R functions for reading images and analyzing colors in images. Demo at http://www.nottamuntown.com/2013/02/reading-images-in-r.html .

015-imageColor.R

library(reshape)
library(ggplot2)
library(ReadImages)

ccodes <- read.csv('http://sandbox.adamtbradley.com/colors/')


#From http://is-r.tumblr.com/post/36586614678
#Read an image from the web and shape it into a data.frame.
#codeColors() uses this--I don't call it directly.
imageLoader <- function(f) { 
  download.file(f, "tempPicture.jpg", mode = "wb")  # Stash image locally
  readImage <- read.jpeg("tempPicture.jpg")
  
  longImage <- melt(readImage)
  rgbImage <- reshape(longImage, timevar = "X3",
                      idvar = c("X1", "X2"), direction = "wide")
  rgbImage$X1 <- -rgbImage$X1
  return(rgbImage)
}

#Use imageLoader() to grab an image, then do some
#postprocessing--rename the columns, get each pixel's 
#RGB color code, round each color to its nearest websafe
#equivalent then figure out what simple color name 
#describes each pixel.
codeColors <- function(image_url) {
  #TODO: some kind of caching.
  img <- imageLoader(image_url)
  #img$X1 = -img$X1
  colnames(img) <- c('Y', 'X', 'R', 'G', 'B')
  
  img$rgb <- rgb(img$R, img$G, img$B)
  
  #Round each pixel color to the nearest "websafe" color.
  img$rounded <- rgb(round(img$R/2, 1)*2, 
                     round(img$G/2, 1)*2,
                     round(img$B/2, 1)*2)

  codes <- data.frame(colorID=sub('#', '', img$rounded))
  codes <- join(codes, ccodes, 'colorID')
  
  img$code <- codes$colorCode
  img
}

#Make a url to an Amazon product image.
amzImage <- function(asin) {
  sprintf('http://images.amazon.com/images/P/%s.01.LZ', asin)
}

#Shortcut to code a book cover by ISBN.
codeCover <- function(isbn) {
  codeColors(amzImage(isbn))
}

#Plot an image. img is the output of codeImage() or codeCover()
plotImage <- function(img, type="colors", min.pct.cols=.2, max.num.cols=F) {
  if ( type == 'orig' ) {
    with(img, plot(X, Y, col=rgb, 
                   asp=1, pch='.', 
                   ann=F, bty='n',
                   xaxt='n', yaxt='n'))
  }
  
  if ( type == 'rounded' ) {
    with(img, plot(X, Y, col=rounded, 
                   asp=1, pch='.', 
                   ann=F, bty='n',
                   xaxt='n', yaxt='n'))
  }
  
  if ( type == 'code' ) {
    #Remember that img$code is a factor.
    with(img, plot(X, Y, col=as.character(code), 
                   asp=1, pch='.', 
                   ann=F, bty='n',
                   xaxt='n', yaxt='n'))
  }
  
  if ( type == 'colors' ) {
    tmpimg <- imageColors(img)
    plotImageColors(tmpimg, min.pct.cols, max.num.cols)
  }
}

#Summarize a data.frame output from codeImage().
#Used by plotImageColors()
imageColors <- function(img) {
  outp <- summary(img$code)
  data.frame(color=names(outp), count=outp, stringsAsFactors=FALSE)
} 

#Called by plotImage(type='colors')
plotImageColors <- function(img, min.pct.cols=.2, max.num.cols=F) {
  col_theme <- theme(
          legend.position="none", 
          axis.title.x = element_blank(),
          panel.background = element_rect(fill="white"),
          axis.line = element_line(color="#000000"),
          panel.grid.major.y = element_line(color="#CCCCCC"),
          panel.grid.minor.y = element_line(color="#EEEEEE"),
          panel.grid.major.y = element_blank(),
          panel.grid.minor.y = element_blank()
  )
  
  if ( is.numeric(min.pct.cols) ) {
    #Ignore colors that are only a tiny fraction of the image.
    img <- img[img$count > (sum(img$count)*(min.pct.cols/100)),]
  }
  
  if ( is.numeric(max.num.cols) ) {
    img <- img[order(img$count, decreasing=T),]
    img <- img[1:max.num.cols,]
    img <- img[order(img$color),]
  }
  
  fillscale = rgb(t(col2rgb(img$color)), maxColorValue=255)
  #A better brown.
  fillscale[img$color=='brown'] = '#4c2722'
  #A darker grey.
  fillscale[img$color=='grey'] = '#888888'

  #Put a grey line around the white bar so we can see it.
  linescale = fillscale
  linescale[linescale=='#FFFFFF'] = '#999999'
  
  plt <- ggplot(img) + col_theme +
         ylab("Number of Pixels") +
         geom_bar(aes(color, count, fill=color,
                      colour=color), 
                  stat="identity") +
         scale_fill_manual(values=fillscale) +
         scale_colour_manual(values=linescale)
  
  plt
}