Use numpy to mask an image with a pattern?

Question

I'm using numpy to build pixel arrays. An 800x600 image is an 3-dimensional array of uint8, 800x600x3. I also have a similar array with a fixed pattern (a checkerboard, see here). I have another array, 800x600 of mask values. Where the mask is zero, I want to copy the pattern pixel to the image pixel. Where the mask is not zero, I want to leave the image pixel alone.

>>> image.shape (800, 600, 3) >>> chex.shape (800, 600, 3) >>> mask.shape (800, 600) 

This feels like it should work:

image[mask == 0,...] = chex 

but gives "ValueError: array is not broadcastable to correct shape".

What do I use to copy chex pixels to image pixels where mask is zero?

Answer 1

idx=(mask==0) image[idx]=chex[idx] 

Note that image has shape (800,600,3), while idx has shape (800,600). The rules for indexing state

if the selection tuple is smaller than n, then as many : objects as needed are added to the end of the selection tuple so that the modified selection tuple has length N.

Thus indexing arrays have a sort of broadcasting ability of their own. idx's shape gets promoted to (800,600,:)

Answer 2

I wanted to illustrate an example using @unutbu answer. In this scenario, I have an image of a cat that I rotate. This rotation causes some black edges that look ugly especially when pasted on a non-black background.

import matplotlib.pyplot as plt from scipy.ndimage import rotate   cat = plt.imread('cat.jpeg') bg = plt.imread('background.jpeg')   rotcat = rotate(cat, angle=8, reshape=True) ## rotating creates some black edges height, width, _ = rotcat.shape  bgcopy = bg.copy() ## create a copy of the background; paste on copy  x, y = 40, 50  bgcopy[x:x+height, y:y+width] = rotcat plt.imsave('cat-on-bg-mask.jpg', bgcopy) 

So, I find the areas of the mask and replace those values with the original background values

mask_ind = (bgcopy == 0) bgcopy[mask_ind] = bg[mask_ind] plt.imsave('cat-on-bg.jpg', bgcopy) 

I should also note that PIL.Image (from the Pillow library) has the ability to paste an image onto another image with fewer steps.