Detect and extract images surrounded by a frame

I want to get the following result images from the input image. The resulting images are surrounded by the frame which has the same border size and type, but the border rectangle size is not same. Are there any ways to do that? I think I need to detect the area surrounded by the border as the first step. But no idea. I'm trying to find it in ImageMagick.

• Input image (input.png)

• Result image (output1.png)

• Result image (output2.png)

• Border

Update 1

This is not perfect but it worked with OpenCV as below.

import cv2 as cv

def main():
    image_file = '/path/to/your/input/image.png'
    src = cv.imread(image_file, cv.IMREAD_COLOR)
    height, width, channels = src.shape
    image_size = height * width
    img_gray = cv.cvtColor(src, cv.COLOR_RGB2GRAY)
    retval, dst = cv.threshold(img_gray, 1000, 255, cv.THRESH_TOZERO_INV)
    dst = cv.bitwise_not(dst)
    retval, dst = cv.threshold(dst, 0, 255, cv.THRESH_BINARY | cv.THRESH_OTSU)
    dst, contours, hierarchy = cv.findContours(
        dst, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

    xxx = 0
    for i, contour in enumerate(contours):
        area = cv.contourArea(contour)
        if area < 50000:
            continue
        if image_size * 0.99 < area:
            continue
        if abs(i - xxx) < 10:
            continue
        xxx = i
        x, y, w, h = cv.boundingRect(contour)
        cut = src[y:y+h, x:x+w]
        detector = cv.FastFeatureDetector_create()
        detector.setNonmaxSuppression(False)
        keypoints = detector.detect(cut)
        cv.imwrite('debug_%d.png' % i, cut)

if __name__ == '__main__':
    main()

Refer from this site:https://angular.io/guide/providers

Update 2

fmw42's way is great but it is not sufficient for my requirement as the following. (I did not mention in the first post) The only blue rectangle is extracted. It it possible that the background color is white.

• Input image (input2.png)

• Actual result image (output.png)