[core] Add `line-gradient` property

Porting of https://github.com/mapbox/mapbox-gl-js/pull/6303
See the link above for the description of the feature and
its limitations).

Based on patch from @lbud (Lauren Budorick).

3 files changed, 37 insertions, 0 deletions

diff --git a/src/mbgl/programs/line_program.cpp b/src/mbgl/programs/line_program.cpp
index faf57ef19b..0533a13c35 100644
--- a/src/mbgl/programs/line_program.cpp
+++ b/src/mbgl/programs/line_program.cpp
@@ -116,4 +116,18 @@ LinePatternProgram::uniformValues(const RenderLinePaintProperties::PossiblyEvalu
     );
 }
 
+LineGradientProgram::UniformValues
+LineGradientProgram::uniformValues(const RenderLinePaintProperties::PossiblyEvaluated& properties,
+                           const RenderTile& tile,
+                           const TransformState& state,
+                           const std::array<float, 2>& pixelsToGLUnits) {
+    return makeValues<LineGradientProgram::UniformValues>(
+        properties,
+        tile,
+        state,
+        pixelsToGLUnits,
+        uniforms::u_image::Value{ 0 }
+    );
+}
+
 } // namespace mbgl
diff --git a/src/mbgl/programs/line_program.hpp b/src/mbgl/programs/line_program.hpp
index da9964e623..632dbe92b9 100644
--- a/src/mbgl/programs/line_program.hpp
+++ b/src/mbgl/programs/line_program.hpp
@@ -4,6 +4,7 @@
 #include <mbgl/programs/attributes.hpp>
 #include <mbgl/programs/uniforms.hpp>
 #include <mbgl/shaders/line.hpp>
+#include <mbgl/shaders/line_gradient.hpp>
 #include <mbgl/shaders/line_pattern.hpp>
 #include <mbgl/shaders/line_sdf.hpp>
 #include <mbgl/util/geometry.hpp>
@@ -159,6 +160,26 @@ public:
                                        float atlasWidth);
 };
 
+class LineGradientProgram : public Program<
+    shaders::line_gradient,
+    gl::Triangle,
+    LineLayoutAttributes,
+    gl::Uniforms<
+        uniforms::u_matrix,
+        uniforms::u_ratio,
+        uniforms::u_gl_units_to_pixels,
+        uniforms::u_image>,
+    RenderLinePaintProperties>
+{
+public:
+    using Program::Program;
+
+    static UniformValues uniformValues(const RenderLinePaintProperties::PossiblyEvaluated&,
+                                       const RenderTile&,
+                                       const TransformState&,
+                                       const std::array<float, 2>& pixelsToGLUnits);
+};
+
 using LineLayoutVertex = LineProgram::LayoutVertex;
 using LineAttributes = LineProgram::Attributes;
 
diff --git a/src/mbgl/programs/programs.hpp b/src/mbgl/programs/programs.hpp
index b703323d9c..7895069112 100644
--- a/src/mbgl/programs/programs.hpp
+++ b/src/mbgl/programs/programs.hpp
@@ -37,6 +37,7 @@ public:
           hillshade(context, programParameters),
           hillshadePrepare(context, programParameters),
           line(context, programParameters),
+          lineGradient(context, programParameters),
           lineSDF(context, programParameters),
           linePattern(context, programParameters),
           raster(context, programParameters),
@@ -64,6 +65,7 @@ public:
     HillshadeProgram hillshade;
     HillshadePrepareProgram hillshadePrepare;
     ProgramMap<LineProgram> line;
+    ProgramMap<LineGradientProgram> lineGradient;
     ProgramMap<LineSDFProgram> lineSDF;
     ProgramMap<LinePatternProgram> linePattern;
     RasterProgram raster;