convertColorSpace: Convert between color spaces

In colordistance: Distance Metrics for Image Color Similarity

Description

Wrapper for convertColor that builds in random sampling, error messages, and removes default illuminant (D65) to enforce manual specification of a reference white.

Usage

convertColorSpace(
  color.coordinate.matrix,
  from = "sRGB",
  to = "Lab",
  sample.size = 1e+05,
  from.ref.white,
  to.ref.white
)

Arguments

color.coordinate.matrix	A color coordinate matrix with rows as colors and channels as columns. If a color histogram (e.g. as returned by getImageHist) is passed, the 'Pct' column is ignored.
from, to	Input and output color spaces, passed to convertColor. See details.
sample.size	Number of pixels to be randomly sampled from filtered pixel array for conversion. If not numeric or larger than number of colors provided (i.e. cluster matrix), all colors are converted. See details.
from.ref.white, to.ref.white	Reference whites passed to convertColor. Unlike convertColor, no default is provided. See details for explanation of different reference whites.

Details

Color spaces are all passed to convertColor, and can be any of: "XYZ", "sRGB", "Apple RGB", "CIE RGB", "Lab", or "Luv".

Lab and Luv color spaces are approximately perceptually uniform, meaning they usually do the best job of reflecting intuitive color distances without the non-linearity problems of more familiar RGB spaces. However, because they describe object colors, they require a reference 'white light' color (dimly and brightly lit photographs of the same object will have very different RGB palettes, but similar Lab palettes if appropriate white references are used). The idea here is that the apparent colors in an image depend not just on the "absolute" color of an object, but also on the available light in the scene. There are seven CIE standardized illuminants available in colordistance (A, B, C, E, and D50, D55, and D65), but the most common are:

• "A": Standard incandescent lightbulb

• "D65": Average daylight

• "D50": Direct sunlight

Color conversions will be highly dependent on the reference white used, which is why no default is provided. Users should look into standard illuminants to choose an appropriate reference for a dataset.

The conversion from RGB to a standardized color space (XYZ, Lab, or Luv) is approximate, non-linear, and relatively time-consuming. Converting a large number of pixels can be computationally expensive, so convertColorSpace will randomly sample a specified number of rows to reduce the time. The default sample size, 100,000 rows, takes about 5 seconds convert from sRGB to Lab space on an early 2015 Macbook with 8 GB of RAM. Time scales about linearly with number of rows converted.

Value

A 3- or 4-column matrix depending on whether color.coordinate.matrix included a 'Pct' column (as from getImageHist), with one column per channel.

Examples

# Convert a single RGB triplet and then back convert it
rgb_color <- c(0, 1, 0)
lab_color <- colordistance::convertColorSpace(rgb_color,
 from="sRGB", to="Lab", to.ref.white="D65")
rgb_again <- colordistance::convertColorSpace(lab_color,
 from="Lab", to="sRGB", from.ref.white="D65")

# Convert pixels from loadImage() function
img <- colordistance::loadImage(system.file("extdata",
"Heliconius/Heliconius_B/Heliconius_07.jpeg", package="colordistance"))
lab_pixels <- colordistance::convertColorSpace(img$filtered.rgb.2d,
from="sRGB", to="XYZ", sample.size=5000)

# Convert clusters
img <- colordistance::loadImage(system.file("extdata",
"Heliconius/Heliconius_B/Heliconius_07.jpeg", package="colordistance"))
img_hist <- colordistance::getImageHist(img, bins=2, plotting=FALSE)
lab_clusters <- colordistance::convertColorSpace(img_hist, to.ref.white="D55")

colordistance documentation built on March 21, 2021, 1:06 a.m.