feat: add camera calibration example (#125)

examples/opencv/.gitignore

@@ -1,3 +1,4 @@
 # pixi environments
 .pixi
 haarcascade_frontalface_default.xml
+*.png

examples/opencv/README.md

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+# OpenCV example
+OpenCV is a powerfull tool to do computer vision and fully opensource.
+
+Here are some example on how to use it with `pixi`.
+
+## Simple face detection algorithm
+```shell
+pixi run start
+```
+![Face detection result](https://github.com/ruben-arts/pixi/assets/12893423/c0151496-caae-407c-9e90-0f71f3c19aa7)
+
+
+## Simple camera calibration script
+```shell
+pixi run calibrate
+```
+
+You'll need a checkerboard for this to work.
+Print this: [![chessboard](https://github.com/opencv/opencv/blob/4.x/doc/pattern.png?raw=true)](https://github.com/opencv/opencv/blob/4.x/doc/pattern.png)
+
+To make a picture for calibration press `SPACE`
+Do this approximately 10 times with the chessboard in view of the camera
+
+After that press `ESC` which will start the calibration.
+
+When the calibration is done the camera will be used again to find the distance to the checkerboard.
+
+![callibrated camera result](https://github.com/ruben-arts/pixi/assets/12893423/f42825d7-5010-4805-9f6b-b02075395413)
+

examples/opencv/calibrate.py

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+import cv2
+import numpy as np
+
+# Termination criteria
+criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 10, 0.001)
+
+# Prepare object points
+# The example chessboard is 9x6.
+CHESSBOARD_X = 6
+CHESSBOARD_Y = 9
+# The example chessboard printed on a A4 paper will approximaly be 24mm in width and height.
+SQUARE_SIZE_MM = 22.5
+
+objp = np.zeros((CHESSBOARD_X * CHESSBOARD_Y, 3), np.float32)
+objp[:,:2] = np.mgrid[0:CHESSBOARD_Y, 0:CHESSBOARD_X].T.reshape(-1, 2) * (SQUARE_SIZE_MM * 0.001)
+
+# Arrays to store object points and image points
+objpoints = []
+imgpoints = []
+
+# Initialize the camera
+cap = cv2.VideoCapture(0)
+
+# Set the frame width and height
+cap.set(cv2.CAP_PROP_FRAME_WIDTH, 1280)
+cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 720)
+
+img_counter = 0
+
+print("Press SPACE to capture an image for calibration.")
+print("Press ESC to calibrate the camera using the previously captured images.")
+
+while True:
+    ret, frame = cap.read()
+    if not ret:
+        break
+
+    frame_clean = cv2.copyTo(frame, None)
+    # Find the chess board corners
+    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
+    ret, corners = cv2.findChessboardCorners(gray, (CHESSBOARD_Y, CHESSBOARD_X), None)
+
+    if ret:
+        cv2.drawChessboardCorners(frame, (CHESSBOARD_Y, CHESSBOARD_X), corners, ret)
+
+    cv2.imshow("Test", frame)
+    k = cv2.waitKey(1)
+
+    if k%256 == 27:
+        # ESC pressed
+        print("Escape hit, closing...")
+        break
+    elif k%256 == 32:
+        # SPACE pressed
+        img_name = "opencv_frame_{}.png".format(img_counter)
+        cv2.imwrite(img_name, frame_clean)
+        print("{} written!".format(img_name))
+        img_counter += 1
+
+        # Convert to grayscale
+        gray = cv2.cvtColor(frame_clean, cv2.COLOR_BGR2GRAY)
+
+        # Find the chess board corners
+        ret, corners = cv2.findChessboardCorners(gray, (CHESSBOARD_Y, CHESSBOARD_X), None)
+
+        # If found, add object points, image points (after refining them)
+        if ret:
+            objpoints.append(objp)
+
+            corners2 = cv2.cornerSubPix(gray, corners, (11,11), (-1,-1), criteria)
+            imgpoints.append(corners2)
+        else:
+            print("No chessboard detected in this image: {img_name}")
+
+cv2.destroyAllWindows()
+
+if len(objpoints) > 0:
+    # Perform camera calibration
+    ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1], None, None)
+
+    # Print out the camera calibration results
+    print("Camera matrix : \n")
+    print(mtx)
+    print("dist : \n")
+    print(dist)
+    print("rvecs : \n")
+    print(rvecs)
+    print("tvecs : \n")
+    print(tvecs)
+
+    while True:
+        ret, frame = cap.read()
+        if not ret:
+            break
+        k = cv2.waitKey(1)
+
+        if k%256 == 27:
+            # ESC pressed
+            print("Escape hit, closing...")
+            break
+
+        # Convert to grayscale
+        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
+
+        # Find the chess board corners
+        ret, corners = cv2.findChessboardCorners(gray, (CHESSBOARD_Y, CHESSBOARD_X), None)
+
+        if ret:
+            corners2 = cv2.cornerSubPix(gray, corners, (11,11), (-1,-1), criteria)
+            imgpoints.append(corners2)
+
+            # Draw and display the corners
+            frame = cv2.drawChessboardCorners(frame, (CHESSBOARD_Y, CHESSBOARD_X), corners2, ret)
+
+            # Estimate pose of pattern
+            _, rvecs, tvecs, _ = cv2.solvePnPRansac(objp, corners2, mtx, dist)
+
+            # Compute distance from camera to pattern
+            x_distance = tvecs[0][0]
+            y_distance = tvecs[1][0]
+            z_distance = tvecs[2][0]
+
+            text = f"X: {x_distance:.2f}m, Y: {y_distance:.2f}m, Z: {z_distance:.2f}m"
+            cv2.putText(frame, text, (10, 30), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2, cv2.LINE_AA)
+
+        cv2.imshow('Result', frame)
+
+else:
+    print("Not enough images where corners were found. Please capture more images.")
+
+cap.release()
+cv2.destroyAllWindows()