The channels argument in eQuimageLab

Many operations in eQuimageLab can be applied to specific channels specified by the channels argument (also see Channels). channels can be:

  • An empty string: Apply the operation to all channels.

  • A combination of “1”, “2”, “3” (or equivalently “R”, “G”, “B” for RGB images): Apply the operation to the first/second/third channel.

  • “V”: Apply the operation to the HSV value (RGB, HSV and grayscale images).

  • “S”: Apply the operation to the HSV saturation (RGB and HSV images).

  • “L’”: Apply the operation to the HSL lightness (RGB, HSL and grayscale images).

  • “S’”: Apply the operation to the HSL saturation (RGB and HSL images).

  • “L”: Apply the operation to the luma (RGB and grayscale images).

  • “Ls”: Apply the operation to the luma, and protect highlights by desaturation (see below).

  • “Lb”: Apply the operation to the luma, and protect highlights by blending (see below).

  • “Ln”: Apply the operation to the luma, and protect highlights by normalization (see below).

  • “L*”: Apply the operation to the CIE lightness \(L^*\) (CIELab and CIELuv images; equivalent to “L*ab” for lRGB and sRGB images).

  • “L*ab”: Apply the operation to the CIE lightness \(L^*\) in the CIELab/Lab color space and model (CIELab, lRGB and sRGB images).

  • “L*uv”: Apply the operation to the CIE lightness \(L^*\) in the CIELuv/Luv color space and model (CIELuv, lRGB and sRGB images).

  • “L*sh”: Apply the operation to the CIE lightness \(L^*\) in the CIELuv/Lsh color space and model (CIELuv, lRGB and sRGB images).

  • “c*”: Apply the operation to the CIE chroma c* (CIELab and CIELuv images).

  • “s*”: Apply the operation to the CIE saturation s* (CIELuv images).

Note on “Ls” and “Lb”:

When applying an operation f to the luma, the RGB components of the image are rescaled by the ratio f (luma)/luma. This preserves the hue and HSV saturation, but may result in out-of-range RGB components even though f(luma) fits within [0, 1]. These out-of-range components can be regularized with three highlights protection methods:

  • “Desaturation”: The out-of-range pixels are desaturated at constant hue and luma (namely, the out-of-range components are decreased while the in-range components are increased so that the hue and luma are preserved). This tends to bleach the out-of-range pixels. f(luma) must fit within [0, 1] to make use of this highlights protection method.

  • “Blending”: The out-of-range pixels are blended with f(RGB) (the same operation applied to the RGB channels). This tends to bleach the out-of-range pixels too. f(RGB) must fit within [0, 1] to make use of this highlights protection method.

  • “Normalization”: The whole output image is rescaled so that all pixels fall back in the [0, 1] range (output → output/max(1., numpy.max(output))).