Alpha channel in image

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Margareta Drozdikova
编辑： Margareta Drozdikova 2017-12-26
Hi, I have a white circle (255) and background is black (0), Picture is saved as png and is in grayscale, size 513x513. I need one thing. The white circle has to be transparent and black part no. How can I do this?
x=imread('filter.png');
[M,N]=size(x);
A=zeros(M,N);
for i=1:M
for j=1:N
if (x(i,j)==255)
A(i,j)=1;
end
end
end
imwrite(x,'filter.png','Alpha',A)
but i dont know if it is OK. I have this picture lay on another picture, so in white circle I will see second picture and in black part I will see black. Thanks for any help

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Walter Roberson
Walter Roberson 2017-12-4
Your basic technique was correct, but your code can be written much more efficiently.
x = imread('filter.png');
A = double(x==0);
imwrite(x, 'newfilter.png', 'Alpha', A);
"I have this picture lay on another picture,"
If you are reading the new image in using imread() then you are probably not handling the alpha correctly at that time:
image(ArrayForImageToGoOnTheBottom);
[newfilter, map, filter_alpha] = imread('newfilter.png');
image(newfilter, 'AlphaData', filter_alpha);
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Margareta Drozdikova
Margareta Drozdikova 2017-12-7
and one more question. What does it mean A = double(x==0); ?
Image Analyst
Image Analyst 2017-12-7
You are checking if x is zero. The result of x==0 will be a logical variable with values of true or false.
Using double() casts the data type from logical to double, so that now A will be a double with a value of 0 or 1, NOT a logical with a value of true or false.

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