Using Custom Images as "pch" values in R

In a previous post I show how one can use custom pch values to represent different data types. I've long wanted to be able to take a custom image and use that to represent the data. I recently discovered the rasterImage function in R, which allows the user to take an image and plot it! There are various examples out there of using RasterImage (such as RJournal_2011-1_Murrell.pdf), but I didn't find anything that did exactly what I wanted it to do

the tricky part is that rasterImage requires you to define the 4 boundaries of the image. The goal is then to be able to define the boundaries by only specifying an x, y coordinate, and have code smart enough to display the image with the appropriate dimensions


Here are the required packages

library(RCurl)  #for reading file from a URL

## Loading required package: bitops

library(png)  #for reading in a .png, use library(jpeg) for a .jpg

The function I wrote takes the image, the (x,y) coordinates of where you want to plot your image, and for now a cex and a pos argument. I intend on expanding the options, and hopefully will improve these functionalities over time.

image_points = function(image, x, y, cex = 1, pos = NULL) {
    if (length(x) != length(y)) {
        stop("length(x)!=length(y): check your data")
    }
    dim.x = dim(image)[2]  #image width
    dim.y = dim(image)[1]  #image height
    if (dim.x == dim.y) {
        # obtian the ratio of width to height or height to width
        ratio.x = ratio.y = 1
    } else if (dim.x < dim.y) {
        ratio.x = dim.x/dim.y
        ratio.y = 1
    } else {
        ratio.x = 1
        ratio.y = dim.y/dim.x
    }
    cex = cex/10  #how large the image should be, divided by 10 so that it matches more closely to plotting points
    pin = par()$pin  #pin provides the width and height of the _active graphic device_
    pin.ratio = pin/max(pin)  #take the ratio
    usr = par()$usr  #usr provides the lower.x, lower.y, upper.x, upper.y values of the plotable region

    # combine the active device dimensions, the image dimensions, and the
    # desired output size
    image.size.y = (usr[4] - usr[3]) * pin.ratio[1] * cex
    image.size.x = (usr[2] - usr[1]) * pin.ratio[2] * cex
    for (i in 1:length(x)) {
        # plot each point pos can be NULL (default) or 1, 2, 3, or 4, corresponding
        # to centered (defualt), bottom, left, top, right, respectively.
        if (is.null(pos)) {
            # centered at (x,y), define the bottom/top and left/right boundaries of the
            # image
            x.pos = c(x[i] - (image.size.x * ratio.x)/2, x[i] + (image.size.x * 
                ratio.x)/2)
            y.pos = c(y[i] - (image.size.y * ratio.y)/2, y[i] + (image.size.y * 
                ratio.y)/2)

            rasterImage(image, x.pos[1], y.pos[1], x.pos[2], y.pos[2])
        } else if (pos == 1) {
            x.pos = c(x[i] - (image.size.x * ratio.x)/2, x[i] + (image.size.x * 
                ratio.x)/2)
            y.pos = c(y[i] - (image.size.y * ratio.y), y[i])
        } else if (pos == 2) {
            x.pos = c(x[i] - (image.size.x * ratio.x), x[i])
            y.pos = c(y[i] - (image.size.y * ratio.y)/2, y[i] + (image.size.y * 
                ratio.y)/2)
        } else if (pos == 3) {
            x.pos = c(x[i] - (image.size.x * ratio.x)/2, x[i] + (image.size.x * 
                ratio.x)/2)
            y.pos = c(y[i], y[i] + (image.size.y * ratio.y))
        } else if (pos == 4) {
            x.pos = c(x[i], x[i] + (image.size.x * ratio.x))
            y.pos = c(y[i] - (image.size.y * ratio.y)/2, y[i] + (image.size.y * 
                ratio.y)/2)
        }

        rasterImage(image, x.pos[1], y.pos[1], x.pos[2], y.pos[2])  #plot image
    }
}

I pulled an image from sweetclipart.com using getURLContent() and readPNG()

URL = ("http://sweetclipart.com/multisite/sweetclipart/files/imagecache/middle/sports_car_2_red.png")  #where the image is located
image = readPNG(getURLContent(URL))  #gets the content of the URL

image is an array of dimensions width \( \times \) height \( \times \) channels. The first 3 channels represent the R, G, and B values (scaled 0 to 1) of each pixel, with the 4th channel representing the alpha value (scaled 0 to 1, 0 representing transparent)
Only pngs have the alpha channel, so jpegs will have 3 channels. Not all pngs have the 4th channel, if they do have the 4th channel the background won't necessarily be transparent. More on alpha values in a bit

dim(image)  #width, height, alpha

## [1] 275 550   4

Using the cars dataset we can plot the distance it took to stop by speed

data(cars)
plot(cars, type = "n", axes = F, xlab = "Speed", ylab = "Distance to Stop", 
    main = "Cars: Distance to Stop by Speed")
axis(1)
axis(2, las = 2)
x = cars$speed
y = cars$dist
image_points(image, x, y, 2)

It turns out that the data is from the 1920s, so using this image might be more appropriate:

library(jpeg)
URL2 = "http://embed.polyvoreimg.com/cgi/img-thing/size/y/tid/4985430.jpg"
image2 = readJPEG(getURLContent(URL2))
plot(cars, type = "n", axes = F, xlab = "Speed", ylab = "Distance to Stop", 
    main = "Cars: Distance to Stop by Speed\n1920's car")
axis(1)
axis(2, las = 2)
image_points(image2, x, y, 2)

Here we have a jpeg file that is a width \( \times \) height matrix with values 0 to 1 representing the black/white/gray scale. We can manipulate the data using rgb() and abind() to create an array with the appropriate R, G, B, and alpha values.
First, on this color scale, values that are closer to 1 are going to be lighter gray to white, so we can assign an alpha value of 0 (transparent) to any values that are greater than 0.8.

alpha.val = matrix(1, nrow(image2), ncol(image2))  #want a matrix of 1s (not transparent) the same size as the image
alpha.val[image2 > 0.8] = 0  #for lighter gray and white values, set alpha to 0 (transparent)

We then want to create a new array, and will set the first dimension to the original values. Because it is gray colors, R=G=B

library(abind)
image2.adj = array(image2, dim = c(nrow(image2), ncol(image2), 1))
image2.adj = abind(image2.adj, image2, image2, alpha.val)

We can then plot it:

plot(cars, type = "n", axes = F, xlab = "Speed", ylab = "Distance to Stop", 
    main = "Cars: Distance to Stop by Speed\n1920's Car w/Transparency")
axis(1)
axis(2, las = 2)
image_points(image2.adj, x, y, 2)

Notice that we have now essentially removed the border around the image, and even though the images overlap, we get a better sense of whwere the points lie.

A few final notes

• If we want the cars to be reversed, we can simply…

image2.adj.reversed = image2.adj[, ncol(image2.adj):1, ]
plot(cars, type = "n", axes = F, xlab = "Speed", ylab = "Distance to Stop", 
    main = "Cars: Distance to Stop by Speed\n1920's car w/Transparency, Reversed")
axis(1)
axis(2, las = 2)
image_points(image2.adj.reversed, x, y, 2)

• Using images with lower resolution will often yield better results, I don't know if there are better ways to display images at a lower resolution
• There might be better ways to do what I've figured out here - perhaps there are some more par values that would help convert the resolution and control the cex value?