An interesting and simple algorithm to blend two images seamlessly.

TL;DR: do not copy-paste the pixel itself, copy-paste the gradient of the pixel, and then solve for the optimal pixel's value subjecting to an energy function, which takes into account the boundary of background image and the gradient of the source image.

More articles on the topic:

• https://www.ctralie.com/Teaching/PoissonImageEditing/
• http://graphics.cs.cmu.edu/courses/15-463/2019_fall/lectures/lecture9.pdf