Accurate position measurement using low-cost cameras and OpenCV

There are many ways to sense the position of an object, and they’re generally either expensive or low-resolution. Laser interferometers are incredibly accurate, but the complex optics & electronics make the price very high. Hand-held laser measures are quite cheap, but they use a time-of-flight measurement method which limits their resolution, as light travels at roughly 1 foot (300 mm) per nanosecond, and making sub-nanosecond measurements isn’t easy. Lidar (light-based radar) is currently quite expensive, and has similar constraints. Ultrasonic methods benefit from the fact that sound waves travel at a much slower speed; they work well in constrained environments, such as measuring the height of liquid in a tank, but multipath reflections are a problem if there is more than one object in view.

Thanks to the smartphone boom, high-resolution camera modules are quite cheap, and I’ve been wondering whether they could be used to sense the position of an object to a reasonable accuracy for everyday measurements (at least 0.5 mm or 0.02 inches).

To test the idea I’ve set up 2 low-cost webcams at right-angles, to sense the X and Y position of an LED. To give a reproducible setup, I’ve engraved a baseboard with 1 cm squares, and laser-cut a LED support, so I can accurately position the LED and see the result.

The webcams are Logitech C270, that can provide an HD video resolution of 720p (i.e. 1280 x 720 pixels). For image analysis I’ll be using Python OpenCV; it has a wide range of sophisticated software tools, that allow you to experiment with some highly advanced methods, but for now I’ll only be using a few basic functions.

The techniques I’m using are equally applicable to single-camera measurements, e.g. tracking the position of the sun in the sky.

Camera input

My camera display application uses PyQt and OpenCV to display camera images, and it is strongly recommended that you start with this, to prove that your cameras will work with the OpenCV drivers. It contains code that can be re-used for this application, so is imported as a module.

Since we’re dealing with multiple cameras and displays, we need a storage class to house the data.

import sys, time, threading, cv2, numpy as np
import cam_display as camdisp

IMG_SIZE    = 1280,720          # 640,480 or 1280,720 or 1920,1080
DISP_SCALE  = 2                 # Scaling factor for display image
DISP_MSEC   = 50                # Delay between display cycles
CAP_API     = cv2.CAP_ANY       # API: CAP_ANY or CAP_DSHOW etc...

# Class to hold capture & display data for a camera
class CamCap(object):
    def __init__(self, cam_num, label, disp):
        self.cam_num, self.label, self.display = cam_num, label, disp
        self.imageq = camdisp.Queue.Queue()
        self.pos = 0
        self.cap = cv2.VideoCapture(self.cam_num-1 + CAP_API)
        self.cap.set(cv2.CAP_PROP_FRAME_WIDTH, IMG_SIZE[0])
        self.cap.set(cv2.CAP_PROP_FRAME_HEIGHT, IMG_SIZE[1])

The main window of the GUI is subclassed from cam_display, with the addition of a second display area, and storage for the camera capture data:

# Main window
class MyWindow(camdisp.MyWindow):
    def __init__(self, parent=None):
        camdisp.MyWindow.__init__(self, parent)
        self.label.setFont(LABEL_FONT)
        self.camcaps = []
        self.disp2 = camdisp.ImageWidget(self)
        self.displays.addWidget(self.disp2)
        self.capturing = True

On startup, 2 cameras are added to the window:

if __name__ == '__main__':
    app = camdisp.QApplication(sys.argv)
    win = MyWindow()
    win.camcaps.append(CamCap(2, 'x', win.disp))
    win.camcaps.append(CamCap(1, 'y', win.disp2))
    win.show()
    win.setWindowTitle(VERSION)
    win.start()
    sys.exit(app.exec_())

As with cam_display, a separate thread is used to fetch data from the cameras:

    # Grab camera images (separate thread)
    def grab_images(self):
        while self.capturing:
            for cam in self.camcaps:
                if cam.cap.grab():
                    retval, image = cam.cap.retrieve(0)
                    if image is not None and cam.imageq.qsize() < 2:
                        cam.imageq.put(image)
                    else:
                        time.sleep(DISP_MSEC / 1000.0)
                else:
                    print("Error: can't grab camera image")
                    self.capturing = False
        for cam in self.camcaps:
            cam.cap.release()

Image display

A timer event is used to fetch the image from the queue, convert it to RGB, do the image processing, and display the result.

