What is the good plan for the gradual change of CSS pictures?

how do I control the transparent gradient of the background image (not solid color) from top to bottom?

how do you make the following obvious boundary disappear and make the two seem to combine?

it doesn"t work even if I look it up a lot.

Jul.30,2021

does not quite understand the effect you want. It has been used before to add a layer of div on top of the picture as a mask, similar to
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