[core] Implement circle-pitch-alignment property

Closes issue #9349.

1 files changed, 23 insertions, 5 deletions

diff --git a/src/mbgl/shaders/circle.cpp b/src/mbgl/shaders/circle.cpp
index 2e0c76122c..953e750776 100644
--- a/src/mbgl/shaders/circle.cpp
+++ b/src/mbgl/shaders/circle.cpp
@@ -9,7 +9,9 @@ const char* circle::name = "circle";
 const char* circle::vertexSource = R"MBGL_SHADER(
 uniform mat4 u_matrix;
 uniform bool u_scale_with_map;
+uniform bool u_pitch_with_map;
 uniform vec2 u_extrude_scale;
+uniform highp float u_camera_to_center_distance;
 
 attribute vec2 a_pos;
 
@@ -121,12 +123,28 @@ void main(void) {
 
     // multiply a_pos by 0.5, since we had it * 2 in order to sneak
     // in extrusion data
-    gl_Position = u_matrix * vec4(floor(a_pos * 0.5), 0, 1);
-
-    if (u_scale_with_map) {
-        gl_Position.xy += extrude * (radius + stroke_width) * u_extrude_scale;
+    vec2 circle_center = floor(a_pos * 0.5);
+    if (u_pitch_with_map) {
+        vec2 corner_position = circle_center;
+        if (u_scale_with_map) {
+            corner_position += extrude * (radius + stroke_width) * u_extrude_scale;
+        } else {
+            // Pitching the circle with the map effectively scales it with the map
+            // To counteract the effect for pitch-scale: viewport, we rescale the
+            // whole circle based on the pitch scaling effect at its central point
+            vec4 projected_center = u_matrix * vec4(circle_center, 0, 1);
+            corner_position += extrude * (radius + stroke_width) * u_extrude_scale * (projected_center.w / u_camera_to_center_distance);
+        }
+
+        gl_Position = u_matrix * vec4(corner_position, 0, 1);
     } else {
-        gl_Position.xy += extrude * (radius + stroke_width) * u_extrude_scale * gl_Position.w;
+        gl_Position = u_matrix * vec4(circle_center, 0, 1);
+
+        if (u_scale_with_map) {
+            gl_Position.xy += extrude * (radius + stroke_width) * u_extrude_scale * u_camera_to_center_distance;
+        } else {
+            gl_Position.xy += extrude * (radius + stroke_width) * u_extrude_scale * gl_Position.w;
+        }
     }
 
     // This is a minimum blur distance that serves as a faux-antialiasing for