simplifyGeometry() simplifies too much for tolerance=0

[Wilkins]

I'm working on a project that use your simplifyGeometry function a lot (https://github.com/Wilkins/gpx-simplify-optimizer).

And I found that sometimes with a tolerance = 0 it simplifies more than it should.

I made a little snippet to reproduce my problem :

var simplifyGeometry = require("./index");

var case1 = [
    [-60.501,47.001,0],
    [-60.323,46.867,0],
    [-60.317,46.861,0], // This value get stripped
    [-60.311,46.855,0],
    [-60.305,46.834,0],
    [-60.606,46.21,0],
    [-60.606,46.21,0],
    [-60.605,46.211,0],
    [-60.6,46.201,0],
    [-60.736,46.057,0],
    [-60.501,47.001,0]
];

// Same as case 1 but narrowed
var case2 = [
    [-60.323,46.867,0],
    [-60.317,46.861,0], // This value get stripped
    [-60.311,46.855,0],
];

var case3 = [
    [45.759,-63.257],
    [45.754,-63.257], // This value get stripped
    [45.743,-63.257]
];

var case4 = [ 
    [45.294,-61.007],
    [45.294,-61.017], // This value get stripped
    [45.294,-61.027]
];


console.log(simplifyGeometry(case1, 0));
console.log(simplifyGeometry(case2, 0));
console.log(simplifyGeometry(case3, 0));
console.log(simplifyGeometry(case4, 0));

I guess this is a geometry reason, but I think it should not strip anything if tolerance=0.
Maybe a simple if (tolerance === 0) { return points; } would correct this particular case.

But maybe it's a more complex geometry problem that need to be solved for other cases.

[seabre]

It's how the algorithm works. This person gives a pretty good explanation of how the tolerance parameter works: http://gis.stackexchange.com/a/11915

PostGIS's implementation is roughly the same as mine. Here's test case 1 as a PostGIS call with tolerance at zero:

SELECT ST_AsText(ST_Simplify(ST_GeomFromText('MULTIPOLYGON (((-60.501 47.001, -60.323 46.867, -60.317 46.861, -60.311 46.855, -60.305 46.834, -60.606 46.21, -60.606 46.21, -60.605 46.211, -60.6 46.201, -60.736 46.057, -60.501 47.001)))'), 0));

And the output:

MULTIPOLYGON(((-60.501 47.001,-60.323 46.867,-60.311 46.855,-60.305 46.834,-60.606 46.21,-60.605 46.211,-60.6 46.201,-60.736 46.057,-60.501 47.001)))

Also test case 4 for good measure:

SELECT ST_AsText(ST_Simplify(ST_GeomFromText('LINESTRING (45.294 -61.007, 45.294 -61.017, 45.294 -61.027)'), 0));

And the output:

LINESTRING(45.294 -61.007,45.294 -61.027)