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Add ol.geom.simplify.schaub

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This commit is contained in:
Tom Payne
2013-12-13 13:51:58 +01:00
parent cbc71686df
commit 5b1bbe15dd
2 changed files with 229 additions and 0 deletions
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164
src/ol/geom/simplifygeom.js
View File

@@ -233,3 +233,167 @@ ol.geom.simplify.radialDistance = function(flatCoordinates, offset, end,
}
return simplifiedOffset;
};
/**
* @param {number} value Value.
* @param {number} tolerance Squared tolerance.
* @return {number} Rounded value.
*/
ol.geom.simplify.snap = function(value, tolerance) {
return tolerance * Math.round(value / tolerance);
};
/**
* Simplifies a line string using an algorithm designed by Tim Schaub.
* Coordinates are snapped to the nearest value in a virtual grid and
* consecutive duplicate coordinates are discarded. This effectively preserves
* topology as the simplification of any subsection of a line string is
* independent of the rest of the line string. This means that, for examples,
* the common edge between two polygons will be simplified to the same line
* string independently in both polygons. This implementation uses a single
* pass over the coordinates and eliminates intermediate collinear points.
* @param {Array.<number>} flatCoordinates Flat coordinates.
* @param {number} offset Offset.
* @param {number} end End.
* @param {number} stride Stride.
* @param {number} tolerance Squared tolerance.
* @param {Array.<number>} simplifiedFlatCoordinates Simplified flat
* coordinates.
* @param {number} simplifiedOffset Simplified offset.
* @return {number} Simplified offset.
*/
ol.geom.simplify.schaub = function(flatCoordinates, offset, end, stride,
tolerance, simplifiedFlatCoordinates, simplifiedOffset) {
// do nothing if the line is empty
if (offset == end) {
return simplifiedOffset;
}
// snap the first coordinate (P1)
var x1 = ol.geom.simplify.snap(flatCoordinates[offset], tolerance);
var y1 = ol.geom.simplify.snap(flatCoordinates[offset + 1], tolerance);
offset += stride;
// add the first coordinate to the output
simplifiedFlatCoordinates[simplifiedOffset++] = x1;
simplifiedFlatCoordinates[simplifiedOffset++] = y1;
// find the next coordinate that does not snap to the same value as the first
// coordinate (P2)
var x2, y2;
do {
x2 = ol.geom.simplify.snap(flatCoordinates[offset], tolerance);
y2 = ol.geom.simplify.snap(flatCoordinates[offset + 1], tolerance);
offset += stride;
if (offset == end) {
// all coordinates snap to the same value, the line collapses to a point
// push the last snapped value anyway to ensure that the output contains
// at least two points
// FIXME should we really return at least two points anyway?
simplifiedFlatCoordinates[simplifiedOffset++] = x2;
simplifiedFlatCoordinates[simplifiedOffset++] = y2;
return simplifiedOffset;
}
} while (x2 == x1 && y2 == y1);
while (offset < end) {
var x3, y3;
// snap the next coordinate (P3)
x3 = ol.geom.simplify.snap(flatCoordinates[offset], tolerance);
y3 = ol.geom.simplify.snap(flatCoordinates[offset + 1], tolerance);
offset += stride;
// skip P3 if it is equal to P2
if (x3 == x2 && y3 == y2) {
continue;
}
// calculate the delta between P1 and P2
var dx1 = x2 - x1;
var dy1 = y2 - y1;
// calculate the delta between P3 and P1
var dx2 = x3 - x1;
var dy2 = y3 - y1;
// if P1, P2, and P3 are colinear and P3 is further from P1 than P2 is from
// P1 in the same direction then P2 is on the straight line between P1 and
// P3
if ((dx1 * dy2 == dy1 * dx2) &&
((dx1 < 0 && dx2 < dx1) || dx1 == dx2 || (dx1 > 0 && dx2 > dx1)) &&
((dy1 < 0 && dy2 < dy1) || dy1 == dy2 || (dy1 > 0 && dy2 > dy1))) {
// discard P2 and set P2 = P3
x2 = x3;
y2 = y3;
continue;
}
// either P1, P2, and P3 are not colinear, or they are colinear but P3 is
// between P3 and P1 or on the opposite half of the line to P2. add P2,
// and continue with P1 = P2 and P2 = P3
simplifiedFlatCoordinates[simplifiedOffset++] = x2;
simplifiedFlatCoordinates[simplifiedOffset++] = y2;
x1 = x2;
y1 = y2;
x2 = x3;
y2 = y3;
}
// add the last point (P2)
simplifiedFlatCoordinates[simplifiedOffset++] = x2;
simplifiedFlatCoordinates[simplifiedOffset++] = y2;
return simplifiedOffset;
};
/**
* @param {Array.<number>} flatCoordinates Flat coordinates.
* @param {number} offset Offset.
* @param {Array.<number>} ends Ends.
* @param {number} stride Stride.
* @param {number} tolerance Squared tolerance.
* @param {Array.<number>} simplifiedFlatCoordinates Simplified flat
* coordinates.
* @param {number} simplifiedOffset Simplified offset.
* @param {Array.<number>} simplifiedEnds Simplified ends.
* @return {number} Simplified offset.
*/
ol.geom.simplify.schaubs = function(
flatCoordinates, offset, ends, stride,
tolerance,
simplifiedFlatCoordinates, simplifiedOffset, simplifiedEnds) {
var i, ii;
for (i = 0, ii = ends.length; i < ii; ++i) {
var end = ends[i];
simplifiedOffset = ol.geom.simplify.schaub(
flatCoordinates, offset, end, stride,
tolerance,
simplifiedFlatCoordinates, simplifiedOffset);
simplifiedEnds.push(simplifiedOffset);
offset = end;
}
return simplifiedOffset;
};
/**
* @param {Array.<number>} flatCoordinates Flat coordinates.
* @param {number} offset Offset.
* @param {Array.<Array.<number>>} endss Endss.
* @param {number} stride Stride.
* @param {number} tolerance Squared tolerance.
* @param {Array.<number>} simplifiedFlatCoordinates Simplified flat
* coordinates.
* @param {number} simplifiedOffset Simplified offset.
* @param {Array.<Array.<number>>} simplifiedEndss Simplified endss.
* @return {number} Simplified offset.
*/
ol.geom.simplify.schaubss = function(
flatCoordinates, offset, endss, stride,
tolerance,
simplifiedFlatCoordinates, simplifiedOffset, simplifiedEndss) {
var i, ii;
for (i = 0, ii = endss.length; i < ii; ++i) {
var ends = endss[i];
var simplifiedEnds = [];
simplifiedOffset = ol.geom.simplify.schaubs(
flatCoordinates, offset, ends, stride,
tolerance,
simplifiedFlatCoordinates, simplifiedOffset, simplifiedEnds);
simplifiedEndss.push(simplifiedEnds);
offset = ends[ends.length - 1];
}
return simplifiedOffset;
};

