Measure Object Size using OpenCV and ArUco Marker

Find the object sizes in an image containing an ArUco marker using OpenCV

In this post, you will use an ArUco marker with a predefined size and ID as a reference to calculate the pixel to inch conversion and then use the conversion ratio to find the dimension of different objects in the image.

Measuring object size using an ArUco marker

ArUco Marker

The ArUco markers are fiducial square markers used for camera pose estimation. An ArUco marker is a synthetic square marker with an internal binary matrix enclosed within a wide black color border with a Unique Identifier.

The ArUco marker consists of predefined dictionaries covering a range of different dictionary sizes and marker sizes. To generate the ArUco marker, you need to specify

• The dictionary size that determines the number of markers in the dictionary
• The marker size which indicates the number of bits in the marker

To know more about ArUco Marker, click here

Object Size Measuring Logic

The following steps are performed to measure the size of different objects in an image.

• Load the image, identify the objects whose size needs to be measured and grab their coordinates
• Detect an ArUco marker in the image; calculate the pixel to inch conversion ratio with the known perimeter for the ArUco marker.
• Compute the object size based on the object co-ordinates and the pixel to inch conversion

Load the image and identify the objects to grab their coordinates

Load the image to find the contours detected for different shapes. To find the different shapes, convert the image to grayscale using cvtColor(), blur it slightly using GaussianBlur() to smoothen the image and remove any Gaussian noise from an image.

Find the edges in an image using the Canny() algorithm and then apply two morphological image processing operations dilate() and erode().

During dilation, the image is expanded by adding the number of pixels to the boundaries of objects, whereas…