[OSM-dev] Osmarender feature on only one side of road

[Martijn van Oosterhout]

On 7/2/07, Frederik Ramm <frederik at remote.org> wrote:
> But wouldn't you have to individually compute the offset for every
> line segment using trigonometric functions if you want a constant
> distance between the original path and the offset path?

I wonder. (If anyone spots errors in the reasoning below, let me know).

Say you have a line going from (a,b) to (c,d). What you really want is
a transformation that translates (0,0) to (a,b) and rotates the world
so (c,d) is somewhere along the x-axis. This rotation requires the
angle and trig functions, or so you think. Assume the length of the
line is L. Then:

sin(theta) = (d-b)/L
cos(theta) = (c-a)/L

Thus with the following SVG transforms:

transform(a,b) scale(1/L) matrix( c-a d-b b-d c-a 0 0 )

You map the line (L,0) to the original line and a line (L,1) will be
parallel and one pixel offset. This requires no trig functions and
should be within the capability of XSLT, right?

(The proof is in the pudding I suppose, I'll try it out this afternoon).

What do you think?
-- 
Martijn van Oosterhout <kleptog at gmail.com> http://svana.org/kleptog/