1. px: pixel, pixel, the most basic unit of a picture or photo on an electronic screen
2. pt: point, point, commonly used unit in the printing industry, equal to 1/72 inch
3. ppi: pixel per inch, the number of pixels per inch, the higher the value, the clearer the image, 1 inch=2.54cm
4. dpi: Density-independent pixel, the higher the value, the clearer the image
5. dp: dip, Density-independent pixel, is the length unit used for Android development, 1dp means 1px length when the screen pixel density is 160ppi