1. I didn't paint the gradation by myself. From the left state, it will be in the right state with one swing. That is the contour line painting (normal painting).
2. Contour line painting (normal painting) is a bucket that creates a gradation between colors.
3. Contour line painting (normal painting) is in the gradation category of the initial tools. The tool name is "Normal Fill" instead of "Contour Line Fill", so it's not intuitive to understand the function, but it's convenient to use.