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keeping the alpha channel in transparent image mat

asked 2016-10-02 20:05:31 -0500

inac gravatar image

How do you keep an image's transparent background or alpha channel?

i.e., Canny input: .png with existing transparent alpha channel background. I'd like to figure out how to get the latter output instead of the first.

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answered 2016-10-02 23:25:23 -0500

berak gravatar image

updated 2016-10-03 00:49:56 -0500

first note, that alpha is irrelevant to computer vision, so opencv won't ever use or try to display it.

step1: (shave off the original alpha channel, and keep it for later)

Mat image = ... // 4 channels bgra

Mat channels[4];
split(image, channels);

Mat alpha = channels[3];

step2: (when you're done with your canny or whatever, make a new Mat with alpha)

Mat canny = ... // 1 channel grayscale !

Mat newchan[4] = { canny, canny, canny, alpha };
Mat final;
merge(newchan, 4, final);

step3: (save it to disk):

imwrite("glasses.png", final);
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Thanks - sorry for asking the question wrongly, when I was in-fact looking for whether the shaving had to be done or not - apparently it had to be!

inac gravatar imageinac ( 2016-10-03 01:29:35 -0500 )edit
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Asked: 2016-10-02 20:05:31 -0500

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Last updated: Oct 03 '16