Masking

by on under R
6 minute read

Summary

This note is a description of how one can make an aesthetically pleasing graph by masking out unwanted elements.

1. Introduction

This is about making nicer bathymetry plots with “R”.

Coves have an irregular geometry. As a result, when fitting bathymetric data to a function, the math introduces spurious results outside the boundaries of the cove. These are seen when one plots the results. An example of how it’s used is shown here. To generate an aesthetically pleasing plot these extraneous results should be covered up by some kind of single color or other mappings. This is called “applying a mask.”

As it turns out, “R” does not have a suitable masking function, or at least I can’t find a properly working one. This note is about creating my own version of a masking function that works for my purposes.

2. Sample Problem

To demonstrate the concept and solution, I’ve put together a sample problem that mimics the conditions. The problem used for the development process is a diamond with a rectangular hole and a set of points of which some are masked off.

“R” defines a plot area on which a graph is generated. The plot area is on a background. All kinds of items can be controlled by settings and functions.

The plot area is defined by a bounding box placed on a background. The sample points are plotted there, as seen in Figure 1. Superimposed we have a couple of polygons, one that defines backdrop, and one that defines a window through we should be able to see all data points except the ones falling on this second backdrop.

points_display.png Figure 1. Background and Plot Area.

Here are definitions of the bounding box, p1, two areas, p1 and p2, and the number of random coordinate sets, nc. I had to go up to 30 random points in order for some to show up in the window.

p0 <- rbind(c(1, 0),  c(3, 2))    # The bounding box or extent
p1 <- rbind(c(1.0, 1.0),  c(2.0, 0.0),  c(3.0, 1.0),   c(2.0, 2.0), c(1.0, 1.0))    # The square
p2 <- rbind(c(1.5, 0.75), c(2.5, 0.75), c(2.5, 1.25), c(1.5, 1.25), c(1.5, 0.75))   # The rectangle
nc <- 30   # The number of test sample coordinates

Figure 2 plots the points and the two closed polygons. The idea is to make a polygon such that only points 5, 8, 12 and 30 show in the inner rectangle and other points outside the outer polygon.

Figure 2. Figure 2. Polygons and Points.

To make this work one must understand how polygon drawing algorithms work. In this case the inner rectangle is defined by points that start on the lower left hand corner and go clockwise. The same was done with the outer polygon. The strategy is to draw one polygon. So in order to get out of the inner polygon one has to start traveling in the opposite direction which would now be clockwise. To make the polygon drawing algorithm work one must start the inner one going clockwise.

Figure 3. Figure 3. How to Make Two Polygons into One.

This methodology has been incorporated into a couple of functions.

# The following function determines whether a dataframe of x any is clockwise or counter-clockwise
# If the result is positive, it is clock-wise.  If it's negative it's counter clock-wise
# REF: https://stackoverflow.com/questions/1165647/how-to-determine-if-a-list-of-polygon-points-are-in-clockwise-order
# Assumes Cartesian coordinates
# Formula only works for CLOSED polygons.
point_order <- function(df){
	x <- df[[1]]
	y <- df[[2]]
	n <- length(x)
	sum <- 0
	for(i in 2:n){
		sum <- sum + (x[i]-x[i-1])*(y[i]+y[i-1])
	}
	sum
}

# The following function makes one closed polygon in such a way that the one defined by df1
# becomes a transparent window inside df2,
# where df1 and df2 are dataframes defining closed polygons.
hole_in_frame <- function(df1, df2){
	q1 <- df1
	q2 <- df2
	s1 <- point_order(df1)
	s2 <- point_order(df2)
	# If TRUE we have to reverse one of the data sets
	if(s1 > 0 && s2 > 0 || s1 < 0 && s2 < 0) {
		q1 <- q1[order(nrow(q1):1), ]    #invert row order
	}
	# We now put the two dataframes together
	q3 <- rbind(q1, q2)
}

The result, shown in Figure 4, is as one hoped for.

Figure 4. Figure 4. A Transparent Window into the Second Polygon.

To put all this together requires the following “R” code:

# Everything, including the points showing through and around
plot(p1, type="n", xlab="", ylab="")
points(cords, pch=20, col="red", cex=4)
textxy(cords$x, cords$y, ptlabs, cex=1)  # Requires package 'textxy'
q3 <- hole_in_frame(q1, q2)
polygon(q3$X1, q3$X2, col="skyblue")     # Requires package 'Polygon'
lines(q3$X1, q3$X2, col="skyblue", lwd=4)
points(q2, type="l", lwd=2)
title(main="The Whole Works", xlab="X", ylab="Y")

These eight lines of code do the following:

  1. Setup the plot with the proper dimensions without adding labels; a drawing board if you like.
  2. Plot all the points.
  3. Add to the points some kind of an ID marker, here a sequential number.
  4. Create a single polygon, q3, from the two q1, and q2 (dataframes of p1 and p2).
  5. Plot the single polygon and fill it with a color (light blue).
  6. Cover the black drawing lines that Polygon produces with light blue ones, the same color as the mask.
  7. Draw the outline of the center box with a heavy black line to give it emphasis and hide some of the pixelation that may occur.
  8. Add title and labels.

The final result of this effort is shown in Figure 5.

Figure 5. Figure 5. The Final Result.

The working pieces of this work are put in a set of two functions that are easily called by a main script. These functions are shown in the next section of code.

point_order <- function(dff){
	x <- dff[[1]]
	y <- dff[[2]]
	n <- length(x)
	sum <- 0
	for(i in 2:n){
		sum <- sum + (x[i]-x[i-1])*(y[i]+y[i-1])
	}
	sum
}

hole_in_frame <- function(df1, df2){
	# Force the names of the two data frames to be the same
	q1 <- df1
	names(q1) <- c("X", "Y")
	q2 <- df2
	names(q2) <- c("X", "Y")
	s1 <- point_order(q1)
	s2 <- point_order(q2)
	# If TRUE we have to reverse one of the data sets
	if(s1 > 0 && s2 > 0 || s1 < 0 && s2 < 0) {
		q1 <- q1[order(nrow(q1):1), ]    #invert row order
	}
	q2 <- rbind(q2, q1[nrow(q1),])
	# We now put the two data frames together
	q3 <- rbind(q1, q2)
	return(q3)
}

These functions come with the following usage description:

Usage:
1) Define two polygons with x/y coordinates, q1 and q2, where q1 is the hole
2) Place in the main script: source(../functions/hole_in_frame.R)
3) Call: q3 <- q3 <- hole_in_frame(q1, q2)
4) Make sure to have loaded 'library(sp)'
5) After plotting interpolated output: polygon(q3[[1]], q3[[2]], col="grey") # Any color

Have fun….


Author: PLV
First Published: 9/11/2017
Last Updated: 9/12/2017
Script Collection:
Ref:
FILE: 2017-09-11-Masking.md
Adapted for this website: 9/21/2017


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© 2017 PLV Some Rights Reserved - To comment on any of my work please email me at: moc.hcetsnes@etep