    # Fetch & display camera images
    def show_images(self):
        for cam in self.camcaps:
            if not cam.imageq.empty():
                image = cam.imageq.get()
                if image is not None and len(image) > 0:
                    img = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)
                    cam.pos = colour_detect(img)
                    self.display_image(img, cam.display, DISP_SCALE)
                    self.show_positions()
    
    # Show position values given by cameras
    def show_positions(self, s=""):
        for cam in self.camcaps:
            s += "%s=%-5.1f " % (cam.label, cam.pos)
        self.label.setText(s)

Image processing

We need to measure the horizontal (left-to-right) position of the LED for each camera. If the LED is brighter than the surroundings, this isn’t difficult; first we create a mask that isolates the LED from the background, then extract the ‘contour’ of the object with the background masked off. The contour is a continuous curve that marks the boundary between the object and the background; for the illuminated LED this will approximate to a circle. To find an exact position, the contour is converted to a true circle, which is drawn in yellow, and the horizontal position of the circle centre is returned.

LOWER_DET   = np.array([240,  0,  0])       # Colour limits for detection
UPPER_DET   = np.array([255,200,200])

# Do colour detection on image
def colour_detect(img):
    mask = cv2.inRange(img, LOWER_DET, UPPER_DET)
    ctrs = cv2.findContours(mask, cv2.RETR_TREE,
                            cv2.CHAIN_APPROX_SIMPLE)[-2]
    if len(ctrs) > 0:
        (x,y),radius = cv2.minEnclosingCircle(ctrs[0])
        radius = int(radius)
        cv2.circle(img, (int(x),int(y)), radius, (255,255,0), 2)
        return x
    return 0

This code is remarkably brief, and if you’re thinking that I may have taken a few short-cuts, you’d be right:

Colour detection: I’ve specified the upper and lower RGB values that are acceptable; because this is a red LED, the red value is higher than the rest, being between 240 and 255 (the maximum is 255). I don’t want to trigger on a pure white background so I’ve set the green and blue values between 0 and 200, so a pure white (255,255,255) will be rejected. This approach is a bit too simplistic; if the LED is too bright it can saturate the sensor and appear completely white, and conversely another bright light source can cause the camera’s auto-exposure to automatically reduce the image intensity, such that the LED falls below the required level. The normal defence against this is to use manual camera exposure, which can be adjusted to your specific environment. Also it might be worth changing the RGB colourspace to HSV for image matching; I haven’t yet tried this.

Multiple contours: the findContours function returns a list of contours, and I’m always taking the first of these. In a real application, there may be several contours in the list, and it will be necessary to check them all, to find the most likely – for example, the size of the circle to see if it is within an acceptable range.

However, the measurement method does show some very positive aspects:

Complex background: as you can see from the image at the top of this blog, it works well in a normal office environment – no need for a special plain-colour background.

No focussing: most optical applications require the camera to be focussed, but in this case there is no need. I’ve deliberately chosen a target distance of approximately 4 inches (100 mm) that results in a blurred image, but OpenCV is still able to produce an accurate position indication.

Sub-pixel accuracy: with regard to measurement accuracy, the main rule for the camera is obviously “the more pixels, the better”, but also OpenCV can compute the position to within a fraction of a pixel. My application displays the position (in pixels) to one decimal place; at 4 inches (100 mm) distance, the Logitech cameras’ field of view is about 3.6 inches (90 mm), so if the position can be measured within, say, 0.2 of a pixel, this would be a resolution of 0.0006 inch (0.015 mm).

Of course these figures are purely theoretical, and the resolution will be much reduced in a real-world application, but all the same, it does suggest the technique may be capable of achieving quite good accuracy, at relatively low cost.

Single camera

With minor modifications, the code can be used in a single-camera application, e.g. tracking the position of the sun in the sky.

