Select hair with Color Order

Today, I processed a photo with a white background. It is found that the hair processing is still not good, especially the hair on the edge, mixed with the white background, a single hair is not pure black, no matter what method, the selected items are white. I am not satisfied with the effort. The tutorials on the Internet are processed by color order, but they are always unsatisfactory. With the extract function, the effect is still good, but it is not too flexible.
It should be easy to think of it. The first hair and background are clearly visible, that is, it cannot be broken down. Why? It is difficult to distinguish between the source and the background. In addition, the PS selection function supports selecting a percentage, instead of selecting hair completely.
In the afternoon, I calmed down and turned out my PS book and studied the color order.
The color level is a display that is distributed according to the brightness. The left side is dark and the right side is bright. Black and white triangles, determine a range, the outer is full black or full White, the middle of the transition from pure black to pure white. The gray triangle controls the transition speed.
For color level processing, vertices out of the range become uniform black or white, and the change in the center is a proportional change that will not suddenly become a brightness. So keep the details (however, I just want to discard the details when dealing with the hair)
Second, we use the blue channel for processing. The original idea was to completely separate black and white and remove black. However, the effect is not ideal, because black and white will be magnified if black and white are to be absolute. The correct method is to keep the hair details and do not change it to solid color. Into a constituency. To put it simply, it is to turn the hair into white or gray-white after the reversal, and the other parts into non-pure black. Then, adjust the color order to make the other parts black. This selection area removes the white background and retains the details of the hair.
The third key point is the use of constituency. If you use a selection area to retrieve the source image, a lot of white will be obtained. Therefore, a reasonable method is to fill the selection area with black or black, which is close to the hair color.