How to insert new column using numpy and conditionally add values to it

Question

I have a numpy array img(640, 400, 3) It basically contains a pixel array of an image loaded via OpenCV library. Some of the pixels are zero [0, 0, 0] and other are non-zero [r, g, b] values

How do I -

insert another column to make the shape of it (640, 400, 4) to add the alpha values, so that the pixels become [r, g, b, a], where a is the alpha value
In this new column, the alpha value needs to be set based on a condition: if [r, g, b] == 0 then alpha == 0 else alpha == 255. i.e, my new pixels should be either [0, 0, 0, 0] or [r, g, b, 255]

How do I do this in numpy itself, I don't want to use OpenCV to re-read the image to generate the alpha values.

GuillemB · Accepted Answer · 2022-02-18 14:36:47Z

1

rgb = np.random.uniform(0, 255, (100, 100, 3)).astype("uint")
# example position with [0,0,0]
rgb[0, 0, :] = 0

# mask where condition is satisfied
msk = np.sum(rgb, axis=2)==0

# Prep the alpha layer
nrows, ncols, _ = rgb.shape
alpha = np.ones((nrows, ncols), dtype="uint") * 255

# Make positions that satisfy condition equal to 0
alpha[msk]= 0

# add extra alpha layer
rgba = np.dstack([rgb, alpha])

edited Feb 18, 2022 at 14:36

answered Feb 18, 2022 at 14:07

GuillemB

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