WIP on curved labels: calculate their zoom adjustment based on the

 label anchor instead of the glyph anchor, so all the glyphs can behave
 consistently.

src/data/bucket/symbol_bucket.js

@@ -32,6 +32,7 @@ const elementArrayType = createElementArrayType();
 
 const layoutAttributes = [
     {name: 'a_pos_offset',  components: 4, type: 'Int16'},
+    {name: 'a_label_pos',   components: 2, type: 'Int16'},
     {name: 'a_data',        components: 4, type: 'Uint16'}
 ];
 
@@ -68,14 +69,18 @@ const symbolInterfaces = {
     }
 };
 
-function addVertex(array, x, y, ox, oy, tx, ty, sizeVertex, minzoom, maxzoom, labelminzoom, labelangle) {
+function addVertex(array, x, y, ox, oy, labelX, labelY, tx, ty, sizeVertex, minzoom, maxzoom, labelminzoom, labelangle) {
     array.emplaceBack(
         // a_pos_offset
         x,
         y,
         Math.round(ox * 64),
         Math.round(oy * 64),
 
+        // a_label_pos
+        labelX,
+        labelY,
+
         // a_data
         tx / 4, // x coordinate of symbol on glyph atlas texture
         ty / 4, // y coordinate of symbol on glyph atlas texture
@@ -608,7 +613,8 @@ class SymbolBucket {
                         textAlongLine,
                         collisionTile.angle,
                         symbolInstance.featureProperties,
-                        symbolInstance.writingModes);
+                        symbolInstance.writingModes,
+                        symbolInstance.anchor);
                 }
             }
 
@@ -629,7 +635,9 @@ class SymbolBucket {
                         layout['icon-keep-upright'],
                         iconAlongLine,
                         collisionTile.angle,
-                        symbolInstance.featureProperties
+                        symbolInstance.featureProperties,
+                        null,
+                        symbolInstance.anchor
                     );
                 }
             }
@@ -639,7 +647,7 @@ class SymbolBucket {
         if (showCollisionBoxes) this.addToDebugBuffers(collisionTile);
     }
 
-    addSymbols(arrays, quads, scale, sizeVertex, keepUpright, alongLine, placementAngle, featureProperties, writingModes) {
+    addSymbols(arrays, quads, scale, sizeVertex, keepUpright, alongLine, placementAngle, featureProperties, writingModes, labelAnchor) {
         const elementArray = arrays.elementArray;
         const layoutVertexArray = arrays.layoutVertexArray;
 
@@ -676,10 +684,10 @@ class SymbolBucket {
             const segment = arrays.prepareSegment(4);
             const index = segment.vertexLength;
 
-            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, tl.x, tl.y, tex.x, tex.y, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
-            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, tr.x, tr.y, tex.x + tex.w, tex.y, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
-            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, bl.x, bl.y, tex.x, tex.y + tex.h, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
-            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, br.x, br.y, tex.x + tex.w, tex.y + tex.h, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
+            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, tl.x, tl.y, labelAnchor.x, labelAnchor.y, tex.x, tex.y, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
+            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, tr.x, tr.y, labelAnchor.x, labelAnchor.y, tex.x + tex.w, tex.y, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
+            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, bl.x, bl.y, labelAnchor.x, labelAnchor.y, tex.x, tex.y + tex.h, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
+            addVertex(layoutVertexArray, anchorPoint.x, anchorPoint.y, br.x, br.y, labelAnchor.x, labelAnchor.y, tex.x + tex.w, tex.y + tex.h, sizeVertex, minZoom, maxZoom, placementZoom, glyphAngle);
 
             elementArray.emplaceBack(index, index + 1, index + 2);
             elementArray.emplaceBack(index + 1, index + 2, index + 3);

src/shaders/symbol_sdf.vertex.glsl

@@ -1,6 +1,7 @@
 const float PI = 3.141592653589793;
 
 attribute vec4 a_pos_offset;
+attribute vec2 a_label_pos;
 attribute vec4 a_data;
 
