Draw 2 parallel lines

How can I calculate the points to draw 2 parallel lines.

I know the start and end points for the centre of the parallel lines. To makes thing a开发者_JS百科 little bit harder, it needs to support straight and Bezier curved lines.

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The question is vague, but here's a possibility. Hope that helps.

For a segment (x1,y1)-(x2,y2) you can calculate another segment, n pixels away in a direction represented by angle a this way

x1b = x1 + n cos a
y1b = y1 - n sin a
x2b = x2 + n cos a
y2b = y2 - n sin a