mmdet中的Normalize

本文尝试解释mmdet中的Normalize
mmdet中的Normalize使用的是MMCV中的imnormalize函数实现的

路径如下:

mmdetection/mmdet/datasets/pipelines/transforms.pymmdetection/mmdet/datasets/pipelines/transforms.py

@PIPELINES.register_module()
class Normalize:
    """Normalize the image.
    Added key is "img_norm_cfg".
    Args:
        mean (sequence): Mean values of 3 channels.
        std (sequence): Std values of 3 channels.
        to_rgb (bool): Whether to convert the image from BGR to RGB,
            default is true.
    """
    def __init__(self, mean, std, to_rgb=True):
        self.mean = np.array(mean, dtype=np.float32)
        self.std = np.array(std, dtype=np.float32)
        self.to_rgb = to_rgb
    def __call__(self, results):
        """Call function to normalize images.
        Args:
            results (dict): Result dict from loading pipeline.
        Returns:
            dict: Normalized results, 'img_norm_cfg' key is added into
                result dict.
        """
        for key in results.get('img_fields', ['img']):
            results[key] = mmcv.imnormalize(results[key], self.mean, self.std,
                                            self.to_rgb)
        results['img_norm_cfg'] = dict(
            mean=self.mean, std=self.std, to_rgb=self.to_rgb)
        return results
    def __repr__(self):
        repr_str = self.__class__.__name__
        repr_str += f'(mean={self.mean}, std={self.std}, to_rgb={self.to_rgb})'
        return repr_str

MMCV中的normalize函数

Normalize an image with mean and std.
mmcv的imnormalize是使用opencv实现的，通过cv2.subtract(img, mean, img)

def imnormalize(img, mean, std, to_rgb=True):
    """Normalize an image with mean and std.
    Args:
        img (ndarray): Image to be normalized.
        mean (ndarray): The mean to be used for normalize.
        std (ndarray): The std to be used for normalize.
        to_rgb (bool): Whether to convert to rgb.
    Returns:
        ndarray: The normalized image.
    """
    img = img.copy().astype(np.float32)
    return imnormalize_(img, mean, std, to_rgb)
def imnormalize_(img, mean, std, to_rgb=True):
    """Inplace normalize an image with mean and std.
    Args:
        img (ndarray): Image to be normalized.
        mean (ndarray): The mean to be used for normalize.
        std (ndarray): The std to be used for normalize.
        to_rgb (bool): Whether to convert to rgb.
    Returns:
        ndarray: The normalized image.
    """
    # cv2 inplace normalization does not accept uint8
    assert img.dtype != np.uint8
    mean = np.float64(mean.reshape(1, -1))
    stdinv = 1 / np.float64(std.reshape(1, -1))
    if to_rgb:
        cv2.cvtColor(img, cv2.COLOR_BGR2RGB, img)  # inplace
    cv2.subtract(img, mean, img)  # inplace
    cv2.multiply(img, stdinv, img)  # inplace
    return img

Imdenormalize

def imdenormalize(img, mean, std, to_bgr=True):
    assert img.dtype != np.uint8
    mean = mean.reshape(1, -1).astype(np.float64)
    std = std.reshape(1, -1).astype(np.float64)
    img = cv2.multiply(img, std)  # make a copy
    cv2.add(img, mean, img)  # inplace
    if to_bgr:
        cv2.cvtColor(img, cv2.COLOR_RGB2BGR, img)  # inplace
    return img