1 files changed, 33 insertions, 3 deletions

diff --git a/src/mbgl/shaders/collision_box.cpp b/src/mbgl/shaders/collision_box.cpp
index 5f733c6a1e..05f306ef65 100644
--- a/src/mbgl/shaders/collision_box.cpp
+++ b/src/mbgl/shaders/collision_box.cpp
@@ -8,44 +8,74 @@ namespace shaders {
 const char* collision_box::name = "collision_box";
 const char* collision_box::vertexSource = R"MBGL_SHADER(
 attribute vec2 a_pos;
+attribute vec2 a_anchor_pos;
 attribute vec2 a_extrude;
 attribute vec2 a_data;
 
 uniform mat4 u_matrix;
 uniform float u_scale;
+uniform float u_pitch;
+uniform float u_collision_y_stretch;
+uniform float u_camera_to_center_distance;
 
 varying float v_max_zoom;
 varying float v_placement_zoom;
+varying float v_perspective_zoom_adjust;
+varying vec2 v_fade_tex;
 
 void main() {
-    gl_Position = u_matrix * vec4(a_pos + a_extrude / u_scale, 0.0, 1.0);
+    vec4 projectedPoint = u_matrix * vec4(a_anchor_pos, 0, 1);
+    highp float camera_to_anchor_distance = projectedPoint.w;
+    highp float collision_perspective_ratio = 1.0 + 0.5 * ((camera_to_anchor_distance / u_camera_to_center_distance) - 1.0);
+
+    highp float incidence_stretch  = camera_to_anchor_distance / (u_camera_to_center_distance * cos(u_pitch));
+    highp float collision_adjustment = max(1.0, incidence_stretch / u_collision_y_stretch);
+
+    gl_Position = u_matrix * vec4(a_pos + a_extrude * collision_perspective_ratio * collision_adjustment / u_scale, 0.0, 1.0);
 
     v_max_zoom = a_data.x;
     v_placement_zoom = a_data.y;
+
+    v_perspective_zoom_adjust = log2(collision_perspective_ratio * collision_adjustment) * 10.0;
+    v_fade_tex = vec2((v_placement_zoom + v_perspective_zoom_adjust) / 255.0, 0.0);
 }
 
 )MBGL_SHADER";
 const char* collision_box::fragmentSource = R"MBGL_SHADER(
 uniform float u_zoom;
+// u_maxzoom is derived from the maximum scale considered by the CollisionTile
+// Labels with placement zoom greater than this value will not be placed,
+// regardless of perspective effects.
 uniform float u_maxzoom;
+uniform sampler2D u_fadetexture;
 
+// v_max_zoom is a collision-box-specific value that controls when line-following
+// collision boxes are used.
 varying float v_max_zoom;
 varying float v_placement_zoom;
+varying float v_perspective_zoom_adjust;
+varying vec2 v_fade_tex;
 
 void main() {
 
     float alpha = 0.5;
 
+    // Green = no collisions, label is showing
     gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0) * alpha;
 
-    if (v_placement_zoom > u_zoom) {
+    // Red = collision, label hidden
+    if (texture2D(u_fadetexture, v_fade_tex).a < 1.0) {
         gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0) * alpha;
     }
 
-    if (u_zoom >= v_max_zoom) {
+    // Faded black = this collision box is not used at this zoom (for curved labels)
+    if (u_zoom >= v_max_zoom + v_perspective_zoom_adjust) {
         gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0) * alpha * 0.25;
     }
 
+    // Faded blue = the placement scale for this label is beyond the CollisionTile
+    // max scale, so it's impossible for this label to show without collision detection
+    // being run again (the label's glyphs haven't even been added to the symbol bucket)
     if (v_placement_zoom >= u_maxzoom) {
         gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0) * alpha * 0.2;
     }