flatten()可以将二维的array展成一维的

a.flatten(order=’C’) 
Return a copy of the array collapsed into one dimension. 
order = 'C' means to flatten in row-major (C-style) order. 
order = 'F' means to flatten in column-major (Fortran-style) order.

flatten('C')和flatten('F')的区别在于行向量和列向量。

图像的超分辨率重建中，计算PSNR的时候，需要将矩阵形式的二维数据变成向量形式的一维向量，这样方便计算。

# PSNR.py
 
import numpy as np
import math
 
 
def psnr(target, ref, scale):
    # target:目标图像  ref:参考图像  scale:尺寸大小
    # assume RGB image
    target_data = np.array(target)
    target_data = target_data[scale:-scale,scale:-scale]
 
    ref_data = np.array(ref)
    ref_data = ref_data[scale:-scale,scale:-scale]
 
    diff = ref_data - target_data
    diff = diff.flatten('C')
    rmse = math.sqrt( np.mean(diff ** 2.) )
    return 20*math.log10(1.0/rmse)