Why can you see the pixel grid with serious jagged edge in the filling color of PS selection, and how can it be delicate?

In PS, the edges of the selection fill color are jagged because the pixels of the document are too low. If you want to be fine and not jagged, the way is to enlarge the pixels.

Pixel is the basic unit of a digital image, and the resolution of the image is usually expressed in pixels per inch (PPI). For example, a resolution of 300x300PPI means that the number of pixels per inch is 300 both horizontally and vertically, or it can mean that there are 90,000 (300x300) pixels per square inch. If you enlarge the image several times, you will find that these continuous tones are actually composed of many small squares with similar colors, which are the smallest unit of the image-pixels.

In PS, when the pixels of the document are too low, the selection will be jagged, as shown in the following figure:

A pixel is a square point. When the edge of the image is vertical or parallel, there will be no sawtooth, but when it is inclined, there will be sawtooth.

Under any pixel condition, there will be sawtooth on the hypotenuse, but when the pixel is enlarged to a certain extent, the sawtooth is too small to be obtained by the naked eye, so there is no sawtooth.

Therefore, enlarging pixels is the only way to solve aliasing.