package clip import ( "github.com/tidwall/geojson" "github.com/tidwall/geojson/geometry" ) // Clip clips the contents of a geojson object and return func Clip( obj geojson.Object, clipper geojson.Object, opts *geometry.IndexOptions, ) (clipped geojson.Object) { switch obj := obj.(type) { case *geojson.Point: return clipPoint(obj, clipper, opts) case *geojson.Rect: return clipRect(obj, clipper, opts) case *geojson.LineString: return clipLineString(obj, clipper, opts) case *geojson.Polygon: return clipPolygon(obj, clipper, opts) case *geojson.Feature: return clipFeature(obj, clipper, opts) case geojson.Collection: return clipCollection(obj, clipper, opts) } return obj } // clipSegment is Cohen-Sutherland Line Clipping // https://www.cs.helsinki.fi/group/goa/viewing/leikkaus/lineClip.html func clipSegment(seg geometry.Segment, rect geometry.Rect) ( res geometry.Segment, rejected bool, ) { startCode := getCode(rect, seg.A) endCode := getCode(rect, seg.B) if (startCode | endCode) == 0 { // trivially accept res = seg } else if (startCode & endCode) != 0 { // trivially reject rejected = true } else if startCode != 0 { // start is outside. get new start. newStart := intersect(rect, startCode, seg.A, seg.B) res, rejected = clipSegment(geometry.Segment{A: newStart, B: seg.B}, rect) } else { // end is outside. get new end. newEnd := intersect(rect, endCode, seg.A, seg.B) res, rejected = clipSegment(geometry.Segment{A: seg.A, B: newEnd}, rect) } return } // clipRing is Sutherland-Hodgman Polygon Clipping // https://www.cs.helsinki.fi/group/goa/viewing/leikkaus/intro2.html func clipRing(ring []geometry.Point, bbox geometry.Rect) ( resRing []geometry.Point, ) { if len(ring) < 4 { // under 4 elements this is not a polygon ring! return } var edge uint8 var inside, prevInside bool var prev geometry.Point for edge = 1; edge <= 8; edge *= 2 { prev = ring[len(ring)-2] prevInside = (getCode(bbox, prev) & edge) == 0 for _, p := range ring { inside = (getCode(bbox, p) & edge) == 0 if prevInside && inside { // Staying inside resRing = append(resRing, p) } else if prevInside && !inside { // Leaving resRing = append(resRing, intersect(bbox, edge, prev, p)) } else if !prevInside && inside { // Entering resRing = append(resRing, intersect(bbox, edge, prev, p)) resRing = append(resRing, p) } /* else { // Stay outside } */ prev, prevInside = p, inside } if len(resRing) > 0 && resRing[0] != resRing[len(resRing)-1] { resRing = append(resRing, resRing[0]) } ring, resRing = resRing, []geometry.Point{} if len(ring) == 0 { break } } resRing = ring return } func getCode(bbox geometry.Rect, point geometry.Point) (code uint8) { code = 0 if point.X < bbox.Min.X { code |= 1 // left } else if point.X > bbox.Max.X { code |= 2 // right } if point.Y < bbox.Min.Y { code |= 4 // bottom } else if point.Y > bbox.Max.Y { code |= 8 // top } return } func intersect(bbox geometry.Rect, code uint8, start, end geometry.Point) ( new geometry.Point, ) { if (code & 8) != 0 { // top new = geometry.Point{ X: start.X + (end.X-start.X)*(bbox.Max.Y-start.Y)/(end.Y-start.Y), Y: bbox.Max.Y, } } else if (code & 4) != 0 { // bottom new = geometry.Point{ X: start.X + (end.X-start.X)*(bbox.Min.Y-start.Y)/(end.Y-start.Y), Y: bbox.Min.Y, } } else if (code & 2) != 0 { //right new = geometry.Point{ X: bbox.Max.X, Y: start.Y + (end.Y-start.Y)*(bbox.Max.X-start.X)/(end.X-start.X), } } else if (code & 1) != 0 { // left new = geometry.Point{ X: bbox.Min.X, Y: start.Y + (end.Y-start.Y)*(bbox.Min.X-start.X)/(end.X-start.X), } } /* else { // should not call intersect with the zero code } */ return }