[examples] Add example programs for AprilTags detection (#4932)

Co-authored-by: Peter Johnson <johnson.peter@gmail.com>

wpilibcExamples/src/main/cpp/examples/AprilTagsVision/cpp/Robot.cpp

@@ -0,0 +1,165 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <cstdio>
+#include <span>
+#include <sstream>
+#include <string>
+#include <thread>
+
+#include <cameraserver/CameraServer.h>
+#include <fmt/format.h>
+#include <frc/TimedRobot.h>
+#include <frc/apriltag/AprilTagDetection.h>
+#include <frc/apriltag/AprilTagDetector.h>
+#include <frc/apriltag/AprilTagPoseEstimator.h>
+#include <frc/geometry/Transform3d.h>
+#include <networktables/IntegerArrayTopic.h>
+#include <networktables/NetworkTableInstance.h>
+#include <opencv2/core/core.hpp>
+#include <opencv2/core/types.hpp>
+#include <opencv2/imgproc/imgproc.hpp>
+#include <units/angle.h>
+#include <units/length.h>
+
+/**
+ * This is a demo program showing the detection of AprilTags.
+ * The image is acquired from the USB camera, then any detected AprilTags
+ * are marked up on the image and sent to the dashboard.
+ *
+ * Be aware that the performance on this is much worse than a coprocessor
+ * solution!
+ */
+class Robot : public frc::TimedRobot {
+#if defined(__linux__) || defined(_WIN32)
+
+ private:
+  static void VisionThread() {
+    frc::AprilTagDetector detector;
+    // look for tag16h5, don't correct any error bits
+    detector.AddFamily("tag16h5", 0);
+
+    // Set up Pose Estimator - parameters are for a Microsoft Lifecam HD-3000
+    // (https://www.chiefdelphi.com/t/wpilib-apriltagdetector-sample-code/421411/21)
+    frc::AprilTagPoseEstimator::Config poseEstConfig = {
+        .tagSize = units::length::inch_t(6.0),
+        .fx = 699.3778103158814,
+        .fy = 677.7161226393544,
+        .cx = 345.6059345433618,
+        .cy = 207.12741326228522};
+    frc::AprilTagPoseEstimator estimator(poseEstConfig);
+
+    // Get the USB camera from CameraServer
+    cs::UsbCamera camera = frc::CameraServer::StartAutomaticCapture();
+    // Set the resolution
+    camera.SetResolution(640, 480);
+
+    // Get a CvSink. This will capture Mats from the Camera
+    cs::CvSink cvSink = frc::CameraServer::GetVideo();
+    // Setup a CvSource. This will send images back to the Dashboard
+    cs::CvSource outputStream =
+        frc::CameraServer::PutVideo("Detected", 640, 480);
+
+    // Mats are very memory expensive. Lets reuse this Mat.
+    cv::Mat mat;
+    cv::Mat grayMat;
+
+    // Instantiate once
+    std::vector<int64_t> tags;
+    cv::Scalar outlineColor{0, 255, 0};
+    cv::Scalar crossColor{0, 0, 255};
+
+    // We'll output to NT
+    auto tagsTable =
+        nt::NetworkTableInstance::GetDefault().GetTable("apriltags");
+    auto pubTags = tagsTable->GetIntegerArrayTopic("tags").Publish();
+
+    while (true) {
+      // Tell the CvSink to grab a frame from the camera and
+      // put it in the source mat.  If there is an error notify the
+      // output.
+      if (cvSink.GrabFrame(mat) == 0) {
+        // Send the output the error.
+        outputStream.NotifyError(cvSink.GetError());
+        // skip the rest of the current iteration
+        continue;
+      }
+
+      cv::cvtColor(mat, grayMat, cv::COLOR_BGR2GRAY);
+
+      cv::Size g_size = grayMat.size();
+      frc::AprilTagDetector::Results detections =
+          detector.Detect(g_size.width, g_size.height, grayMat.data);
+
+      // have not seen any tags yet
+      tags.clear();
+
+      for (const frc::AprilTagDetection* detection : detections) {
+        // remember we saw this tag
+        tags.push_back(detection->GetId());
+
+        // draw lines around the tag
+        for (int i = 0; i <= 3; i++) {
+          int j = (i + 1) % 4;
+          const frc::AprilTagDetection::Point pti = detection->GetCorner(i);
+          const frc::AprilTagDetection::Point ptj = detection->GetCorner(j);
+          line(mat, cv::Point(pti.x, pti.y), cv::Point(ptj.x, ptj.y),
+               outlineColor, 2);
+        }
+
+        // mark the center of the tag
+        const frc::AprilTagDetection::Point c = detection->GetCenter();
+        int ll = 10;
+        line(mat, cv::Point(c.x - ll, c.y), cv::Point(c.x + ll, c.y),
+             crossColor, 2);
+        line(mat, cv::Point(c.x, c.y - ll), cv::Point(c.x, c.y + ll),
+             crossColor, 2);
+
+        // identify the tag
+        putText(mat, std::to_string(detection->GetId()),
+                cv::Point(c.x + ll, c.y), cv::FONT_HERSHEY_SIMPLEX, 1,
+                crossColor, 3);
+
+        // determine pose
+        frc::Transform3d pose = estimator.Estimate(*detection);
+
+        // put pose into NT
+        frc::Rotation3d rotation = pose.Rotation();
+        tagsTable->GetEntry(fmt::format("pose_{}", detection->GetId()))
+            .SetDoubleArray(
+                {{ pose.X().value(),
+                   pose.Y().value(),
+                   pose.Z().value(),
+                   rotation.X().value(),
+                   rotation.Y().value(),
+                   rotation.Z().value() }});
+      }
+
+      // put list of tags onto NT
+      pubTags.Set(tags);
+
+      // Give the output stream a new image to display
+      outputStream.PutFrame(mat);
+    }
+  }
+#endif
+
+  void RobotInit() override {
+    // We need to run our vision program in a separate thread. If not, our robot
+    // program will not run.
+#if defined(__linux__) || defined(_WIN32)
+    std::thread visionThread(VisionThread);
+    visionThread.detach();
+#else
+    std::fputs("Vision only available on Linux or Windows.\n", stderr);
+    std::fflush(stderr);
+#endif
+  }
+};
+
+#ifndef RUNNING_FRC_TESTS
+int main() {
+  return frc::StartRobot<Robot>();
+}
+#endif

wpilibcExamples/src/main/cpp/examples/I2CCommunication/cpp/Robot.cpp

@@ -5,7 +5,6 @@
 #include "Robot.h"
 
 #include <fmt/format.h>
-
 #include <frc/DriverStation.h>
 #include <frc/Timer.h>
 

wpilibcExamples/src/main/cpp/examples/examples.json

@@ -259,6 +259,17 @@
     "gradlebase": "cpp",
     "commandversion": 2
   },
+  {
+    "name": "AprilTags Vision",
+    "description": "on-roboRIO detection of AprilTags using an attached USB camera.",
+    "tags": [
+      "Vision",
+      "AprilTags"
+    ],
+    "foldername": "AprilTagsVision",
+    "gradlebase": "cpp",
+    "commandversion": 2
+  },
   {
     "name": "I2C Communication",
     "description": "An example program that communicates with external devices (such as an Arduino) using the roboRIO's I2C port",