I have saved png file with imwrite like:

================================
>>[t,map,alpha]=imread('test.png');
....
>>imwrite(t,'test.png','Alpha',alpha);
================================

It worked fine. however, if the alpha is not assigned by reading from
an exsited png file,
there will be some problems.

tArea is the area I need to save.
alpha is a matrix with the same size as tArea

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alpha = zeros(size(tArea));
alpha(tArea~=0) = 1;
imwrite(tArea,'tArea.png','Alpha',alpha); % the saved image is totally
blank
================================

How can I fix it?