Android 12 Developer Preview is here! Try it out, and give us your feedback!

Rgb

class Rgb : ColorSpace
kotlin.Any
   ↳ androidx.compose.ui.graphics.colorspace.ColorSpace
   ↳ androidx.compose.ui.graphics.colorspace.Rgb

An RGB color space is an additive color space using the RGB color model (a color is therefore represented by a tuple of 3 numbers).

A specific RGB color space is defined by the following properties:

  • Three chromaticities of the red, green and blue primaries, which define the gamut of the color space.
  • A white point chromaticity that defines the stimulus to which color space values are normalized (also just called "white").
  • An opto-electronic transfer function, also called opto-electronic conversion function or often, and approximately, gamma function.
  • An electro-optical transfer function, also called electo-optical conversion function or often, and approximately, gamma function.
  • A range of valid RGB values (most commonly [0..1]).

The most commonly used RGB color space is sRGB.

Primaries and white point chromaticities

In this implementation, the chromaticity of the primaries and the white point of an RGB color space is defined in the CIE xyY color space. This color space separates the chromaticity of a color, the x and y components, and its luminance, the Y component. Since the primaries and the white point have full brightness, the Y component is assumed to be 1 and only the x and y components are needed to encode them.

For convenience, this implementation also allows to define the primaries and white point in the CIE XYZ space. The tristimulus XYZ values are internally converted to xyY.

sRGB primaries and white point

Transfer functions

A transfer function is a color component conversion function, defined as a single variable, monotonic mathematical function. It is applied to each individual component of a color. They are used to perform the mapping between linear tristimulus values and non-linear electronic signal value.

The opto-electronic transfer function (OETF or OECF) encodes tristimulus values in a scene to a non-linear electronic signal value. An OETF is often expressed as a power function with an exponent between 0.38 and 0.55 (the reciprocal of 1.8 to 2.6).

The electro-optical transfer function (EOTF or EOCF) decodes a non-linear electronic signal value to a tristimulus value at the display. An EOTF is often expressed as a power function with an exponent between 1.8 and 2.6.

Transfer functions are used as a compression scheme. For instance, linear sRGB values would normally require 11 to 12 bits of precision to store all values that can be perceived by the human eye. When encoding sRGB values using the appropriate OETF (see sRGB for an exact mathematical description of that OETF), the values can be compressed to only 8 bits precision.

When manipulating RGB values, particularly sRGB values, it is safe to assume that these values have been encoded with the appropriate OETF (unless noted otherwise). Encoded values are often said to be in "gamma space". They are therefore defined in a non-linear space. This in turns means that any linear operation applied to these values is going to yield mathematically incorrect results (any linear interpolation such as gradient generation for instance, most image processing functions such as blurs, etc.).

To properly process encoded RGB values you must first apply the EOTF to decode the value into linear space. After processing, the RGB value must be encoded back to non-linear ("gamma") space. Here is a formal description of the process, where f is the processing function to apply:

See RGB equation

If the transfer functions of the color space can be expressed as an ICC parametric curve as defined in ICC.1:2004-10, the numeric parameters can be retrieved from transferParameters. This can be useful to match color spaces for instance.

Some RGB color spaces, such as ColorSpaces.Aces and scRGB, are said to be linear because their transfer functions are the identity function: f(x) = x. If the source and/or destination are known to be linear, it is not necessary to invoke the transfer functions.

Range

Most RGB color spaces allow RGB values in the range [0..1]. There are however a few RGB color spaces that allow much larger ranges. For instance, scRGB is used to manipulate the range [-0.5..7.5] while ACES can be used throughout the range [-65504, 65504].

Extended sRGB and its large range

Converting between RGB color spaces

Conversion between two color spaces is achieved by using an intermediate color space called the profile connection space (PCS). The PCS used by this implementation is CIE XYZ. The conversion operation is defined as such:

See RGB equation

Where Tsrc is the RGB to XYZ transform of the source color space and Tdst^-1 the XYZ to RGB transform of the destination color space.

Many RGB color spaces commonly used with electronic devices use the standard illuminant D65. Care must be take however when converting between two RGB color spaces if their white points do not match. This can be achieved by either calling adapt to adapt one or both color spaces to a single common white point. This can be achieved automatically by calling ColorSpace.connect, which also handles non-RGB color spaces.

To learn more about the white point adaptation process, refer to the documentation of Adaptation.

Summary

Public constructors
<init>(name: String, toXYZ: FloatArray, oetf: (Double) -> Double, eotf: (Double) -> Double)

Creates a new RGB color space using a 3x3 column-major transform matrix.

<init>(name: String, primaries: FloatArray, whitePoint: WhitePoint, oetf: (Double) -> Double, eotf: (Double) -> Double, min: Float, max: Float)

Creates a new RGB color space using a specified set of primaries and a specified white point.

<init>(name: String, toXYZ: FloatArray, function: TransferParameters)

Creates a new RGB color space using a 3x3 column-major transform matrix.

<init>(name: String, primaries: FloatArray, whitePoint: WhitePoint, function: TransferParameters)

Creates a new RGB color space using a specified set of primaries and a specified white point.

<init>(name: String, toXYZ: FloatArray, gamma: Double)

Creates a new RGB color space using a 3x3 column-major transform matrix.

<init>(name: String, primaries: FloatArray, whitePoint: WhitePoint, gamma: Double)

Creates a new RGB color space using a specified set of primaries and a specified white point.

Public methods