为什么在IPython笔记本上使用matplotlib的plt.imshow将bmp图像显示为错误的颜色? [英] Why bmp image displayed as wrong color with plt.imshow of matplotlib on IPython-notebook?

问题描述

有一个bmp图像，如第一个图像波纹管所示，其信息列在第二个图像波纹管中.但是，当在IPython笔记本上使用matplotlib的plt.imshow()函数显示时，它的颜色错误，就像下面的第三个图片一样.我能知道原因吗?
谢谢！

There is a bmp image just as shown the first picture bellow, and its information is list as the second picture bellow. But when display with plt.imshow() function of matplotlib on IPython-notebook, it has the wrong color, just as the third picture bellow. So can I know the reason?
Thanks!

原始文件已在Dropbox https://dl.dropboxusercontent.com/上共享. u/26518813/test2.bmp

The original file has shared at dropbox https://dl.dropboxusercontent.com/u/26518813/test2.bmp

在IPython笔记本上显示图像的代码为:

And the code to show image on IPython-notebook is:

%pylab inline --no-import-all
from PIL import Image
plt.imshow(Image.open("./test/test2.bmp")) 

推荐答案

之所以会发生这种情况，是因为您实际上是使用matplotlib.pyplot将图像绘制为矩阵. Matplotlib本机不支持.bmp，因此我认为默认cmap会有一些错误.在您的特定情况下，您有一个灰度图像.因此，实际上您可以使用cmap="gray"将颜色图更改为灰度.

This happen because you are actually plotting the image as matrix with matplotlib.pyplot. Matplotlib doesn't support .bmpnatively so I think there are some error with the default cmap. In your specific case you have a grayscale image. So in fact you can change the color map to grayscale with cmap="gray".

from PIL import Image
img= Image.open(r'./test/test2.bmp')
plt.imshow(img,cmap='gray',vmin=0,vmax=255)

请注意，如果要再现原始图像的相同亮度，则必须设置vmin和vmax，否则我认为python默认情况下会拉伸到min max值. 这是不导入PIL的解决方案:

Note you have to set vmin and vmax if you want to reproduce the same luminance of your original image otherwise I think python by default will stretch to min max the values. This is a solution without importing PIL:

img=matplotlib.image.imread(r'/Users/giacomo/Downloads/test2.bmp')
plt.imshow(img,cmap='gray',vmin=0,vmax=255)

或者，您可以使用PIL显示图像，也可以将图像转换为.png.

Alternatively you can use PIL to show the image or you can convert your image to a .png before.

如果要使用PIL.Image显示图像，可以使用以下方法:

If you want show the image with PIL.Image you can use this:

from PIL import Image
img= Image.open( r'./test/test2.bmp')
img.show()

请注意，如果您使用的是I-Python Notebook，则图像会显示在新的外部窗口中

另一种选择是将图像的模式更改为'P'(调色板编码:每个像素一个字节，使用ImagePalette类的调色板将像素转换为颜色).使用.convert，然后使用matplotlib plt.imshow绘制图像:

Another option is to change the mode of the image to 'P' (Palette encoding: one byte per pixel, with a palette of class ImagePalette translating the pixels to colors). With .convert and then plot the image with matplotlib plt.imshow:

convertedimg=img.convert('P')
plt.imshow(convertedimg)

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