Webcam Ruler

This instructable explains how to measure object dimensions remotely using a camera, a ruler, and simple ratios.

The supplied software is easy to use which makes it an ideal learning tool.

All you require is a webcam, Processing 3 software, and a ruler.

Excluding the webcam, the cost of this project is less than $5.00

Images

• The length of the red line in the cover photo, as measured by the webcam, is 8.5 inches
• Photo 2 confirms that the distance is indeed 8.5 inches
• Photo 3 is a screen shot of a dimension being measured in mm

Video

• The video shows the webcam ruler in operation

Step 1: Parts Required

The following parts are required:

• 1 only 12 inch / 300mm ruler
• 1 only webcam ... I am using a Logitech Quickcam Pro 9000 but any webcam should work
• Processing 3 software from https://processing.org/download/

Step 2: Theory

The image sensor within a camera may be regarded as a sheet of paper that is divided into a number of small squares called pixels (picture elements).

Assume that your webcam can see a standard ruler.

To measure an object dimensions in inches:

• record the pixel location for zero inches on the ruler
• record the pixel location for 12 inches on the ruler
• subtract these pixel locations to get the ruler-length in pixels
• repeat steps 1-3 to get the object-length in pixels
• the object length in inches is equal to:

object-length (inches) = object-length (pixels) / ruler-length (pixels) * 12 ......... (1)

To measure an object dimensions in millimetres:

• record the pixel location for zero on the ruler
• record the pixel location for 300mm on the ruler
• subtract these pixel locations to get the ruler-length in pixels
• repeat steps 1-3 to get the object-length in pixels
• the object length in mm is equal to:

object-length (mm) = object-length (pixels) / ruler-length (pixels) * 300 .............. (2)

Measurement accuracy depends on your camera resolution ... the more pixels the better.

The camera should be positioned directly overhead without any trapezoid distortion.

Step 3: Software

Installation

• Download and install Processing 3 from https://processing.org/download/
• Download the attached file “webcam_ruler.pde”
• Launch Processing 3 ... an empty sketch similar to Arduino will appear.
• Copy the contents of “webcam_ruler.pde” to the sketch ... use a text editor NOT a word processor
• Save the sketch as “webcam_ruler” (without the quotes)
• Left-click the top-left button to run [1]

Note:

[1]

This sketch requires that the following video library is installed:

• Left-click "Sketch | Import Library ... | Add library" (photo1)
• Type video in the search bar (photo 2)
• Select "Video | GSteamer-based video library for Processing"
• Now left-click "Install"

A green tick will appear when the process is complete.

Step 4: Configuring Your Camera

The software will automatically recognise your camera and list various configurations. Each configuration comes with a number inside brackets.

If your camera doesn’t start, choose a configuration and substitute the [73] shown in photo 1 with the chosen camera configuration number .

For best enjoyment replace size(960, 720) in setup() with the image dimensions listed for your chosen camera configuration.

Step 5: Measuring

Calibration

• RIGHT-click each end of the ruler to calibrate
• Green circles will appear with each mouse click.
• Keep clicking until you are satisfied that the circles are in the correct place.

Measuring

• LEFT-click one end of the object.
• A red line will follow your mouse as it moves to the other end
• LEFT-click this distant end
• The distance between the two left-clicks will appear on the screen
• Press “1” for inches or “2” for millimetres
• Repeat this process to take another measurement ... there is no need to recalibrate

Step 6: Summary

The attached software allows you to measure objects remotely using a webcam.

The software uses simple proportion (ratios) to measure distance. As such it is a fun teaching tool.

Calibration is achieved by right-clicking both ends of a 12 inch / 300mm ruler.

Calibration is only required once prior to measuring.

Left-click both ends of an object to measure its length ... the length will appear on-screen.

You can switch between inches and millimetres using the ‘1’ and ‘2’ keys.

  Click here   to view my other instructables.