recluster {recolorize}

Recluster color centers based on color similarity

Description

Color mapping (as with k-means or binning) often requires over-clustering in order to recover details in an image. This can result in larger areas of relatively uniform color being split into multiple colors, or in regions with greater variation (due to lighting, shape, reflection, etc) being split into multiple colors. This function clusters the color centers by visual similarity (in CIE Lab space), then returns the re-clustered object. Users can either set a similarity cutoff or a final number of colors. See examples.

Usage

recluster(
  recolorize_obj,
  dist_method = "euclidean",
  hclust_method = "complete",
  channels = 1:3,
  color_space = "Lab",
  ref_white = "D65",
  cutoff = 60,
  n_final = NULL,
  plot_hclust = TRUE,
  refit_method = "impose",
  resid = FALSE,
  plot_final = TRUE,
  color_space_fit = "sRGB"
)

Arguments

recolorize_obj

A recolorize object from recolorize, recluster, or imposeColors.

dist_method

Method passed to stats::dist for calculating distances between colors. One of "euclidean", "maximum", "manhattan", "canberra", "binary" or "minkowski".

hclust_method

Method passed to stats::hclust for clustering colors by similarity. One of "ward.D", "ward.D2", "single", "complete", "average" (= UPGMA), "mcquitty" (= WPGMA), "median" (= WPGMC) or "centroid" (= UPGMC).

channels

Numeric: which color channels to use for clustering. Probably some combination of 1, 2, and 3, e.g., to consider only luminance and blue-yellow (b-channel) distance in CIE Lab space, channels = c(1, 3) (L and b).

color_space

Color space in which to cluster centers, passed to grDevices{convertColor}. One of "sRGB", "Lab", or "Luv". Default is "Lab", a perceptually uniform (for humans) color space.

ref_white

Reference white for converting to different color spaces. D65 (the default) corresponds to standard daylight.

cutoff

Numeric similarity cutoff for grouping color centers together. The range and value will depend on the chosen color space (see below), but the default is in absolute Euclidean distance in CIE Lab space, which means it is greater than 0-100, but cutoff values between 20 and 80 will usually work best. See details.

n_final

Final number of desired colors; alternative to specifying a similarity cutoff. Overrides cutoff if provided.

plot_hclust

Logical. Plot the hierarchical clustering tree for color similarity? Helpful for troubleshooting a cutoff.

refit_method

Method for refitting the image with the new color centers. One of either "impose" or "merge". imposeColors refits the original image using the new colors (slow but often better results). mergeLayers merges the layers of the existing recolored image. This is faster since it doesn't require a new fit, but can produce messier results.

resid

Logical. Get final color fit residuals with colorResiduals?

plot_final

Logical. Plot the final color fit?

color_space_fit

Passed to imposeColors. What color space should the image be reclustered in?

Details

This function is fairly straightforward: the RGB color centers of the recolorize object are converted to CIE Lab color space (which is approximately perceptually uniform for human vision), clustered using hclust, then grouped using cutree. The resulting groups are then passed as the assigned color centers to imposeColors, which re-fits the original image using the new centers.

The similarity cutoff does not require the user to specify the final number of colors, unlike k-means or n_final, meaning that the same cutoff could be used for multiple images (with different numbers of colors) and produce relatively good fits. Because the cutoff is in absolute Euclidean distance in CIE Lab space for sRGB colors, the possible range of distances (and therefore cutoffs) is from 0 to >200. The higher the cutoff, the more dissimilar colors will be grouped together. There is no universally recommended cutoff; the same degree of color variation due to lighting in one image might be biologically relevant in another.

Value

A recolorize object with the re-fit color centers.

Examples

# get an image
corbetti <- system.file("extdata/corbetti.png", package = "recolorize")

# too many color centers
recolored_corbetti <- recolorize(corbetti, bins = 2)

# just enough!
# check previous plot for clustering cutoff
recluster_obj <- recluster(recolored_corbetti,
                           cutoff = 45,
                           plot_hclust = TRUE,
                           refit_method = "impose")

# we get the same result by specifying n_final = 5
recluster_obj <- recluster(recolored_corbetti,
                           n_final = 5,
                           plot_hclust = TRUE)