The code scans all the cameras in the ‘camcaps’ list, so will automatically adapt if there is only one.

The colour_detect function currently returns the horizontal position only; this can be changed to return the vertical as well. The show_positions method can be changed to display both of the returned values from the single camera.

Then you just need a wide-angle lens, and a suitable filter to stop the image sensor being overloaded. Sundial, anyone?

Source code

The ‘campos’ source code is available here, and is compatible with Windows and Linux, Python 2.7 and 3.x, PyQt v4 and v5. It imports my cam_display application, and I strongly recommended that you start by running that on its own, to check compatibility. If it fails, read the Image Capture section of that blog, which contains some pointers that might be of help.

Copyright (c) Jeremy P Bentham 2019. Please credit this blog if you use the information or software in it.

PC / RPi camera display using PyQt and OpenCV

OpenCV is an incredibly powerful image-processing tool, but it can be difficult to know where to start – how do you grab an image from a camera, and display it in a user-friendly GUI? This post describes such an application, that runs unmodified on a PC or Raspberry Pi, Windows or Linux, Python 2.7 or 3.x, and PyQt v4 or v5.

Installation

On Windows, the OpenCV and PyQt5 libraries can be installed using pip:

pip install numpy opencv-python PyQt5

If pip isn’t available, you should be able to run the module from the command line by invoking Python, e.g. for Python 3:

py -3 -m pip install numpy opencv-python PyQt5

Installing on a Raspberry Pi is potentially a lot more complicated; it is generally recommended to install from source, and for opencv-python, this is a bit convoluted. Fortunately there is a simpler option, if you don’t mind using versions that are a few years old, namely to load the binary image from the standard repository, e.g.

sudo apt update
sudo apt install python3-opencv python3-pyqt5 

At the time of writing, the most recent version of Raspbian Linux is ‘buster’, and that has OpenCV 3.2, which is quite usable. The previous ‘stretch’ distribution has python-opencv version 2.4, which is a bit too old: my code isn’t compatible with it.

With regard to cameras, all the USB Webcams I’ve tried have worked fine on Windows without needing to have any extra driver software installed; they also work on the Raspberry Pi, as well as the standard Pi camera with the ribbon-cable interface.

PyQt main window

Being compatible with PyQt version 4 and 5 requires some boilerplate code to handle the way some functions have been moved between libraries:

import sys, time, threading, cv2
try:
    from PyQt5.QtCore import Qt
    pyqt5 = True
except:
    pyqt5 = False
if pyqt5:
    from PyQt5.QtCore import QTimer, QPoint, pyqtSignal
    from PyQt5.QtWidgets import QApplication, QMainWindow, QTextEdit, QLabel
    from PyQt5.QtWidgets import QWidget, QAction, QVBoxLayout, QHBoxLayout
    from PyQt5.QtGui import QFont, QPainter, QImage, QTextCursor
else:
    from PyQt4.QtCore import Qt, pyqtSignal, QTimer, QPoint
    from PyQt4.QtGui import QApplication, QMainWindow, QTextEdit, QLabel
    from PyQt4.QtGui import QWidget, QAction, QVBoxLayout, QHBoxLayout
    from PyQt4.QtGui import QFont, QPainter, QImage, QTextCursor
try:
    import Queue as Queue
except:
    import queue as Queue

The main window is subclassed from PyQt, with a simple arrangement of a menu bar, video image, and text box:

class MyWindow(QMainWindow):
    text_update = pyqtSignal(str)

    # Create main window
    def __init__(self, parent=None):
        QMainWindow.__init__(self, parent)

        self.central = QWidget(self)
        self.textbox = QTextEdit(self.central)
        self.textbox.setFont(TEXT_FONT)
        self.textbox.setMinimumSize(300, 100)
        self.text_update.connect(self.append_text)
        sys.stdout = self
        print("Camera number %u" % camera_num)
        print("Image size %u x %u" % IMG_SIZE)
        if DISP_SCALE > 1:
            print("Display scale %u:1" % DISP_SCALE)

