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Visual Response Equivalent Gray
The Visual Response Equivalent Gray ("VREI") is a single number that represents the equivalent "gray" luminance of a 3-component color. The VREI is computed from an RGB triplet as the dot product of a standard weight vector (0.292, 0.594, 0.114) and the luminance values of the R, G, and B components.
Suppose an orange color is specified as (255,140,5). The resulting gray intensity is 158.19. If this gray value were assigned to the pixel of an image of integer data type, the pixel value would be truncated to 158.