csI {gdi}

Calculates the second moment of area (=area moment of inertia, Ix and Iy) and polar moment of inertia (Iz or J) for a cross-section given as an image.

Description

Calculates the second moment of area (=area moment of inertia, Ix and Iy) and polar moment of inertia (Iz or J) for a cross-section given as an image.

Usage

csI(
  image_file,
  threshold = 0.5,
  channel = 4,
  method = "greater",
  scale = 1,
  return = "total"
)

Arguments

image_file

Image to be read. Images can be jpeg or png files, or a previously read image saved as an object in R.

threshold

Reference value for color criterium after which pixels that are part of the silhouette should be differentiated from the background.

channel

color channel to which to apply the threshold criterium. Default is 4 (alpha channel of rgba image). Channel setting needs to be adjusted depending on the color mode of the image used (e.g. there are two channels to choose from in a greyscale image, and 3 in an rgb image).

method

Method for determining which pixels to count. Default "greater" counts pixels with value greater than threshold (e.g. higher opacity, in the case of an alpha channel). "less" counts pixels with a value less than the threshold. "not" counts all pixels not precisely matching threshold. Any other character string results in only pixels exactly matching the value given as threshold being counted.

scale

Optional scale of the image (number of pixels per linear unit).

return

What to return, defaults to returning both x and y second moments of area and polar moment of inertia for the entire shape (if return=="total"), otherwise returns raw data matrix for all pixels.

Value

A numeric vector containing Ix, Iy and Iz for the shape (default), or a matrix containing area moments and coordinates for each pixel in the image, as well as area moments converted relative to the common centroid of the shape using parallel axis theorem.

Examples

fdir <- system.file(package="gdi")
csI(file.path(fdir,"exdata","cross_section.png"))