ImageMagick get image side only - not a edge detection problem

Question

This is not a finding edge problem but I am trying to find the background color of the image, in order to find it, I am trying to get the edge of the image and guess the best color.

In order to do so, I want to remove the center of the image and just get image like a frame with transparent center.

Is it possible through ImageMagick?

For example

If want

Answer 1

Try this tested on Windows with IM V7:

magick "input.png" ( -size 200x200 xc:black ) -gravity center -compose DstOut -composite "output.png"

Takes the original photo, creates a rectangular mask, places it over the middle of the image and "cuts" out the rectangle

If the input is not a png ( for instance a jpg ) you will have to do some work setting the alpha channel

EDIT: This seems to work for both jpg and png images

magick "input.png" -alpha on ( -size 200x200 xc:black ) -gravity center -compose DstOut -composite "output.png"