Histogram

histogram returns the distribution of channel values across all the pixels of an image. It is available on any image.

val hist = image.histogram()

The returned Histogram exposes one array per channel. Each is a 256-element long[] where index i holds the number of pixels whose value in that channel is exactly i:

val red: LongArray = hist.red()
val green: LongArray = hist.green()
val blue: LongArray = hist.blue()
val alpha: LongArray = hist.alpha()

There is also a luminance() distribution, computed per pixel from the Rec. 709 weights (0.2126*R + 0.7152*G + 0.0722*B). Unlike the individual channels, its counts sum to the total number of pixels in the image:

val luminance: LongArray = hist.luminance()

For example, to find how many pixels are fully black in the blue channel:

val fullyBlackBlue = image.histogram().blue()[0]

Note

Each getter returns a fresh copy of the underlying counts, so mutating the returned array does not affect the histogram or subsequent calls.