Matplotlib: What is the cmap function in imshow?

Question

I am trying to learn opencv using python and came across this code below:

import cv2 import numpy as np from matplotlib import pyplot as plt BLUE = [255,0,0] img1 = cv2.imread('opencv_logo.png') replicate = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_REPLICATE) reflect = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_REFLECT) reflect101 = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_REFLECT_101) wrap = cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_WRAP) constant= cv2.copyMakeBorder(img1,10,10,10,10,cv2.BORDER_CONSTANT,value=BLUE) plt.subplot(231),plt.imshow(img1,'gray'),plt.title('ORIGINAL') plt.subplot(232),plt.imshow(replicate,'gray'),plt.title('REPLICATE') plt.subplot(233),plt.imshow(reflect,'gray'),plt.title('REFLECT') plt.subplot(234),plt.imshow(reflect101,'gray'),plt.title('REFLECT_101') plt.subplot(235),plt.imshow(wrap,'gray'),plt.title('WRAP') plt.subplot(236),plt.imshow(constant,'gray'),plt.title('CONSTANT') plt.show() 

source: http://docs.opencv.org/master/doc/py_tutorials/py_core/py_basic_ops/py_basic_ops.html#exercises

What does plt.imshow (img1, 'gray') do? I tried searching on Google, and all I could understand was that the “gray” argument was the color map. But my image (pic is present on the site, see the Link) does not appear in shades of gray. I tried to remove the second argument. So the code was similar to plt.imshow (img1). He is running. The image remains the same as before. Then what makes the second argument “gray”? Can someone explain all this to me? Any help appreciated. Thanks.

PS. I am completely new to Matplotlib

Answer 1

When img1 is of the form (M,N,3) or (M,N,4) , the values ​​in img1 interpreted as RGB or RGBA values. In this case, cmap is ignored. Per help(plt.imshow) docstring :

cmap: ~matplotlib.colors.Colormap , optional, default: None

If None, the default value is rc image.cmap . cmap ignored when X has RGB (A) information

However, if img was an array of form (M,N) , then cmap controls the color used to display the values.

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 import numpy as np import matplotlib.pyplot as plt import mpl_toolkits.axes_grid1 as axes_grid1 np.random.seed(1) data = np.random.randn(10, 10) fig = plt.figure() grid = axes_grid1.AxesGrid( fig, 111, nrows_ncols=(1, 2), axes_pad = 0.5, cbar_location = "right", cbar_mode="each", cbar_size="15%", cbar_pad="5%",) im0 = grid[0].imshow(data, cmap='gray', interpolation='nearest') grid.cbar_axes[0].colorbar(im0) im1 = grid[1].imshow(data, cmap='jet', interpolation='nearest') grid.cbar_axes[1].colorbar(im1) plt.savefig('/tmp/test.png', bbox_inches='tight', pad_inches=0.0, dpi=200,)