Table 1.

List of features extracted from a plant root image. Features extracted from broken roots only are marked with (*), and from whole root images only are marked with (**).

Feature extracted	Description
Number of Root Tips	The number of root tips is pixels in identified root topology that have only one neighbouring skeletal pixel.
Total Root Length (px, mm)	The sum of the Euclidean distances between the connected skeletal pixels in the entire root topology of the plant root image.
Network Area (px2, mm2)	The total number of pixels in the segmented image.
Average, Median and Maximum Diameter (px, mm)	The distance transform value at each skeletal pixel is the radius at that pixel and is doubled to give the diameter. The average, median and maximum diameters are computed across all these skeleton pixels.
Perimeter (px, mm)	The sum of the Euclidean distances between the connected contour pixels in the entire segmented image of the plant root.
Volume (px3, mm3) and Surface Area (px2, mm2)	Using the radii for each skeletal pixel determined earlier, the volume at the skeletal pixel is calculated as the length of the pixel multiplied by the cross-sectional area of the root at that pixel. Similarly, the surface area is calculated as the length of the pixel multiplied by the circumference of the cross-section of the root at that pixel. Volume and surface area are calculated as the sum of values from all skeletal pixels.
Computation Time (s)	The time taken to analyse a plant root image is noted as computation time in seconds.
Root Length, Projected Area, Surface Area and Volume histograms	For each skeletal pixel, the root length, projected area on the surface of the image plane, surface area and volume of the root at that pixel are computed. These values are binned according to the diameter ranges specified by the user.
Number of Branch Points*	The number of branch points is pixels in the identified root topology that have at least three neighbouring skeletal root segments.
Branching Frequency (px−1, mm−1)*	The number of branch points per unit root length.
Median and Maximum number of roots**	For each row in the segmented image, a horizontal line scanning is performed that records the number of pixel transitions from a background to a root pixel. This list of pixel transitions is sorted and the median and the maximum number of roots are noted.
Depth, Maximum Width (px, mm)**	The maximum vertical and horizontal distance the root crown grew at the time of imaging are noted as Depth and Maximum Width, respectively.
Width-to-Depth Ratio**	The ratio of Maximum Width to Depth is noted as the Width-to-Depth ratio.
Convex Area (px2, mm2) and Solidity**	The area of the convex hull that is fit to include the entire root crown system is noted as Convex Area. Solidity is the ratio of the Network Area to Convex Area.
Lower Root Area (px2, mm2)**	For a root crown image, the skeletal pixel that has the maximum radius is located. The network area of the root system that is below the skeletal pixel located above is noted as Lower Root Area.
Holes and Average Hole Size (px2, mm2)**	Holes are background components between roots in a segmented image. These holes are counted, and their average size is determined.
Steep Angle Frequency, Medium Angle Frequency, Shallow Angle Frequency and Average Root Orientation**	Within 40 × 40 pixel locality for every skeletal pixel in the centre, we get the coordinates of all the skeletal pixels in that locality and compute the angular orientation for the locality. We group these orientations in bins of 0–30°, 30–60° and 60–90° and note the frequencies as Steep, Medium and Shallow Angle Frequencies. The average of these orientations is noted as Average Root Orientation.