How can I stretch images while preserving corners using ImageMagick?

Question

I want to stretch images, while preserving 4 corners intact. Similar stuff in iOS is setting capInsets, for UIImage.

Answer 1

Imagine a hash symbol (tic-tac-toe grid) on your image, with the corner squares being the part you don't want to distort.

1. Divide your image into 3 subimages at the vertical lines with the -crop option.
2. Stretch (or compress) the middle image horizontally.
3. Reassemble the 3 parts into one image (+append)
4. Divide the image into 3 subimages at the horizontal lines with -crop.
5. Stretch the middle piece vertically.
6. Reassemble (-append).