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A character vector for tells the raster function that extract the signal via rgb stack, default nothing means just extract the raster data via the image pixel its brightness value, otherwise this parameter should be a character of of value combination of chars: r, g and b. example as: + rgb.stack = ['r'] means just extract the red channel as the raster data + rgb.stack = ['g', 'b'] means extract the raster data via green and blue channel, the raster scale value will be evaluated as g * 10 + b andalso this parameter value could be a .net clr color height map ruler object, which could be used for mapping a color sequence to a scale level.