图片按照原来比例进行缩放

import cv2

# 按照原来的h/w比例进行缩放，周围用color值进行填充
def letterbox(im, new_shape=(224, 640), color=(0, 0, 0)):
    # 获取h/w
    shape = im.shape[:2]  # current shape [height, width]
    if isinstance(new_shape, int):
        new_shape = (new_shape, new_shape)

    # 获取new/old缩放比例
    r = min(new_shape[0] / shape[0], new_shape[1] / shape[1])

    # Compute padding
    ratio = r, r  # width, height ratios

    # 缩放的h/w
    new_unpad = int(round(shape[1] * r)), int(round(shape[0] * r))

    # h/w的补充大小
    dw, dh = new_shape[1] - new_unpad[0], new_shape[0] - new_unpad[1]  # wh padding

    dw /= 2  # divide padding into 2 sides
    dh /= 2

    # 缩放大小跟原来大小不相等，则进行缩放
    if shape[::-1] != new_unpad:  # resize
        im = cv2.resize(im, new_unpad, interpolation=cv2.INTER_LINEAR)
    top, bottom = int(round(dh - 0.1)), int(round(dh + 0.1))
    left, right = int(round(dw - 0.1)), int(round(dw + 0.1))

    # 对缩放之后的图片进行前后左右进行补充
    im = cv2.copyMakeBorder(im, top, bottom, left, right, cv2.BORDER_CONSTANT, value=color)
    return im


if __name__ == '__main__':
    img = cv2.imread(r"172.png", -1)
    result = letterbox(img)
    cv2.imwrite(r"172.png", result)