mirror of https://github.com/tidwall/tile38.git
122 lines
2.9 KiB
Go
122 lines
2.9 KiB
Go
package geojson
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// Cohen-Sutherland Line Clipping
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// https://www.cs.helsinki.fi/group/goa/viewing/leikkaus/lineClip.html
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func ClipSegment(start, end Position, bbox BBox) (resStart, resEnd Position, rejected bool) {
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startCode := getCode(bbox, start)
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endCode := getCode(bbox, end)
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if (startCode | endCode) == 0 {
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// trivially accept
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resStart, resEnd = start, end
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} else if (startCode & endCode) != 0 {
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// trivially reject
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rejected = true
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} else if startCode != 0 {
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// start is outside. get new start.
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newStart := intersect(bbox, startCode, start, end)
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resStart, resEnd, rejected = ClipSegment(newStart, end, bbox)
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} else {
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// end is outside. get new end.
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newEnd := intersect(bbox, endCode, start, end)
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resStart, resEnd, rejected = ClipSegment(start, newEnd, bbox)
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}
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return
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}
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// Sutherland-Hodgman Polygon Clipping
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// https://www.cs.helsinki.fi/group/goa/viewing/leikkaus/intro2.html
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func ClipRing(ring[] Position, bbox BBox) (resRing []Position) {
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if len(ring) < 4 {
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// under 4 elements this is not a polygon ring!
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return
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}
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var edge uint8
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var inside, prevInside bool
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var prev Position
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for edge = 1; edge <= 8; edge *=2 {
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prev = ring[len(ring) - 2]
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prevInside = (getCode(bbox, prev) & edge) == 0
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for _, p := range ring {
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inside = (getCode(bbox, p) & edge) == 0
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if prevInside && inside {
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// Staying inside
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resRing = append(resRing, p)
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} else if prevInside && !inside {
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// Leaving
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resRing = append(resRing, intersect(bbox, edge, prev, p))
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} else if !prevInside && inside {
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// Entering
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resRing = append(resRing, intersect(bbox, edge, prev, p))
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resRing = append(resRing, p)
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} else {
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// Staying outside
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}
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prev, prevInside = p, inside
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}
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if resRing[0] != resRing[len(resRing)-1] {
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resRing = append(resRing, resRing[0])
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}
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ring, resRing = resRing, []Position{}
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}
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resRing = ring
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return
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}
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func getCode(bbox BBox, point Position) (code uint8) {
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code = 0
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if point.X < bbox.Min.X {
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code |= 1 // left
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} else if point.X > bbox.Max.X {
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code |= 2 // right
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}
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if point.Y < bbox.Min.Y {
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code |= 4 // bottom
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} else if point.Y > bbox.Max.Y {
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code |= 8 // top
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}
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return
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}
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func intersect(bbox BBox, code uint8, start, end Position) (new Position) {
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if (code & 8) != 0 { // top
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new = Position{
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X: start.X + (end.X - start.X) * (bbox.Max.Y - start.Y) / (end.Y - start.Y),
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Y: bbox.Max.Y,
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}
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} else if (code & 4) != 0 { // bottom
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new = Position{
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X: start.X + (end.X - start.X) * (bbox.Min.Y - start.Y) / (end.Y - start.Y),
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Y: bbox.Min.Y,
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}
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} else if (code & 2) != 0 { //right
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new = Position{
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X: bbox.Max.X,
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Y: start.Y + (end.Y - start.Y) * (bbox.Max.X - start.X) / (end.X - start.X),
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}
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} else if (code & 1) != 0 { // left
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new = Position{
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X: bbox.Min.X,
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Y: start.Y + (end.Y - start.Y) * (bbox.Min.X - start.X) / (end.X - start.X),
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}
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} else { // should not call intersect with the zero code
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}
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return
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}
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