I need to cut this image into three parts using PIL and pick the middle part.

How do I do it?

解决方案

If the boxes are not known on before hand I would run a simple edge finding filter over the image (both x and y directions) to find the boundaries of the box.

A simple approach would be:

Run horizontal edge filter over image. You now have an image where each pixel describes the changes in intensity left and right of that pixel. I.e. it will "find" vertical lines.

For each column in the horizontal-edge-image get the average absolute magnitude of its rows. In the resulting 1 x WIDTH sized array you will find the vertical lines at the positions of highest value. Since the lines are more than one pixel wide yo might have to be a bit clever here.

Do the same for the other axis to find the horizontal lines.

You could do some pre processing by first extracting only pixels that are black (or near black) if you believe that the borders of the boxes will always be black. But I doubt it'd be necessary since the above method should be very stable.