CyprUS CyprUS - 1 month ago 13
Python Question

matplotlib image shows in black and white, but I wanted gray

I have a small code sample to plot images in matplotlib, and the image is shown as this :

enter image description here

Notice the image in the black box has black background, while my desired output is this :

enter image description here

My code to plot the image is this :

plt.subplot(111)
plt.imshow(np.abs(img), cmap = 'gray')
plt.title('Level 0'), plt.xticks([]), plt.yticks([])
plt.show()


My understanding is that
cmap=grey
should display it in grayscale. Below is a snippet of the matrix
img
being plotted :

[[ 192.77504036 +1.21392817e-11j 151.92357434 +1.21278246e-11j
140.67585733 +6.71014111e-12j 167.76903747 +2.92050743e-12j
147.59664180 +2.33718944e-12j 98.27986577 +3.56896094e-12j
96.16252035 +5.31530804e-12j 112.39194666 +5.86689097e-12j....


What am I missing here ?

Answer

For my case, the color (gray) which I wanted is actually "negative" pixels. Subtracting 128 from the image matrix brings the range of pixels from 0-255 to -128 to +127. The negative pixels is displayed in the "gray" color by the package matplotlib.

val = np.subtract(imageMatrix,128) plt.subplot('111') plt.imshow(np.abs(val), cmap=plt.get_cmap('gray'),vmin=0,vmax=255) plt.title('Image'), plt.xticks([]), plt.yticks([]) plt.show()

I will mark my own answer as accepted, as the answer accepted earlier does not talk about treating the pixels on negative scale.

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