[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.

3 files changed, 23 insertions, 5 deletions

diff --git a/src/mbgl/gl/context.cpp b/src/mbgl/gl/context.cpp
index 1b4d6fbcb7..3508b92a79 100644
--- a/src/mbgl/gl/context.cpp
+++ b/src/mbgl/gl/context.cpp
@@ -164,15 +164,20 @@ void Context::verifyProgramLinkage(ProgramID program_) {
     throw std::runtime_error("program failed to link");
 }
 
-UniqueBuffer Context::createVertexBuffer(const void* data, std::size_t size) {
+UniqueBuffer Context::createVertexBuffer(const void* data, std::size_t size, const BufferUsageType usage) {
     BufferID id = 0;
     MBGL_CHECK_ERROR(glGenBuffers(1, &id));
     UniqueBuffer result { std::move(id), { this } };
     vertexBuffer = result;
-    MBGL_CHECK_ERROR(glBufferData(GL_ARRAY_BUFFER, size, data, GL_STATIC_DRAW));
+    MBGL_CHECK_ERROR(glBufferData(GL_ARRAY_BUFFER, size, data, static_cast<GLenum>(usage)));
     return result;
 }
 
+void Context::updateVertexBuffer(UniqueBuffer& buffer, const void* data, std::size_t size) {
+    vertexBuffer = buffer;
+    MBGL_CHECK_ERROR(glBufferSubData(GL_ARRAY_BUFFER, 0, size, data));
+}
+
 UniqueBuffer Context::createIndexBuffer(const void* data, std::size_t size) {
     BufferID id = 0;
     MBGL_CHECK_ERROR(glGenBuffers(1, &id));
diff --git a/src/mbgl/gl/context.hpp b/src/mbgl/gl/context.hpp
index 4f5b4c797c..f1f0ac7f8a 100644
--- a/src/mbgl/gl/context.hpp
+++ b/src/mbgl/gl/context.hpp
@@ -65,13 +65,19 @@ public:
     optional<std::pair<BinaryProgramFormat, std::string>> getBinaryProgram(ProgramID) const;
 
     template <class Vertex, class DrawMode>
-    VertexBuffer<Vertex, DrawMode> createVertexBuffer(VertexVector<Vertex, DrawMode>&& v) {
+    VertexBuffer<Vertex, DrawMode> createVertexBuffer(VertexVector<Vertex, DrawMode>&& v, const BufferUsageType usage=BufferUsageType::StaticDraw) {
         return VertexBuffer<Vertex, DrawMode> {
             v.vertexSize(),
-            createVertexBuffer(v.data(), v.byteSize())
+            createVertexBuffer(v.data(), v.byteSize(), usage)
         };
     }
 
+    template <class Vertex, class DrawMode>
+    void updateVertexBuffer(VertexBuffer<Vertex, DrawMode>& buffer, VertexVector<Vertex, DrawMode>&& v) {
+        assert(v.vertexSize() == buffer.vertexCount);
+        updateVertexBuffer(buffer.buffer, v.data(), v.byteSize());
+    }
+
     template <class DrawMode>
     IndexBuffer<DrawMode> createIndexBuffer(IndexVector<DrawMode>&& v) {
         return IndexBuffer<DrawMode> {
@@ -239,7 +245,8 @@ private:
     State<value::PointSize> pointSize;
 #endif // MBGL_USE_GLES2
 
-    UniqueBuffer createVertexBuffer(const void* data, std::size_t size);
+    UniqueBuffer createVertexBuffer(const void* data, std::size_t size, const BufferUsageType usage);
+    void updateVertexBuffer(UniqueBuffer& buffer, const void* data, std::size_t size);
     UniqueBuffer createIndexBuffer(const void* data, std::size_t size);
     UniqueTexture createTexture(Size size, const void* data, TextureFormat, TextureUnit);
     void updateTexture(TextureID, Size size, const void* data, TextureFormat, TextureUnit);
diff --git a/src/mbgl/gl/types.hpp b/src/mbgl/gl/types.hpp
index 74ce67fba6..8997fcbf31 100644
--- a/src/mbgl/gl/types.hpp
+++ b/src/mbgl/gl/types.hpp
@@ -96,5 +96,11 @@ enum class UniformDataType : uint32_t {
     SamplerCube = 0x8B60,
 };
 
+enum class BufferUsageType : uint32_t {
+    StreamDraw = 0x88E0,
+    StaticDraw = 0x88E4,
+    DynamicDraw = 0x88E8,
+};
+
 } // namespace gl
 } // namespace mbgl