[rolling] image_publisher: add field of view parameter (#985)

Currently, the default value for focal length when no camera info is
provided defaults to `1.0` rendering whole approximate intrinsics and
projection matrices useless. Based on [this
article](https://learnopencv.com/approximate-focal-length-for-webcams-and-cell-phone-cameras/),
I propose a better approximation of the focal length based on the field
of view of the camera.

For most of the use cases, users will either know the field of view of
the camera the used, or they already calibrated it ahead of time.

If there is some documentation to fill. please let me know.

This PR should be straightforward to port it to `Humble`, `Iron` and
`Jazzy`.

---------

Co-authored-by: Alejandro Hernández Cordero <[email protected]>

image_publisher/doc/components.rst

@@ -14,6 +14,7 @@ Published Topics
 Parameters
 ^^^^^^^^^^
  * **filename** (string, default: ""): Name of image file to be published.
+ * **field_of_view** (double, default: 0): Camera field of view (deg) used to calculate focal length for camera info topic.
  * **flip_horizontal** (bool, default: false): Flip output image horizontally.
  * **flip_vertical** (bool, default: false): Flip output image vertically.
  * **frame_id** (string, default: "camera") Frame id inserted in published

image_publisher/include/image_publisher/image_publisher.hpp

@@ -65,6 +65,7 @@ class ImagePublisher : public rclcpp::Node
   rclcpp::Node::OnSetParametersCallbackHandle::SharedPtr on_set_parameters_callback_handle_;
 
   std::string filename_;
+  double field_of_view_;
   bool flip_horizontal_;
   bool flip_vertical_;
   bool image_flipped_;

image_publisher/src/image_publisher.cpp

@@ -31,7 +31,9 @@
 // ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 // POSSIBILITY OF SUCH DAMAGE.
 
+#include <cmath>
 #include <chrono>
+#include <limits>
 #include <string>
 #include <thread>
 #include <vector>
@@ -60,6 +62,7 @@ ImagePublisher::ImagePublisher(
   std::string topic_name = node_base->resolve_topic_or_service_name("image_raw", false);
   pub_ = image_transport::create_camera_publisher(this, topic_name);
 
+  field_of_view_ = this->declare_parameter("field_of_view", static_cast<double>(0));
   flip_horizontal_ = this->declare_parameter("flip_horizontal", false);
   flip_vertical_ = this->declare_parameter("flip_vertical", false);
   frame_id_ = this->declare_parameter("frame_id", std::string("camera"));
@@ -79,6 +82,11 @@ ImagePublisher::ImagePublisher(
           RCLCPP_INFO(get_logger(), "Reset filename as '%s'", filename_.c_str());
           ImagePublisher::onInit();
           return result;
+        } else if (parameter.get_name() == "field_of_view") {
+          field_of_view_ = parameter.as_double();
+          RCLCPP_INFO(get_logger(), "Reset field_of_view as '%f'", field_of_view_);
+          ImagePublisher::onInit();
+          return result;
         } else if (parameter.get_name() == "flip_horizontal") {
           flip_horizontal_ = parameter.as_bool();
           RCLCPP_INFO(get_logger(), "Reset flip_horizontal as '%d'", flip_horizontal_);
@@ -221,10 +229,16 @@ void ImagePublisher::onInit()
   camera_info_.height = image_.rows;
   camera_info_.distortion_model = "plumb_bob";
   camera_info_.d = {0, 0, 0, 0, 0};
-  camera_info_.k = {1, 0, static_cast<float>(camera_info_.width / 2), 0, 1,
+
+  double f_approx = 1.0;  // FOV equal to 0 disables the approximation
+  if (std::abs(field_of_view_) > std::numeric_limits<double>::epsilon()) {
+    // Based on https://learnopencv.com/approximate-focal-length-for-webcams-and-cell-phone-cameras/
+    f_approx = (camera_info_.width / 2) / std::tan((field_of_view_ * M_PI / 180) / 2);
+  }
+  camera_info_.k = {f_approx, 0, static_cast<float>(camera_info_.width / 2), 0, f_approx,
     static_cast<float>(camera_info_.height / 2), 0, 0, 1};
   camera_info_.r = {1, 0, 0, 0, 1, 0, 0, 0, 1};
-  camera_info_.p = {1, 0, static_cast<float>(camera_info_.width / 2), 0, 0, 1,
+  camera_info_.p = {f_approx, 0, static_cast<float>(camera_info_.width / 2), 0, 0, f_approx,
     static_cast<float>(camera_info_.height / 2), 0, 0, 0, 1, 0};
 
   timer_ = this->create_wall_timer(