        self.vlayout = QVBoxLayout()        # Window layout
        self.displays = QHBoxLayout()
        self.disp = ImageWidget(self)    
        self.displays.addWidget(self.disp)
        self.vlayout.addLayout(self.displays)
        self.label = QLabel(self)
        self.vlayout.addWidget(self.label)
        self.vlayout.addWidget(self.textbox)
        self.central.setLayout(self.vlayout)
        self.setCentralWidget(self.central)

        self.mainMenu = self.menuBar()      # Menu bar
        exitAction = QAction('&Exit', self)
        exitAction.setShortcut('Ctrl+Q')
        exitAction.triggered.connect(self.close)
        self.fileMenu = self.mainMenu.addMenu('&File')
        self.fileMenu.addAction(exitAction)

There is a horizontal box layout called ‘displays’, that seems to be unnecessary as it only has one display widget in it. This is intentional, since much of my OpenCV experimentation requires additional displays to show the image processing in action; this can easily be done by creating more ImageWidgets, and adding them to the ‘displays’ layout.

Similarly, there is a redundant QLabel below the displays, which isn’t currently used, but is handy for displaying static text below the images.

Text display

It is convenient to redirect the ‘print’ output to the text box, rather than appearing on the Python console. This is done using the ‘text_update’ signal that was defined above:

    # Handle sys.stdout.write: update text display
    def write(self, text):
        self.text_update.emit(str(text))
    def flush(self):
        pass

    # Append to text display
    def append_text(self, text):
        cur = self.textbox.textCursor()     # Move cursor to end of text
        cur.movePosition(QTextCursor.End) 
        s = str(text)
        while s:
            head,sep,s = s.partition("\n")  # Split line at LF
            cur.insertText(head)            # Insert text at cursor
            if sep:                         # New line if LF
                cur.insertBlock()
        self.textbox.setTextCursor(cur)     # Update visible cursor

The use of a signal means that print() calls can be scattered about the code, without having to worry about which thread they’re in.

Image capture

A separate thread is used to capture the camera images, and put them in a queue to be displayed. The camera may produce images faster than they can be displayed, so it is necessary to check how many images are already in the queue; if more than 1, the new image is discarded. This prevents a buildup of unwanted images.

IMG_SIZE    = 1280,720          # 640,480 or 1280,720 or 1920,1080
CAP_API     = cv2.CAP_ANY       # or cv2.CAP_DSHOW, etc...
EXPOSURE    = 0                 # Non-zero for fixed exposure

# Grab images from the camera (separate thread)
def grab_images(cam_num, queue):
    cap = cv2.VideoCapture(cam_num-1 + CAP_API)
    cap.set(cv2.CAP_PROP_FRAME_WIDTH, IMG_SIZE[0])
    cap.set(cv2.CAP_PROP_FRAME_HEIGHT, IMG_SIZE[1])
    if EXPOSURE:
        cap.set(cv2.CAP_PROP_AUTO_EXPOSURE, 0)
        cap.set(cv2.CAP_PROP_EXPOSURE, EXPOSURE)
    else:
        cap.set(cv2.CAP_PROP_AUTO_EXPOSURE, 1)
    while capturing:
        if cap.grab():
            retval, image = cap.retrieve(0)
            if image is not None and queue.qsize() < 2:
                queue.put(image)
            else:
                time.sleep(DISP_MSEC / 1000.0)
        else:
            print("Error: can't grab camera image")
            break
    cap.release()

The choice of image size will depend on the camera used; all cameras support VGA size (640 x 480 pixels), more modern versions the high-definition standards of 720p (1280 x 720) or 1080p (1920 x 1080).

The camera number refers to the position in the list of cameras collected by the operating system; I’ve defined the first camera as number 1, but the OpenCV call defines the first as 0, so the number has to be adjusted.

The same parameter is also used to define the capture API setting; by default this is ‘any’, which usually works well; my Windows 10 system defaults to the MSMF (Microsoft Media Foundation) backend, while the Raspberry Pi defaults to Video for Linux (V4L). Sometimes you may need to force a particular API to be used, for example, I have a Logitech C270 webcam that works fine on Windows 7, but fails on Windows 10 with an ‘MSMF grab error’. Forcing the software to use the DirectShow API (using the cv2.CAP_DSHOW option) fixes the problem.

