Reduce Dimensions of PNG Image using PCA

ReduceDimsPNG.R

library(png)
library(abind)

ProjectImage <- function(prcomp.obj, pc.num) {
    # project image onto n principal components
    scores <- prcomp.obj$x
    loadings <- prcomp.obj$rotation
    img.proj <- scores[, c(1:pc.num)] %*% t(loadings[, c(1:pc.num)])
    return(img.proj)
}

SplitImage <- function(rgb.array) {
    # decompose image into RGB elements
    rgb.list <- list()
    for (i in 1:dim(rgb.array)[3]) {
        rgb.list[[i]] <- rgb.array[, , i]
    }
    return(rgb.list)
}

ReduceDimsPNG <- function(png.file, pc.num, display.only=TRUE) {
    # reduce dimensions of PNG image
    rgb.array <- readPNG(png.file)
    rgb.list <- SplitImage(rgb.array)
    # apply pca and reproject onto new principal components
    rgb.list <- lapply(rgb.list, prcomp, center=FALSE)
    rgb.proj <- lapply(rgb.list, ProjectImage, pc.num=pc.num)
    # restore original dimensions
    restored.img <- abind(rgb.proj, along=3)
    
    filename <- sprintf("reduced_pc%s_%s", pc.num, png.file)
    writePNG(restored.img, filename)
    grid.raster(readPNG(filename, native=TRUE))
    if (display.only){
        file.remove(filename)
    }
}