Use terse Eigen aliases for small Vectors

Summary:
Execute T250116725 (Use terse Eigen aliases for small Vectors and square-Matrices) on arvr/projects/ariane/.

# Why:
Makes code terser.

# Context:
It seems `Eigen::Vector3<Scalar>` is a thing (also for sizes 2,4 and small square matrices).
Terser and closer to `Eigen::Vector3{f/d}` we use when not templated.

Reviewed By: aforster

Differential Revision: D89955586

fbshipit-source-id: e1d5a52d6bd9c393264d5d3f14e4e5b13eea9d88

Author: Jesus Briales

core/calibration/camera_projections/CameraProjection.cpp

@@ -92,9 +92,9 @@ int CameraProjectionTemplated<Scalar>::numDistortionParameters() const {
 }
 
 template <typename Scalar>
-Eigen::Vector<Scalar, 2> CameraProjectionTemplated<Scalar>::getFocalLengths() const {
+Eigen::Vector2<Scalar> CameraProjectionTemplated<Scalar>::getFocalLengths() const {
   return std::visit(
-      [this](auto&& projection) -> Eigen::Vector<Scalar, 2> {
+      [this](auto&& projection) -> Eigen::Vector2<Scalar> {
         using T = std::decay_t<decltype(projection)>;
         int focalXIdx = T::kFocalXIdx;
         int focalYIdx = T::kFocalYIdx;
@@ -104,9 +104,9 @@ Eigen::Vector<Scalar, 2> CameraProjectionTemplated<Scalar>::getFocalLengths() co
 }
 
 template <typename Scalar>
-Eigen::Vector<Scalar, 2> CameraProjectionTemplated<Scalar>::getPrincipalPoint() const {
+Eigen::Vector2<Scalar> CameraProjectionTemplated<Scalar>::getPrincipalPoint() const {
   return std::visit(
-      [this](auto&& projection) -> Eigen::Vector<Scalar, 2> {
+      [this](auto&& projection) -> Eigen::Vector2<Scalar> {
         using T = std::decay_t<decltype(projection)>;
         int principalPointColIdx = T::kPrincipalPointColIdx;
         int principalPointRowIdx = T::kPrincipalPointRowIdx;
@@ -116,8 +116,8 @@ Eigen::Vector<Scalar, 2> CameraProjectionTemplated<Scalar>::getPrincipalPoint()
 }
 
 template <typename Scalar>
-Eigen::Vector<Scalar, 2> CameraProjectionTemplated<Scalar>::project(
-    const Eigen::Vector<Scalar, 3>& pointInCamera,
+Eigen::Vector2<Scalar> CameraProjectionTemplated<Scalar>::project(
+    const Eigen::Vector3<Scalar>& pointInCamera,
     Eigen::Ref<Eigen::Matrix<Scalar, 2, 3>> jacobianWrtPoint,
     Eigen::Ref<Eigen::Matrix<Scalar, 2, Eigen::Dynamic>> jacobianWrtParams) const {
   return std::visit(
@@ -133,8 +133,8 @@ Eigen::Vector<Scalar, 2> CameraProjectionTemplated<Scalar>::project(
 }
 
 template <typename Scalar>
-Eigen::Vector<Scalar, 3> CameraProjectionTemplated<Scalar>::unproject(
-    const Eigen::Vector<Scalar, 2>& cameraPixel) const {
+Eigen::Vector3<Scalar> CameraProjectionTemplated<Scalar>::unproject(
+    const Eigen::Vector2<Scalar>& cameraPixel) const {
   return std::visit(
       [&](auto&& projection) {
         using T = std::decay_t<decltype(projection)>;

core/calibration/camera_projections/CameraProjection.h

@@ -102,25 +102,25 @@ struct CameraProjectionTemplated {
    *
    * @return The 2D pixel coordinates of the projected point in the image space.
    */
-  Eigen::Vector<Scalar, 2> project(
-      const Eigen::Vector<Scalar, 3>& pointInCamera,
+  Eigen::Vector2<Scalar> project(
+      const Eigen::Vector3<Scalar>& pointInCamera,
       Eigen::Ref<Eigen::Matrix<Scalar, 2, 3>> jacobianWrtPoint = NullRef(),
       Eigen::Ref<Eigen::Matrix<Scalar, 2, Eigen::Dynamic>> jacobianWrtParams = NullRef()) const;
 
   /**
    * @brief unprojects a 2d pixel in the image space to a 3d world point in homogenous coordinate.
    * No checks performed in this process.
    */
-  Eigen::Vector<Scalar, 3> unproject(const Eigen::Vector<Scalar, 2>& cameraPixel) const;
+  Eigen::Vector3<Scalar> unproject(const Eigen::Vector2<Scalar>& cameraPixel) const;
 
   /**
    * @brief returns principal point location as {cx, cy}
    */
-  Eigen::Vector<Scalar, 2> getPrincipalPoint() const;
+  Eigen::Vector2<Scalar> getPrincipalPoint() const;
   /**
    * @brief returns focal lengths as {fx, fy}
    */
-  Eigen::Vector<Scalar, 2> getFocalLengths() const;
+  Eigen::Vector2<Scalar> getFocalLengths() const;
 
   /**
    * @brief scales the projection parameters as the image scales without the offset changing

core/image/Image.h

@@ -208,8 +208,8 @@ struct Zero<Eigen::Matrix<T, M, N, Opts>> {
 } // namespace
 
 template <typename Scalar, std::enable_if_t<std::is_floating_point<Scalar>::value, int> = 0>
-Eigen::Vector<Scalar, 2> imageCenter(size_t imageWidth, size_t imageHeight) {
-  return Eigen::Vector<Scalar, 2>{
+Eigen::Vector2<Scalar> imageCenter(size_t imageWidth, size_t imageHeight) {
+  return Eigen::Vector2<Scalar>{
       static_cast<Scalar>(imageWidth) / Scalar(2.0) - Scalar(0.5),
       static_cast<Scalar>(imageHeight) / Scalar(2.0) - Scalar(0.5)};
 }