图像颜色数

import numpy as np
from PIL import Image
import matplotlib.pylab as pylab
from skimage.io import imread, imshow, show
def signaltonoise(a,axis,ddof=0): 
    a=np.asanyarray(a)
    m=a.mean(axis)
    sd=a.std(axis=axis,ddof=ddof)
    return np.where(sd==0,0,m/sd)
img=Image.open('C:/Users/xpp/Desktop/Lena.png')#原始图像
pylab.figure(figsize=(30,45))
num_colors_list=[1 << n for n in range(8,0,-1)]#图像颜色数
snr_list=[]
i=1
for num_colors in num_colors_list:
    im1=img.convert('P',palette=Image.ADAPTIVE,colors=num_colors)
    snr_list.append(signaltonoise(im1,axis=None))
    i+=1
pylab.plot(num_colors_list,snr_list,'r.-')
pylab.xlabel('Max# colors in the image')
pylab.ylabel('SNR')
pylab.title('Change in SNR w.r.t. # colors')
pylab.xscale('log',basex=2)
pylab.gca().invert_xaxis()