[core] reset line distance when it is near max

port https://github.com/mapbox/mapbox-gl-js/commit/77ce4b45b24e8a0ed1bb739da436d19415c7e5e4

fix #4492

5 files changed, 36 insertions, 8 deletions

diff --git a/src/mbgl/geometry/line_buffer.cpp b/src/mbgl/geometry/line_buffer.cpp
index b35d3e8f47..75b9af7e20 100644
--- a/src/mbgl/geometry/line_buffer.cpp
+++ b/src/mbgl/geometry/line_buffer.cpp
@@ -22,7 +22,7 @@ GLsizei LineVertexBuffer::add(vertex_type x, vertex_type y, float ex, float ey,
     // The z component's first bit, as well as the sign bit is reserved for the direction,
     // so we need to shift the linesofar.
     extrude[6] = ((dir < 0) ? -1 : 1) * ((dir ? 1 : 0) | static_cast<int8_t>((linesofar << 1) & 0x7F));
-    extrude[7] = (linesofar >> 6) & 0x7F;
+    extrude[7] = ((linesofar >> 6) & 0xFF) - 128;
 
     return idx;
 }
diff --git a/src/mbgl/renderer/line_bucket.cpp b/src/mbgl/renderer/line_bucket.cpp
index 06daf3bbc3..1e8ba0ecb5 100644
--- a/src/mbgl/renderer/line_bucket.cpp
+++ b/src/mbgl/renderer/line_bucket.cpp
@@ -41,6 +41,17 @@ void LineBucket::addGeometry(const GeometryCollection& geometryCollection) {
 const float COS_HALF_SHARP_CORNER = std::cos(75.0 / 2.0 * (M_PI / 180.0));
 const float SHARP_CORNER_OFFSET = 15.0f;
 
+// The number of bits that is used to store the line distance in the buffer.
+const int LINE_DISTANCE_BUFFER_BITS = 14;
+
+// We don't have enough bits for the line distance as we'd like to have, so
+// use this value to scale the line distance (in tile units) down to a smaller
+// value. This lets us store longer distances while sacrificing precision.
+const float LINE_DISTANCE_SCALE = 1.0 / 2.0;
+
+// The maximum line distance, in tile units, that fits in the buffer.
+const float MAX_LINE_DISTANCE = std::pow(2, LINE_DISTANCE_BUFFER_BITS) / LINE_DISTANCE_SCALE;
+
 void LineBucket::addGeometry(const GeometryCoordinates& vertices) {
     const GLsizei len = [&vertices] {
         GLsizei l = static_cast<GLsizei>(vertices.size());
@@ -363,7 +374,7 @@ void LineBucket::addGeometry(const GeometryCoordinates& vertices) {
 
 void LineBucket::addCurrentVertex(const GeometryCoordinate& currentVertex,
                                   float flip,
-                                  double distance,
+                                  double &distance,
                                   const vec2<double>& normal,
                                   float endLeft,
                                   float endRight,
@@ -375,7 +386,7 @@ void LineBucket::addCurrentVertex(const GeometryCoordinate& currentVertex,
     vec2<double> extrude = normal * flip;
     if (endLeft)
         extrude = extrude - (util::perp(normal) * endLeft);
-    e3 = vertexBuffer.add(currentVertex.x, currentVertex.y, extrude.x, extrude.y, tx, 0, endLeft, distance)
+    e3 = vertexBuffer.add(currentVertex.x, currentVertex.y, extrude.x, extrude.y, tx, 0, endLeft, distance * LINE_DISTANCE_SCALE)
          - startVertex;
     if (e1 >= 0 && e2 >= 0) {
         triangleStore.emplace_back(e1, e2, e3);
@@ -386,13 +397,22 @@ void LineBucket::addCurrentVertex(const GeometryCoordinate& currentVertex,
     extrude = normal * (-flip);
     if (endRight)
         extrude = extrude - (util::perp(normal) * endRight);
-    e3 = vertexBuffer.add(currentVertex.x, currentVertex.y, extrude.x, extrude.y, tx, 1, -endRight, distance)
+    e3 = vertexBuffer.add(currentVertex.x, currentVertex.y, extrude.x, extrude.y, tx, 1, -endRight, distance * LINE_DISTANCE_SCALE)
          - startVertex;
     if (e1 >= 0 && e2 >= 0) {
         triangleStore.emplace_back(e1, e2, e3);
     }
     e1 = e2;
     e2 = e3;
+
+    // There is a maximum "distance along the line" that we can store in the buffers.
+    // When we get close to the distance, reset it to zero and add the vertex again with
+    // a distance of zero. The max distance is determined by the number of bits we allocate
+    // to `linesofar`.
+    if (distance > MAX_LINE_DISTANCE / 2.0f) {
+        distance = 0;
+        addCurrentVertex(currentVertex, flip, distance, normal, endLeft, endRight, round, startVertex, triangleStore);
+    }
 }
 
