public static float luma( Color color ) {
		// see https://en.wikipedia.org/wiki/Luma_(video)
		// see https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
		// see https://github.com/less/less.js/blob/master/packages/less/src/less/tree/color.js
		float r = gammaCorrection( color.getRed() / 255f );
		float g = gammaCorrection( color.getGreen() / 255f );
		float b = gammaCorrection( color.getBlue() / 255f );
		return (0.2126f * r) + (0.7152f * g) + (0.0722f * b);
	}
@Test
	void luma() {
		assertEquals( 0, ColorFunctions.luma( Color.black ) );
		assertEquals( 1, ColorFunctions.luma( Color.white ) );

		assertEquals( 0.2126f, ColorFunctions.luma( Color.red ) );
		assertEquals( 0.7152f, ColorFunctions.luma( Color.green ) );
		assertEquals( 0.0722f, ColorFunctions.luma( Color.blue ) );

		assertEquals( 0.9278f, ColorFunctions.luma( Color.yellow ) );
		assertEquals( 0.7874f, ColorFunctions.luma( Color.cyan ) );

		assertEquals( 0.051269464f, ColorFunctions.luma( Color.darkGray ) );
		assertEquals( 0.21586052f, ColorFunctions.luma( Color.gray ) );
		assertEquals( 0.52711517f, ColorFunctions.luma( Color.lightGray ) );
	}