 // contents of a_size vary based on the type of property value
@@ -99,8 +100,10 @@ void main() {
 
     float fontScale = u_is_text ? v_size / 24.0 : v_size;
 
-    highp float perspective_ratio = 1.0;
-    highp float camera_to_anchor_distance;
+    vec4 projectedPoint = u_matrix * vec4(a_label_pos, 0, 1);
+    highp float camera_to_anchor_distance = projectedPoint.w;
+    highp float perspective_ratio = 1.0 + (1.0 - u_pitch_scale)*((camera_to_anchor_distance / u_camera_to_center_distance) - 1.0);;
+
     //mediump float z = clipUnusedGlyphAngles(v_size, layoutSize, a_minzoom, a_maxzoom);
     // pitch-alignment: map
     // rotation-alignment: map | viewport
@@ -110,13 +113,9 @@ void main() {
         lowp float acos = cos(angle);
         mat2 RotationMatrix = mat2(acos, asin, -1.0 * asin, acos);
         vec2 offset = RotationMatrix * a_offset;
-        vec2 extrude = fontScale * u_extrude_scale * (offset / 64.0);
-        vec4 projectedPoint = u_matrix * vec4(a_pos, 0, 1);
-        camera_to_anchor_distance = projectedPoint.w;
+        vec2 extrude = fontScale * u_extrude_scale * perspective_ratio * (offset / 64.0);
 
-        perspective_ratio += (1.0 - u_pitch_scale)*((camera_to_anchor_distance / u_camera_to_center_distance) - 1.0);
-        extrude *= perspective_ratio;
-        gl_Position+= u_matrix * vec4(a_pos + extrude, 0, 1);
+        gl_Position = u_matrix * vec4(a_pos + extrude, 0, 1);
         gl_Position.z += clipUnusedGlyphAngles(v_size*perspective_ratio, layoutSize, a_minzoom, a_maxzoom) * gl_Position.w;
     // pitch-alignment: viewport
     // rotation-alignment: map
@@ -140,25 +139,16 @@ void main() {
 
         vec2 offset = RotationMatrix * (vec2(foreshortening, 1.0) * a_offset);
         vec2 extrude = fontScale * u_extrude_scale * (offset / 64.0);
-        gl_Position = u_matrix * vec4(a_pos, 0, 1);
-        camera_to_anchor_distance = gl_Position.w;
 
-        perspective_ratio += (1.0 - u_pitch_scale)*((camera_to_anchor_distance / u_camera_to_center_distance) - 1.0);
-        extrude *= perspective_ratio;
-        gl_Position += vec4(extrude, 0, 0);
+        //extrude *= perspective_ratio;
 
+        gl_Position = u_matrix * vec4(a_pos, 0, 1) + vec4(extrude, 0, 0);
         gl_Position.z += clipUnusedGlyphAngles(v_size*perspective_ratio, layoutSize, a_minzoom, a_maxzoom) * gl_Position.w;
     // pitch-alignment: viewport
     // rotation-alignment: viewport
     } else {
-        gl_Position = u_matrix * vec4(a_pos, 0, 1);
-        camera_to_anchor_distance = gl_Position.w;
-
-        vec2 extrude = fontScale * u_extrude_scale * (a_offset / 64.0);
-        perspective_ratio += (1.0 - u_pitch_scale)*((camera_to_anchor_distance / u_camera_to_center_distance) - 1.0);
-        extrude *= perspective_ratio;
-
-        gl_Position += vec4(extrude, 0, 0);
+        vec2 extrude = fontScale * u_extrude_scale * perspective_ratio * (a_offset / 64.0);
+        gl_Position = u_matrix * vec4(a_pos, 0, 1) + vec4(extrude, 0, 0);
     }
 
     v_gamma_scale = gl_Position.w / perspective_ratio;