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package clip
import (
"github.com/paulmach/orb"
)
// Code based on https://github.com/mapbox/lineclip
// line will clip a line into a set of lines
// along the bounding box boundary.
func line(box orb.Bound, in orb.LineString, open bool) orb.MultiLineString {
if len(in) == 0 {
return nil
}
var out orb.MultiLineString
line := 0
var codeA int
if open {
codeA = bitCodeOpen(box, in[0])
} else {
codeA = bitCode(box, in[0])
}
loopTo := len(in)
for i := 1; i < loopTo; i++ {
a := in[i-1]
b := in[i]
var codeB int
if open {
codeB = bitCodeOpen(box, b)
} else {
codeB = bitCode(box, b)
}
endCode := codeB
// loops through all the intersection of the line and box.
// eg. across a corner could have two intersections.
for {
if codeA|codeB == 0 {
// both points are in the box, accept
out = push(out, line, a)
if codeB != endCode { // segment went outside
out = push(out, line, b)
if i < loopTo-1 {
line++
}
} else if i == loopTo-1 {
out = push(out, line, b)
}
break
} else if codeA&codeB != 0 {
// both on one side of the box.
// segment not part of the final result.
break
} else if codeA != 0 {
// A is outside, B is inside, clip edge
a = intersect(box, codeA, a, b)
codeA = bitCode(box, a)
} else {
// B is outside, A is inside, clip edge
b = intersect(box, codeB, a, b)
codeB = bitCode(box, b)
}
}
codeA = endCode // new start is the old end
}
return out
}
func push(out orb.MultiLineString, i int, p orb.Point) orb.MultiLineString {
if i >= len(out) {
out = append(out, orb.LineString{})
}
out[i] = append(out[i], p)
return out
}
// ring will clip the Ring into a smaller ring around the bounding box boundary.
func ring(box orb.Bound, in orb.Ring) orb.Ring {
var out orb.Ring
if len(in) == 0 {
return in
}
f := in[0]
l := in[len(in)-1]
initClosed := false
if f == l {
initClosed = true
}
for edge := 1; edge <= 8; edge <<= 1 {
out = out[:0]
loopTo := len(in)
// if we're not a nice closed ring, don't implicitly close it.
prev := in[loopTo-1]
if !initClosed {
prev = in[0]
}
prevInside := bitCode(box, prev)&edge == 0
for i := 0; i < loopTo; i++ {
p := in[i]
inside := bitCode(box, p)&edge == 0
// if segment goes through the clip window, add an intersection
if inside != prevInside {
i := intersect(box, edge, prev, p)
out = append(out, i)
}
if inside {
out = append(out, p)
}
prev = p
prevInside = inside
}
if len(out) == 0 {
return nil
}
in, out = out, in
}
out = in // swap back
if initClosed {
// need to make sure our output is also closed.
if l := len(out); l != 0 {
f := out[0]
l := out[l-1]
if f != l {
out = append(out, f)
}
}
}
return out
}
// bitCode returns the point position relative to the bbox:
// left mid right
// top 1001 1000 1010
// mid 0001 0000 0010
// bottom 0101 0100 0110
func bitCode(b orb.Bound, p orb.Point) int {
code := 0
if p[0] < b.Min[0] {
code |= 1
} else if p[0] > b.Max[0] {
code |= 2
}
if p[1] < b.Min[1] {
code |= 4
} else if p[1] > b.Max[1] {
code |= 8
}
return code
}
func bitCodeOpen(b orb.Bound, p orb.Point) int {
code := 0
if p[0] <= b.Min[0] {
code |= 1
} else if p[0] >= b.Max[0] {
code |= 2
}
if p[1] <= b.Min[1] {
code |= 4
} else if p[1] >= b.Max[1] {
code |= 8
}
return code
}
// intersect a segment against one of the 4 lines that make up the bbox
func intersect(box orb.Bound, edge int, a, b orb.Point) orb.Point {
if edge&8 != 0 {
// top
return orb.Point{a[0] + (b[0]-a[0])*(box.Max[1]-a[1])/(b[1]-a[1]), box.Max[1]}
} else if edge&4 != 0 {
// bottom
return orb.Point{a[0] + (b[0]-a[0])*(box.Min[1]-a[1])/(b[1]-a[1]), box.Min[1]}
} else if edge&2 != 0 {
// right
return orb.Point{box.Max[0], a[1] + (b[1]-a[1])*(box.Max[0]-a[0])/(b[0]-a[0])}
} else if edge&1 != 0 {
// left
return orb.Point{box.Min[0], a[1] + (b[1]-a[1])*(box.Min[0]-a[0])/(b[0]-a[0])}
}
panic("no edge??")
}
|
