Yolo V5 V7 前处理

```
import cv2
import numpy as np

def letterbox(img, new_shape=(640, 640), color=(114, 114, 114)):
    shape = img.shape[:2]  # current shape [height, width]
    if isinstance(new_shape, int):
        new_shape = (new_shape, new_shape)

r = min(new_shape[0] / shape[0], new_shape[1] / shape[1])
    ratio = r, r  # width, height ratios
    new_unpad = int(round(shape[1] * r)), int(round(shape[0] * r))
    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

if shape[::-1] != new_unpad:
        img = cv2.resize(img, 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))
    img = cv2.copyMakeBorder(img, top, bottom, left, right, cv2.BORDER_CONSTANT, value=color)
    return img, ratio, (dw, dh)

def preprocess_v5_v7(img_path, img_size=640):
    img = cv2.imread(img_path)
    img, _, _ = letterbox(img, new_shape=img_size)
    img = img[:, :, ::-1].transpose(2, 0, 1)  # BGR to RGB, to 3x640x640
    img = img[np.newaxis, :, :, :] / 255.0  # Add batch dimension, scale (0, 1)
    img = np.ascontiguousarray(img, dtype=np.float32)  # Convert to float32
    return img
```