If you want to check which backend is being used, try:

print("Backend '%s'" % cap.getBackendName())

Unfortunately this only works on the later revisions of OpenCV.

Manual exposure setting can be a bit hit-and-miss, depending on the camera and API you are using; the default is automatic operation, and setting EXPOSURE non-zero (e.g. to a value of -3) generally works, however it can be difficult to set a webcam back to automatic operation: sometimes I’ve had to use another application to do this. So it is suggested that you keep auto-exposure enabled if possible.

[Supplementary note: it seems that these parameter values aren’t standardised across the backends. For example, the CAP_PROP_AUTO_EXPOSURE value in my source code is correct for the MSMF backend; a value of 1 enables automatic exposure, 0 disables it. However, the V4L backend on the Raspberry Pi uses the opposite values: automatic is 0, and manual is 1. So it looks like my code is incorrect for Linux. I haven’t yet found any detailed documentation for this, so had to fall back on reading the source code, namely the OpenCV videoio ‘cap’ files such as cap_msmf.cpp and cap_v4l.cpp.]

Image display

The camera image is displayed in a custom widget:

# Image widget
class ImageWidget(QWidget):
    def __init__(self, parent=None):
        super(ImageWidget, self).__init__(parent)
        self.image = None

    def setImage(self, image):
        self.image = image
        self.setMinimumSize(image.size())
        self.update()

    def paintEvent(self, event):
        qp = QPainter()
        qp.begin(self)
        if self.image:
            qp.drawImage(QPoint(0, 0), self.image)
        qp.end()

A timer event is used to trigger a scan of the image queue. This contains images in the camera format, which must be converted into the PyQt display format:

DISP_SCALE  = 2                 # Scaling factor for display image

    # Start image capture & display
    def start(self):
        self.timer = QTimer(self)           # Timer to trigger display
        self.timer.timeout.connect(lambda: 
                    self.show_image(image_queue, self.disp, DISP_SCALE))
        self.timer.start(DISP_MSEC)         
        self.capture_thread = threading.Thread(target=grab_images, 
                    args=(camera_num, image_queue))
        self.capture_thread.start()         # Thread to grab images

    # Fetch camera image from queue, and display it
    def show_image(self, imageq, display, scale):
        if not imageq.empty():
            image = imageq.get()
            if image is not None and len(image) > 0:
                img = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)
                self.display_image(img, display, scale)

    # Display an image, reduce size if required
    def display_image(self, img, display, scale=1):
        disp_size = img.shape[1]//scale, img.shape[0]//scale
        disp_bpl = disp_size[0] * 3
        if scale > 1:
            img = cv2.resize(img, disp_size, 
                             interpolation=cv2.INTER_CUBIC)
        qimg = QImage(img.data, disp_size[0], disp_size[1], 
                      disp_bpl, IMG_FORMAT)
        display.setImage(qimg)

This demonstrates the power of OpenCV; with one function call we convert the image from BGR to RGB format, then another is used to resize the image using cubic interpolation. Finally a PyQt function is used to convert from OpenCV to PyQt format.

Running the application

Make sure you’re using the Python version that has the OpenCV and PyQt installed, e.g. for the Raspberry Pi:

python3 cam_display.py

There is an optional argument that can be used if there are multiple cameras; the default first camera is number 1.

On Linux, some USB Webcams cause a constant stream of JPEG format errors to be printed on the console, complaining about extraneous bytes in the data. There is some discussion online as to the cause of the error, and the cure seems to involve rebuilding the libraries from source; I’m keen to avoid that, so used the simple workaround of suppressing the errors by redirecting STDERR to null:

python3 cam_display.py 2> /dev/null

Fortunately this workaround is only needed with some USB cameras; the standard Raspberry Pi camera with the CSI ribbon-cable interface works fine.

Source code

Full source code is available here.

For a more significant OpenCV application, take a look at this post.

Copyright (c) Jeremy P Bentham 2019. Please credit this blog if you use the information or software in it.