines "real".

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up vote 16 down vote accepted

You're right, the shorten > syntax doesn't "actually" extend the line, but works on a lower level. In your case, I would use \usetikzlibrary{calc}, which gives you access to a lot of nifty coordinate calculations.

Your desired result could be achieved with the syntax

\draw (A) -- ($(B)!-1cm!(A)$) coordinate (C);

where the ($...$) encloses the calc syntax, and the (<first node>)!<distance>!(<second node>) specifies the coordinate that lies at <distance> along the line from <first node> to <second node>.

\fill (0,0) circle [radius=2pt] node (A) [label=A] {};
\fill (2,1) circle [radius=2pt] node (B) [label=B] {};
\fill (3.2,1) circle [radius=2pt] node (C) [label=C] {};
\fill (4,0.5) circle [radius=2pt] node (D) [label=D] {};
\draw [name path=AB] (A) -- ($(B)!-1cm!(A)$);
\draw [name path=CD] (D) -- ($(C)!-1cm!(D)$);

\fill [red,name intersections={of={AB and CD}}] (intersection-1) circle [radius=2pt];
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thanks a lot. that's "actually" helpful! :) – Ting Sep 25 '11 at 3:16

If you use tkz-euclide (documentation only in French, though), this is much easier:




\tkzDefPoint(0,0){A} \tkzDefPoint(2,1){B}
\tkzDefPoint(3.2,1){C} \tkzDefPoint(4,0.5){D}

\tkzInterLL(A,B)(C,D) \tkzGetPoint{I}
\tkzDrawLines(A,I D,I)
\tkzDrawPoints[color=blue](A,B,C,D) \tkzDrawPoint[color=red](I)


You could also write a piece of code to check the coordinates and automate the choice points for the \tkzDrawLines command and then use arbitrary coordinates, if you need, but I was too lazy to do it. :)

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Consider this a side-answer to the original question, since the solution is provided using pstricks.

pstricks-add provides this point of intersection by means of the macro \psIntersectionPoint(<P0>)(<P1>)(<P2>)(<P3>){<node name>} where P0 and P1 define one straight line (say 0-1) and P2 and P3 define another straight line (say 2-3). <node name> is the name of the saved node at the intersection of 0-1 and 2-3. The following example is taken directly from the pstricks-add package documentation:

\usepackage{pstricks-add}% http://ctan.org/pkg/pstricks-add}

Intersection between two lines in 2D using PStricks

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