Question

I’m using numpy to build pixel arrays. An 800×600 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, 800×600 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,:)