[OSM-dev] looking for some spline code to adapt

[Christoph Hormann]

On Sunday 02 July 2017, Rich Morin wrote:
> I'm generating sets of tactile maps for a blind friend, as follows:
>
>   OSM export files -(Ruby)-> PostScript -(lpr)-> swell paper
>
> I'm using straight lines for the roads, using the PostScript moveto
> and lineto operators.  However, I'd like to use the curveto operator:
>
>   x1 y1 x2 y2 x3 y3 curveto -
>
>   This operator draws a curve from the current point to the point
> (x3, y3) using points (x1, y1) and (x2, y2) as control points. The
> curve is a Bézier cubic curve. In such a curve, the tangent of the
> curve at the current point will be a line segment running from the
> current point to (x1, y1) and the tangent at (x3, y3) is the line
> running from (x3, y3) to (x2, y2).
>
> [...]

You can certainly do so but you need to expect that your results are 
largely wrong.

The problem is that in OSM we do not record any supplemental information 
on ways regarding if the corners at the nodes are actual corners or if 
they are just approximations for a smooth form of some sort - what you 
would usually do with a kind of tension parameter.  Neither can you 
rely on the fact that when two roads connect this is not a smooth 
transit.  It is not even defined if when a way approximates a smooth 
form the nodes of the way are supposed to be on the curve or if they 
should be placed so the average distance of the way to the curve is 
minimized.

-- 
Christoph Hormann
http://www.imagico.de/