65
test/spec/ol/geom/simplifygeom.test.js
View File

@@ -287,6 +287,71 @@ describe('ol.geom.simplify', function() {
});
describe('ol.geom.simplify.schaub', function() {
it('handles empty coordinates', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[], 0, 0, 2, 2, simplifiedFlatCoordinates, 0)).to.be(0);
expect(simplifiedFlatCoordinates).to.be.empty();
});
it('expands points to a zero-length line', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0, 0, 0, 0], 0, 4, 2, 2, simplifiedFlatCoordinates, 0)).to.be(4);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 0, 0]);
});
it('snaps near-by points to the same value', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0.1, 0, 0, 0.1], 0, 4, 2, 2, simplifiedFlatCoordinates, 0)).to.be(4);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 0, 0]);
});
it('eliminates duplicate snapped points', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0.1, 0, 2, 0, 2.1, 0, 2, 0.1, 1.9, 0, 2, -0.1], 0, 12, 2, 2,
simplifiedFlatCoordinates, 0)).to.be(4);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 2, 0]);
});
it('eliminates horizontal colinear points', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0, 0, 2, 0, 4, 0, 6, 0], 0, 8, 2, 2,
simplifiedFlatCoordinates, 0)).to.be(4);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 6, 0]);
});
it('eliminates vertical colinear points', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0, 0, 0, -2, 0, -4, 0, -6], 0, 8, 2, 2,
simplifiedFlatCoordinates, 0)).to.be(4);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 0, -6]);
});
it('eliminates diagonal colinear points', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0, 0, 2, -2, 4, -4, 6, -6], 0, 8, 2, 2,
simplifiedFlatCoordinates, 0)).to.be(4);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 6, -6]);
});
it('handles switchbacks', function() {
var simplifiedFlatCoordinates = [];
expect(ol.geom.simplify.schaub(
[0, 0, 2, 0, 0, 0, 4, 0], 0, 8, 2, 2,
simplifiedFlatCoordinates, 0)).to.be(8);
expect(simplifiedFlatCoordinates).to.eql([0, 0, 2, 0, 0, 0, 4, 0]);
});
});
});