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Note: Why do we increase the color saturation? どうして　色の飽和を上げて色を出そうとするのか？
The increase of color saturation is attempted because the real color saturation was reduced due to the tone curve manipulation. In our case, we have two problems. First, when the RGB peaks are made close to each other, the saturation is altered. For example, if the night sky has a strong red component, reducing this component results in reducing the red color of the objects; stars and nebulae. Note the color of the stars is not altered by the　red glow of the night sky. In addition, the effect of the background light is negated by moving the left most point of the tone curve. This is equivalent to reducing the intensity by proportionally lowering the intensity. A proper processing needs to eliminate the intensity by subtraction while maintaining the degree of color saturation.