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（一）图像非形变操作

>>>>>>Python代码：

import os
import cv2
import numpy as np
import time
# 非形变图像处理函数
def letterbox(img,height = 416,augment = False, color = (127.5,127.5,127.5)):
    # Resize a rectangular image to a padded square
    shape = img.shape[:2] # shape = [height,width]
    ratio = float(height) / max(shape) # ratio = old / new 得到缩放比例
    new_shape = (round(shape[1] * ratio),round(shape[0] * ratio))
    dw = (height - new_shape[0]) / 2 # width padding
    dh = (height - new_shape[1]) / 2 # height padding
    top,bottom = round(dh - 0.1),round(dh + 0.1)
    left,right = round(dw - 0.1),round(dw + 0.1)
    # resize img
    if augment: # 数据增强
        interpolation = np.random.choice([None,cv2.INTER_NEAREST,cv2.INTER_LINEAR,
                                          None,cv2.INTER_NEAREST,cv2.INTER_LINEAR,
                                          cv2.INTER_AREA,cv2.INTER_CUBIC,cv2.INTER_LANCZOS4])

        if interpolation is None:
            img = cv2.resize(img,new_shape)
        else:
            img = cv2.resize(img,new_shape,interpolation = interpolation)
    else:
        img = cv2.resize(img,new_shape,interpolation=cv2.INTER_NEAREST)
    # print("resize time:",time.time()-s1)

    img = cv2.copyMakeBorder(img,top,bottom,left,right,cv2.BORDER_CONSTANT,value = color) # padded square
    return img,ratio,dw,dh

if __name__ == '__main__':
    path = './images/' # 图片路径
    img_size = 416 # 生成非形变图像的最终的尺寸，长和宽都是相等的
    for f_ in os.listdir(path):# 遍历文件夹中的图片
        img_o = cv2.imread(path+f_) # 加载图片文件
        img,_,_,_ = letterbox(img_o,height=img_size) # 非形变图像处理
        print("------------------->>>>")
        print("img_o shape:",img_o.shape)
        print("img shape:",img.shape)
        cv2.namedWindow("img_o",0) # 图像窗口的申明，参数: 窗口的尺寸可以通过鼠标去拉取改变，参数1:则固定为原尺寸，不可改变
        cv2.imshow("img_o",img_o)
        cv2.namedWindow("img",0)
        cv2.imshow("img",img)
        cv2.waitKey(0)

实验效果：

原图                                   补边图

 

           原图                                                    补边图

（二）图像归一化：

img = img/255. 

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