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  • Musharraf Azeem thinks this is interesting:

luminance value based on the red component of an input color

From

Cover of OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Note

Used when we have an 8-bit grayscale scale stored in the first component of color:
if 8-bit grayscale level is interpreted as GL_LUMINANCE, then:
vec4 color = vec4(color.r, color.r, color.r, 1);
if its treated as an intensity value:
vec4 color = vec4(color.r, color.r, color.r, color.r);