downsampling for difference selected areas with different sizes but keep size of sampled image the same

As the images shown， I want to do the downsampling for the orange area, I have calculated the coordinates for four corner points of the center square.

How to do the downsampling for the orange squares to get same size of sampled images? Is there any good API or function to do this?

I hope the downsampling could work as a kind of convlolution pooling ,which means that for a downsamling subarea, we calculate the average or max value of that subarea as the following image shows.