How to do the brightness contrast 9 tone map?

How to make a 9-tone chart of lightness contrast;

Do three groups of high brightness, medium brightness and low brightness. The brightness of each group is divided into nine comparative exercises: long tone, middle tone and short tone.

Brightness is the perception of the light source and the surface of the object by the eyes, which is mainly a visual experience determined by the intensity of light. Generally speaking, the stronger the light, the brighter it looks; The weaker the light, the darker it looks.

Brightness also refers to the brightness of colors. Colors with the same hue may have different hues. For example, both magenta and pink contain red, but the former is dark and the latter is bright.

RGB lightness calculation formula: L=(2.2 times) √ [(r/255) 2.2+(1.5g/255) 2.2+(0.6b/255) 2.2]/(1+1.5. The range of l is 0~ 1.

Note that the 2.2 times and 2.2 times root and RGB color values here cannot be simply added directly, and must be converted into physical optical power by 2.2 times. Because the relationship between RGB value and power is not a simple linear relationship, but a power function relationship, the exponent of this function is called Gamma value, generally 2.2, and this conversion process is called Gamma correction.