[core, ios] replace `altitude` with `fov`

ported from -js: eb6c6596c6a7a61363d30356674e0002153b1d19

`altitude` was a terribly-named variable that was used to indirectly
control the fov. This should eliminate some confusion.

`altitude` was equivalent to `cameraToCenterDistance / height`

3 files changed, 4 insertions, 4 deletions

diff --git a/platform/darwin/src/MGLGeometry.mm b/platform/darwin/src/MGLGeometry.mm
index 70f00afd2f..57922e30b9 100644
--- a/platform/darwin/src/MGLGeometry.mm
+++ b/platform/darwin/src/MGLGeometry.mm
@@ -5,8 +5,8 @@
 /** Vertical field of view, measured in degrees, for determining the altitude
     of the viewpoint.
     
-    TransformState::getProjMatrix() assumes a vertical field of view of
-    2 arctan ⅓ rad ≈ 36.9°, but MapKit uses a vertical field of view of 30°.
+    TransformState::getProjMatrix() has a variable vertical field of view that
+    defaults to 2 arctan ⅓ rad ≈ 36.9° but MapKit uses a vertical field of view of 30°.
     flyTo() assumes a field of view of 2 arctan ½ rad. */
 const CLLocationDegrees MGLAngularFieldOfView = 30;
 
diff --git a/platform/darwin/src/MGLOpenGLStyleLayer.h b/platform/darwin/src/MGLOpenGLStyleLayer.h
index 7cb6b147c2..8fbbd91854 100644
--- a/platform/darwin/src/MGLOpenGLStyleLayer.h
+++ b/platform/darwin/src/MGLOpenGLStyleLayer.h
@@ -14,7 +14,7 @@ typedef struct MGLStyleLayerDrawingContext {
     double zoomLevel;
     CLLocationDirection direction;
     CGFloat pitch;
-    CGFloat perspectiveSkew;
+    CGFloat fieldOfView;
 } MGLStyleLayerDrawingContext;
 
 @interface MGLOpenGLStyleLayer : MGLStyleLayer
diff --git a/platform/darwin/src/MGLOpenGLStyleLayer.mm b/platform/darwin/src/MGLOpenGLStyleLayer.mm
index f109ea85b0..745eacbf4e 100644
--- a/platform/darwin/src/MGLOpenGLStyleLayer.mm
+++ b/platform/darwin/src/MGLOpenGLStyleLayer.mm
@@ -34,7 +34,7 @@ void MGLDrawCustomStyleLayer(void *context, const mbgl::style::CustomLayerRender
         .zoomLevel = params.zoom,
         .direction = mbgl::util::wrap(params.bearing, 0., 360.),
         .pitch = static_cast<CGFloat>(params.pitch),
-        .perspectiveSkew = static_cast<CGFloat>(params.altitude),
+        .fieldOfView = static_cast<CGFloat>(params.fieldOfView),
     };
     [layer drawInMapView:layer.mapView withContext:drawingContext];
 }