 void LineBucket::addPieSliceVertex(const GeometryCoordinate& currentVertex,
@@ -405,7 +425,7 @@ void LineBucket::addPieSliceVertex(const GeometryCoordinate& currentVertex,
     int8_t ty = lineTurnsLeft;
 
     auto flippedExtrude = extrude * (flip * (lineTurnsLeft ? -1 : 1));
-    e3 = vertexBuffer.add(currentVertex.x, currentVertex.y, flippedExtrude.x, flippedExtrude.y, 0, ty, 0, distance)
+    e3 = vertexBuffer.add(currentVertex.x, currentVertex.y, flippedExtrude.x, flippedExtrude.y, 0, ty, 0, distance * LINE_DISTANCE_SCALE)
          - startVertex;
     if (e1 >= 0 && e2 >= 0) {
         triangleStore.emplace_back(e1, e2, e3);
diff --git a/src/mbgl/renderer/line_bucket.hpp b/src/mbgl/renderer/line_bucket.hpp
index 5b6509a5ee..1586bef0a7 100644
--- a/src/mbgl/renderer/line_bucket.hpp
+++ b/src/mbgl/renderer/line_bucket.hpp
@@ -43,7 +43,7 @@ private:
         TriangleElement(uint16_t a_, uint16_t b_, uint16_t c_) : a(a_), b(b_), c(c_) {}
         uint16_t a, b, c;
     };
-    void addCurrentVertex(const GeometryCoordinate& currentVertex, float flip, double distance,
+    void addCurrentVertex(const GeometryCoordinate& currentVertex, float flip, double& distance,
             const vec2<double>& normal, float endLeft, float endRight, bool round,
             GLint startVertex, std::vector<LineBucket::TriangleElement>& triangleStore);
     void addPieSliceVertex(const GeometryCoordinate& currentVertex, float flip, double distance,
diff --git a/src/mbgl/shader/linepattern.vertex.glsl b/src/mbgl/shader/linepattern.vertex.glsl
index 5b3c468be0..6fa499106c 100644
--- a/src/mbgl/shader/linepattern.vertex.glsl
+++ b/src/mbgl/shader/linepattern.vertex.glsl
@@ -6,6 +6,10 @@
 // #define scale 63.0
 #define scale 0.015873016
 
+// We scale the distance before adding it to the buffers so that we can store
+// long distances for long segments. Use this value to unscale the distance.
+#define LINE_DISTANCE_SCALE 2.0
+
 attribute vec2 a_pos;
 attribute vec4 a_data;
 
@@ -30,7 +34,7 @@ varying float v_gamma_scale;
 void main() {
     vec2 a_extrude = a_data.xy;
     float a_direction = sign(a_data.z) * mod(a_data.z, 2.0);
-    float a_linesofar = abs(floor(a_data.z / 2.0)) + a_data.w * 64.0;
+    float a_linesofar = (abs(floor(a_data.z / 2.0)) + (a_data.w + 128.0) * 64.0) * LINE_DISTANCE_SCALE;
 
     // We store the texture normals in the most insignificant bit
     // transform y so that 0 => -1 and 1 => 1
diff --git a/src/mbgl/shader/linesdf.vertex.glsl b/src/mbgl/shader/linesdf.vertex.glsl
index 80516f97c0..f24d8399b8 100644
--- a/src/mbgl/shader/linesdf.vertex.glsl
+++ b/src/mbgl/shader/linesdf.vertex.glsl
@@ -6,6 +6,10 @@
 // #define scale 63.0
 #define scale 0.015873016
 
+// We scale the distance before adding it to the buffers so that we can store
+// long distances for long segments. Use this value to unscale the distance.
+#define LINE_DISTANCE_SCALE 2.0
+
 attribute vec2 a_pos;
 attribute vec4 a_data;
 
@@ -34,7 +38,7 @@ varying float v_gamma_scale;
 void main() {
     vec2 a_extrude = a_data.xy;
     float a_direction = sign(a_data.z) * mod(a_data.z, 2.0);
-    float a_linesofar = abs(floor(a_data.z / 2.0)) + a_data.w * 64.0;
+    float a_linesofar = (abs(floor(a_data.z / 2.0)) + (a_data.w + 128.0) * 64.0) * LINE_DISTANCE_SCALE;
 
     // We store the texture normals in the most insignificant bit
     // transform y so that 0 => -1 and 1 => 1