[core] improve legibility of labels that follow lines

port https://github.com/mapbox/mapbox-gl-js/pull/4781

This improves legibility of labels that follow lines in pitched views.
The previous approach used the limited information in the shader to
calculate put the glyph in approximatelyright place. The new approach
does this more accurately by doing it on the cpu where we have access to
the entire line geometry.

1 files changed, 31 insertions, 8 deletions

diff --git a/src/mbgl/programs/symbol_program.cpp b/src/mbgl/programs/symbol_program.cpp
index bd43237b8f..8790adcc63 100644
--- a/src/mbgl/programs/symbol_program.cpp
+++ b/src/mbgl/programs/symbol_program.cpp
@@ -2,6 +2,7 @@
 #include <mbgl/renderer/render_tile.hpp>
 #include <mbgl/map/transform_state.hpp>
 #include <mbgl/style/layers/symbol_layer_impl.hpp>
+#include <mbgl/layout/symbol_projection.hpp>
 #include <mbgl/tile/tile.hpp>
 #include <mbgl/util/enum.hpp>
 #include <mbgl/math/clamp.hpp>
@@ -10,7 +11,7 @@ namespace mbgl {
 
 using namespace style;
 
-static_assert(sizeof(SymbolLayoutVertex) == 20, "expected SymbolLayoutVertex size");
+static_assert(sizeof(SymbolLayoutVertex) == 16, "expected SymbolLayoutVertex size");
 
 std::unique_ptr<SymbolSizeBinder> SymbolSizeBinder::create(const float tileZoom,
                                                     const style::DataDrivenPropertyValue<float>& sizeProperty,
@@ -33,6 +34,7 @@ Values makeValues(const bool isText,
                   const style::SymbolPropertyValues& values,
                   const Size& texsize,
                   const std::array<float, 2>& pixelsToGLUnits,
+                  const bool alongLine,
                   const RenderTile& tile,
                   const TransformState& state,
                   Args&&... args) {
@@ -46,21 +48,41 @@ Values makeValues(const bool isText,
             pixelsToGLUnits[1] * state.getCameraToCenterDistance()
         }};
     }
+
+    const float pixelsToTileUnits = tile.id.pixelsToTileUnits(1.0, state.getZoom());
+    const bool pitchWithMap = values.pitchAlignment == style::AlignmentType::Map;
+    const bool rotateWithMap = values.rotationAlignment == style::AlignmentType::Map;
+
+    mat4 labelPlaneMatrix;
+    if (alongLine) {
+        // For labels that follow lines the first part of the projection is handled on the cpu.
+        // Pass an identity matrix because no transformation needs to be done in the vertex shader.
+        matrix::identity(labelPlaneMatrix);
+    } else {
+        labelPlaneMatrix = getLabelPlaneMatrix(tile.matrix, pitchWithMap, rotateWithMap, state, pixelsToTileUnits);
+    }
+
+    mat4 glCoordMatrix = getGlCoordMatrix(tile.matrix, pitchWithMap, rotateWithMap, state, pixelsToTileUnits);
         
     return Values {
         uniforms::u_matrix::Value{ tile.translatedMatrix(values.translate,
                                    values.translateAnchor,
                                    state) },
+        uniforms::u_label_plane_matrix::Value{labelPlaneMatrix},
+        uniforms::u_gl_coord_matrix::Value{ tile.translateVtxMatrix(glCoordMatrix,
+                                            values.translate,
+                                            values.translateAnchor,
+                                            state,
+                                            true) },
         uniforms::u_extrude_scale::Value{ extrudeScale },
         uniforms::u_texsize::Value{ texsize },
-        uniforms::u_zoom::Value{ float(state.getZoom()) },
-        uniforms::u_rotate_with_map::Value{ values.rotationAlignment == AlignmentType::Map },
         uniforms::u_texture::Value{ 0 },
         uniforms::u_fadetexture::Value{ 1 },
         uniforms::u_is_text::Value{ isText },
         uniforms::u_collision_y_stretch::Value{ tile.tile.yStretch() },
         uniforms::u_camera_to_center_distance::Value{ state.getCameraToCenterDistance() },
         uniforms::u_pitch::Value{ state.getPitch() },
+        uniforms::u_pitch_with_map::Value{ pitchWithMap },
         uniforms::u_max_camera_distance::Value{ values.maxCameraDistance },
         std::forward<Args>(args)...
     };
@@ -71,6 +93,7 @@ SymbolIconProgram::uniformValues(const bool isText,
                                  const style::SymbolPropertyValues& values,
                                  const Size& texsize,
                                  const std::array<float, 2>& pixelsToGLUnits,
+                                 const bool alongLine,
                                  const RenderTile& tile,
                                  const TransformState& state)
 {
@@ -79,6 +102,7 @@ SymbolIconProgram::uniformValues(const bool isText,
         values,
         texsize,
         pixelsToGLUnits,
+        alongLine,
         tile,
         state
     );
@@ -90,25 +114,24 @@ typename SymbolSDFProgram<PaintProperties>::UniformValues SymbolSDFProgram<Paint
       const style::SymbolPropertyValues& values,
       const Size& texsize,
       const std::array<float, 2>& pixelsToGLUnits,
+      const bool alongLine,
       const RenderTile& tile,
       const TransformState& state,
       const SymbolSDFPart part)
 {
     const float gammaScale = (values.pitchAlignment == AlignmentType::Map
-                              ? std::cos(state.getPitch())
-                              : 1.0) * state.getCameraToCenterDistance();
+                              ? std::cos(state.getPitch()) * state.getCameraToCenterDistance()
+                              : 1.0);
     
     return makeValues<SymbolSDFProgram<PaintProperties>::UniformValues>(
         isText,
         values,
         texsize,
         pixelsToGLUnits,
+        alongLine,
         tile,
         state,
         uniforms::u_gamma_scale::Value{ gammaScale },
-        uniforms::u_bearing::Value{ -1.0f * state.getAngle() },
-        uniforms::u_aspect_ratio::Value{ (state.getSize().width * 1.0f) / (state.getSize().height * 1.0f) },
-        uniforms::u_pitch_with_map::Value{ values.pitchAlignment == AlignmentType::Map },
         uniforms::u_is_halo::Value{ part == SymbolSDFPart::Halo }
     );
 }