Add Camera::LookAt

This is identical to the POV-Ray look_at keyword for cameras. In fact, I stole the code almost directly from them… It's super handy for panning and tilting the camera towards a particular point, and saves me having to do dubiously accurate direction vector calculations by hand.

I still need to figure out what exactly this math is doing.

src/camera.cc

@@ -93,6 +93,31 @@ Camera::SetUp(const Vector3& up)
     mUp = up;
 }
 
+
+void
+Camera::LookAt(const Vector3& pt)
+{
+    const Double directionLength = mDirection.length();
+    const Double rightLength = mRight.length();
+    const Double upLength = mUp.length();
+    /* TODO: What does this actually do? */
+    const Double handedness = mUp.cross(mDirection).dot(mRight);
+
+    mDirection = (pt - mOrigin).normalize();
+    /* TODO: Check for zero length direction vector. */
+
+    mRight = Vector3::Y.cross(mDirection).normalize();
+    mUp = mDirection.cross(mRight);
+    mDirection *= directionLength;
+
+    if (handedness > 0.0) {
+        mRight *= rightLength;
+    } else {
+        mRight *= -rightLength;
+    }
+    mUp *= upLength;
+}
+
 #pragma mark - Perspective Camera
 
 PerspectiveCamera::PerspectiveCamera()

src/camera.h

@@ -35,6 +35,8 @@ struct Camera
     const Vector3& GetUp() const;
     void SetUp(const Vector3& up);
 
+    void LookAt(const Vector3& pt);
+
     virtual Ray compute_primary_ray(const int x, const int width,
                                     const int y, const int height) const = 0;
 

src/yaml/cameraParser.cc

@@ -32,6 +32,7 @@ CameraParser::HandleKeyEvent(const std::string& key)
 {
     static const std::map<std::string, Section> sSections = {
         {"direction", DirectionSection},
+        {"lookAt", LookAtSection},
         {"origin", OriginSection},
         {"right", RightSection},
         {"type", TypeSection},
@@ -54,6 +55,9 @@ CameraParser::HandleValueEvent(yaml_event_t& event)
         case DirectionSection:
             HandleDirectionEvent(event);
             break;
+        case LookAtSection:
+            HandleLookAtEvent(event);
+            break;
         case OriginSection:
             HandleOriginEvent(event);
             break;
@@ -92,6 +96,25 @@ CameraParser::HandleDirectionEvent(yaml_event_t& event)
 }
 
 
+void
+CameraParser::HandleLookAtEvent(yaml_event_t& event)
+{
+    if (event.type != YAML_SEQUENCE_START_EVENT) {
+        /* TODO: Clean this up. */
+        assert(event.type != YAML_SEQUENCE_START_EVENT);
+        return;
+    }
+
+    auto onDone = [this](Vector3 lookAt) {
+        mCamera->LookAt(lookAt);
+        mSection = NoSection;
+        SetShouldExpectKey(true);
+    };
+
+    GetParsers().push(new Vector3Parser(GetScene(), GetParsers(), onDone));
+}
+
+
 void
 CameraParser::HandleOriginEvent(yaml_event_t& event)
 {

src/yaml/cameraParser.hh

@@ -28,6 +28,7 @@ private:
     enum Section {
         NoSection,
         DirectionSection,
+        LookAtSection,
         OriginSection,
         RightSection,
         TypeSection,
@@ -40,6 +41,7 @@ private:
     };
 
     void HandleDirectionEvent(yaml_event_t& event);
+    void HandleLookAtEvent(yaml_event_t& event);
     void HandleOriginEvent(yaml_event_t& event);
     void HandleRightEvent(yaml_event_t& event);
     void HandleTypeEvent(yaml_event_t& event);