[core] Added Viewport mode

Satisfies embedding platforms that requires the viewport coordinate
systems to be set according to its standards e.g. viewport mirrored
vertically.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/mbgl/map/transform_state.cpp b/src/mbgl/map/transform_state.cpp
index 2813d6334e..0b8d831e54 100644
--- a/src/mbgl/map/transform_state.cpp
+++ b/src/mbgl/map/transform_state.cpp
@@ -7,8 +7,9 @@
 
 namespace mbgl {
 
-TransformState::TransformState(ConstrainMode constrainMode_)
+TransformState::TransformState(ConstrainMode constrainMode_, ViewportMode viewportMode_)
     : constrainMode(constrainMode_)
+    , viewportMode(viewportMode_)
 {
 }
 
@@ -39,7 +40,8 @@ void TransformState::getProjMatrix(mat4& projMatrix) const {
 
     // After the rotateX, z values are in pixel units. Convert them to
     // altitude unites. 1 altitude unit = the screen height.
-    matrix::scale(projMatrix, projMatrix, 1, -1, 1.0f / (rotatedNorth() ? getWidth() : getHeight()));
+    const bool flippedY = viewportMode == ViewportMode::FlippedY;
+    matrix::scale(projMatrix, projMatrix, 1, flippedY ? 1 : -1, 1.0f / (rotatedNorth() ? getWidth() : getHeight()));
 
     using NO = NorthOrientation;
     switch (getNorthOrientation()) {
@@ -89,6 +91,12 @@ ConstrainMode TransformState::getConstrainMode() const {
     return constrainMode;
 }
 
+#pragma mark - ViewportMode
+
+ViewportMode TransformState::getViewportMode() const {
+    return viewportMode;
+}
+
 #pragma mark - Position
 
 LatLng TransformState::getLatLng(LatLng::WrapMode wrapMode) const {