Looking at the histogram of the picture with PS, there are many sharp thorns, like hedgehog thorns ... what is the reason?

Tone separation will occur when there is a sword stab in the histogram, or the middle is discontinuous and the neutral reaction directly occurs.

Theoretically, the human eye can perceive twice the color of RGB color system, but because the human eye is an analog system, it is difficult to quantify the exact differences between colors (it varies from person to person, and some are only sensory differences). Histogram is a method of quantifying colors and displaying them through icons. The horizontal axis of histogram is brightness and the vertical axis is quantity (usually in linear coordinate system). So histograms are often divided into RGB histograms and brightness histograms. The horizontal axis in the brightness histogram describes the comprehensive brightness (B in the HSB color system or L in the Lab system), while the RGB histogram is composed of three histograms of R, G and B, and each histogram represents the brightness of R, G and B respectively (that is, 0-255). According to the brightness of the histogram, the integral of the histogram curve (the area of the lower part of the curve) is 1. Brightness histogram can be used to monitor whether there are highlights and shadows overflowing in the picture (that is, colors exceeding 255, 255 and 255 will not be displayed, but will be uniformly identified as 255, 255 and 255). 0, 0, below 0 can't be represented, so it is marked as 0, 0, 0. There will be fading). At the same time, you can also monitor whether the picture is too bright (bending to the right) or too dark (bending to the left). Note: Generally speaking, the right direction of the horizontal axis of the histogram is the positive direction of brightness. The function of color histogram (RGB) is mainly used to observe whether the picture has color cast. By definition, if the picture is a grayscale image (without any color), then the three curves of RGB should overlap. If the picture is red, the red curve is farther to the right than the other two curves, and the white balance or the tone of the whole picture can be preliminarily distinguished.

Similarly, by definition, if the color or brightness in the picture is discontinuous, then the horizontal axis discontinuity will appear in the histogram. On the other hand, if there are discontinuous curves, it means that the color of the original picture is discontinuous. There are many reasons for discontinuity, such as over-adjustment of curves (nonlinear mapping of local colors), over-compression of image quality (JPG), using non-true color image formats such as GIF, mosaic of images, etc ... But the final result is a decrease in the number of colors and a decline in image quality.