python - Matplotlib - 无法以与原始图像相同的分辨率保存图像

Question

我无法以初始分辨率 (1037x627) 保存没有白色边框的图像

import numpy as np
import matplotlib.pyplot as plt
from matplotlib import pyplot, lines
import matplotlib.image as mpimg
from matplotlib.patches import Ellipse
x=[0,0,0,0,0]
y=[0,0,0,0,0]
a=10**1.3*15
inc=25
b=np.cos(np.radians(inc))*a
x[0],y[0]=516.667,313.021
x[1],y[1]=x[0]-a,y[0]
x[2],y[2]=x[0]+a,y[0]
x[3],y[3]=x[0],y[0]+b
x[4],y[4]=x[0],y[0]-b
for pa in range(0,10,5):
    fig, ax = plt.subplots()
    img=mpimg.imread('IC342.png')
    imgplot = plt.imshow(img)
    x[1],y[1]=x[0]-a/2*np.cos(np.radians(pa)),y[0]-a/2*np.sin(np.radians(pa))
    x[2],y[2]=x[0]+a/2*np.cos(np.radians(pa)),y[0]+a/2*np.sin(np.radians(pa))
    x[3],y[3]=x[0]+b/2*np.cos(np.radians(pa+90)),y[0]+b/2*np.sin(np.radians(pa+90))
    x[4],y[4]=x[0]-b/2*np.cos(np.radians(pa+90)),y[0]-b/2*np.sin(np.radians(pa+90))
    ell = Ellipse(xy=[516.667,313.021], width=a, height=b, angle=pa, edgecolor='b',lw=4, alpha=0.5, facecolor='none')
    name='plt'+str(pa)+'.png'
    leg='PA='+str(pa)
    #ax.text(10, 10, leg, fontsize=15,color='white')
    ax.add_artist(ell)
    xn=[x[1],x[2],x[0]]
    yn=[y[1],y[2],y[0]]
    xnw=[x[3],x[4],x[0]]
    ynw=[y[3],y[4],y[0]]
    line = lines.Line2D(xn, yn, linestyle='-.',lw=5., color='r', alpha=0.4)
    line1 = lines.Line2D(xnw, ynw, linestyle='-.',lw=5., color='g', alpha=0.4)
    ax.add_line(line)
    ax.add_line(line1)
    plt.axis('off')
    fig.savefig(name, transparent=True, bbox_inches='tight', pad_inches=0,dpi=150 )

初始图像

结果

此外，我需要在图像上显示白色文本 PA=something 而不更改分辨率。据我了解，添加另一个数字(如文本)可能会自动更改分辨率。

感谢您的宝贵时间!

Answer 1

这里有两个因素在起作用:

1. 默认情况下，Axes 不会占据整个Figure
2. 在matplotlib中，图形的大小是固定的，内容会被拉伸(stretch)/压缩/插值以适合图形。您希望Figure的大小由其内容定义。

要做你想做的事，需要三个步骤:

1. 根据图像大小和设定的 DPI 创建图形
2. 添加占据整个图形的子图/轴
3. 使用用于计算图窗尺寸的 DPI 保存图窗

让我们使用来自 Nasa 的随机哈勃图像 http://www.nasa.gov/sites/default/files/thumbnails/image/hubble_friday_12102015.jpg 。这是一个 1280x1216 像素的图像。

这里有一个被大量评论的示例来引导您完成它:

import matplotlib.pyplot as plt

# On-screen, things will be displayed at 80dpi regardless of what we set here
# This is effectively the dpi for the saved figure. We need to specify it,
# otherwise `savefig` will pick a default dpi based on your local configuration
dpi = 80

im_data = plt.imread('hubble_friday_12102015.jpg')
height, width, nbands = im_data.shape

# What size does the figure need to be in inches to fit the image?
figsize = width / float(dpi), height / float(dpi)

# Create a figure of the right size with one axes that takes up the full figure
fig = plt.figure(figsize=figsize)
ax = fig.add_axes([0, 0, 1, 1])

# Hide spines, ticks, etc.
ax.axis('off')

# Display the image.
ax.imshow(im_data, interpolation='nearest')

# Add something...
ax.annotate('Look at This!', xy=(590, 650), xytext=(500, 500),
            color='cyan', size=24, ha='right',
            arrowprops=dict(arrowstyle='fancy', fc='cyan', ec='none'))

# Ensure we're displaying with square pixels and the right extent.
# This is optional if you haven't called `plot` or anything else that might
# change the limits/aspect.  We don't need this step in this case.
ax.set(xlim=[-0.5, width - 0.5], ylim=[height - 0.5, -0.5], aspect=1)

fig.savefig('test.jpg', dpi=dpi, transparent=True)
plt.show()

保存的test.jpg将恰好是1280x1216像素。当然，由于我们对输入和输出都使用有损压缩格式，因此由于压缩伪影，您将无法获得完美的像素匹配。不过，如果您使用无损输入和输出格式，则应该使用无损输入和输出格式。