Table 4 List of theoretical and empirical RGB image transformations.

Original

Red

R

R channel from RGB color space

Raw values were adjusted by contrast stretching. Values ranged from 0 to 255.

¹⁷

Green

G

G channel from RGB color space

Blue

B

B channel from RGB color space

Theoretical transformation

X

X

X channel from CIE 1931 XYZ color space

CIE 1931 2° Standard Observer; CIE Standard Illuminant D65

⁷⁷

Y

Y

Y channel from CIE 1931 XYZ color space

Z

Z

Z channel from CIE 1931 XYZ color space

L-star

L^*

L^* channel from CIE 1976 L^*a^*b^* color space

CIE Standard Illuminant D65

¹⁷

a-star

a^*

a^* channel from CIE 1976 L^*a^*b^* color space

b-star

b^*

b^* channel from CIE 1976 L^*a^*b^* color space

Hue

H

H channel from HSI color space

^{17, 78}

Saturation

S

S channel from HSI color space

Intensity

I

I channel from HSI color space

Y-prime

Y’

Y’ channel from Y’CbCr color space

⁷⁹

Cb

Cb

Cb channel from Y’CbCr color space

Cr

Cr

Cr channel from Y’CbCr color space

Empirical transformation

Normalized red

NR

\({\rm{NR}}=\frac{{\rm{R}}}{{\rm{R}}+{\rm{G}}+{\rm{B}}}\)

Equations simplified. Abbreviations also known as r, g, b.

²⁶

Normalized green

NG

\({\rm{NG}}=\frac{{\rm{G}}}{{\rm{R}}+{\rm{G}}+{\rm{B}}}\)

Normalized blue

NB

\({\rm{NB}}=\frac{{\rm{B}}}{{\rm{R}}+{\rm{G}}+{\rm{B}}}\)

Excess red

ExR

\({\rm{ExR}}=\frac{{\rm{1}}{\rm{.4R}}-{\rm{G}}}{{\rm{R}}+{\rm{G}}+{\rm{B}}}\)

Equation simplified.

⁸⁰

Excess blue

ExB

\({\rm{ExB}}=\frac{{\rm{1}}{\rm{.4B}}-{\rm{G}}}{{\rm{R}}+{\rm{G}}+{\rm{B}}}\)

Equation simplified.

⁸¹

Excess green red

ExGR

\({\rm{ExGR}}=\frac{{\rm{3G}}-{\rm{2}}{\rm{.4R}}-{\rm{B}}}{{\rm{R}}+{\rm{G}}+{\rm{B}}}\)

Equation simplified.

⁸²

Green blue difference

GBD

\({\rm{GBD}}={\rm{G}}-{\rm{B}}\)

⁸³

Red blue difference

RBD

\({\rm{RBD}}={\rm{R}}-{\rm{B}}\)

Red green difference

RGD

\({\rm{RGD}}={\rm{R}}-{\rm{G}}\)

Green red ratio

GRR

\({\rm{GRR}}=\frac{{\rm{G}}}{{\rm{R}}}\)

^{14, 84}

Green blue ratio

GBR

\({\rm{GBR}}=\frac{{\rm{G}}}{{\rm{B}}}\)

⁸³

Normalized green red difference

NGRD

\({\rm{NGRD}}=\frac{{\rm{G}}-{\rm{R}}}{{\rm{G}}+{\rm{R}}}\)

Also known as normalized difference index (NDI) or green red vegetation index (GRVI).

^{11, 15}

Normalized green blue difference

NGBD

\({\rm{NGBD}}=\frac{{\rm{G}}-{\rm{B}}}{{\rm{G}}+{\rm{B}}}\)

^{84, 85}

Modified normalized green red difference

MNGRD

\({\rm{MNGRD}}=\frac{{{\rm{G}}}^{2}-{{\rm{R}}}^{2}}{{{\rm{G}}}^{2}+{{\rm{R}}}^{2}}\)

Also known as modified green red vegetation index (MGRVI).

⁸⁶

Visible band difference

VD

\({\rm{VD}}=\frac{{\rm{2G}}-{\rm{B}}-{\rm{R}}}{{\rm{2G}}+{\rm{B}}+{\rm{R}}}\)

Also known as green leaf index (GLI).

^{87, 88}

Red green blue vegetation index

RGBVI

\({\rm{RGBVI}}=\frac{{{\rm{G}}}^{2}-{\rm{B}}\times {\rm{R}}}{{{\rm{G}}}^{2}+{\rm{B}}\times {\rm{R}}}\)

⁸⁶

Crust index

CI

\({\rm{CI}}=\frac{{\rm{2B}}}{{\rm{R}}+{\rm{B}}}\)

Equation simplified.

⁸³

Color index of vegetation extraction

CIVE

\({\rm{CIVE}}={\rm{0}}{\rm{.441R}}-{\rm{0}}{\rm{.811G}}+{\rm{0}}{\rm{.385B}}+{\rm{18.78745}}\)

⁸⁹

Triangular greenness index

TGI

\({\rm{TGI}}={\rm{95G}}-{\rm{35R}}-{\rm{60B}}\)

Equation simplified.

¹⁶

Modified excess green

MExG

\(\mathrm{MExG}\,=1{\rm{.262G}}-{\rm{0}}{\rm{.884R}}-{\rm{0}}{\rm{.311B}}\)

⁹⁰