pixel_rule: Creates a rule object

Description

Creates an object of class pixel_rule from a line in rgb space, defined by the user, and a relational operator.

Usage

pixel_rule(rule_name, x_axis, y_axis, line_points, comp_op)
is.rule(x)
# S3 method for pixel_rule
summary(object, ...)

Value

An object of class "pixel_rule" containing these elements:

rule_name: a character string containing the rule name.
rule_text: a character string containing the mathematical expression of the rule.
comp_op: a character string containing the comparison operator used in the rule.
a: a numerical vector containing the parameter a (slope) of the line.
c: a numerical vector containing the parameter c (intercept) of the line.
x_axis: a character string containing the colour variable selected as x axis (one of "r", "g" or "b").
y_axis: a character string containing the colour variable selected as y axis.
first_point: a numerical vector containing the coordinates of the first point used to estimate the line equation.
second_point: a numerical vector containing the coordinates of the second point.

Arguments

rule_name: a character string containing the name of the rule.
x_axis: a character string selecting the colour variable used as x axis, one of "r", "g" or "b".
y_axis: a character string selecting the colour variable used as y axis, one of "r", "g" or "b".
line_points: either an object of of class "rule_points" created with function place_rule(), or a list containing the coordinates of two points defining the line. The coordinates are two-element numeric vectors.
comp_op: a character string containing one of the comparison operators ">", ">=", "<", "<=".
x: the R object being tested
object: an object of class "pixel_category".
...: additional parameters to pass to the function.

Details

This function estimates the slope (a) and intercept (c) of the line y = ax + c from the coordinates of two points on the line. x and y are two colour variables selected by the user (r, g, or b). The line divides the plane in two subsets and the comparison operator selects the subset that contains the points (pixels) of interest.

When the line is defined by a list containing a couple of points, it is first converted into an object of class "pixel_rule_points" (see place_rule, and the examples below).

The lines are mathematical objects that extend without bound, i.e. all along the x axis. The pair of points do not set the line limits, they only allow the estimation of the line parameters. Therefore, they are not constrained to be inside the triangular area occupied by the pixels, and the points can be selected in the most convenient way, provided that the line divides correctly the categories. Convenience in this context means that the line should seem nice in the plot, if this matters.

Because the variables were transformed into proportions, the pixels in the plot are always inside the triangle defined by the points (0, 0), (1, 0), (0, 1). So, the sides of this triangle can be considered as implicit rules which do not need to be created explicitly. In this way, a single line creates two polygons by cutting the triangle in two. Usually, the implicit rules reduce the number of rules to create.

Examples

Run this code

# Creating the line by passing the coordinates of two points on the line:
rule01 <- pixel_rule("rule01", "g", "b",
                      list(c(0.35, 0.30), c(0.45, 0.10)),">")

# A vertical line as a rule; note that the equation will be simplified
rule02 <- pixel_rule("rule02", "g", "b",
                      list(c(0.35, 0.30), c(0.35, 0.00)), ">")
if (FALSE) {
# Creating the rule by passing an object of type rule_point:
rule_points01 <- place_rule("g", "b")
rule03 <- pixel_rule("rule03", "g", "b", rule_points01,">")

# Note that the creation of the intermediate object can be avoided:
rule04 <- pixel_rule("rule04", "g", "b", place_rule("g", "b"),">")
}