[raster] St_Grayscale

[nclay]

Returns a single band 8BUI representation of a RGB raster.

[robe]

Milestone renamed

[dustymugs]

This can be written as a set of MapAlgebra callback functions (one per particular algorithm) and a ST_Grayscale user (wrapper) function.

Basic implementation should just use the following:

Y = 0.2989 * R + 0.5870 * G + 0.1140 * B

Map Algebra callback function

ST_Grayscale4ma(double precision[][][] value, integer[][] pos, text[] VARIADIC userargs)

User function

ST_Grayscale(raster rast, integer redband=1, integer blueband=2, integer greenband=3)

Bands provided should be preprocessed before grayscaling to make sure input pixel types are 8BUI (via ST_Reclass if necessary)

[dustymugs]

Equation from algorithm section of ​https://www.mathworks.com/help/matlab/ref/rgb2gray.html

[dustymugs]

Done as of r16207