Making White Pixels in an Opaque TIF Image Transparent and Saving as a PNG

Hi everyone,

I have a TIF file stored in the .mat file named 'Opaque_Image.mat'. I have attached this .mat file for your reference. It is a 944x944x3 uint8 type of image and contains a colorful circle surrounded by white pixels. I wrote the .m file below to turn this image into a PNG file which would have the white pixels in the original TIF file replaced with transparent pixels. Unfortunately I end up getting a PNG file which has opaque black pixels instead of transparent pixels. Could you please help me solve this problem? Thank you for your answers in advance.

clc
clear all
load('Opaque_Image.mat')

[K L M]=size(opaque_image);

A = ones(K,L);

for i=1:K
   for j=1:L
      if opaque_image(i,j,1) == 255 && opaque_image(i,j,2) == 255 && opaque_image(i,j,3) == 255   
         A(i,j) = 0;      
      end
   end
end

AA = logical(A);
mask = cast(AA, class(opaque_image)); 
transparent_image = opaque_image .* repmat(mask, [1 1 3]);
imwrite(transparent_image,'Transparent_Image.png');

Answer 1

[DGM]

Yeah imwrite() is really clumsy about handling certain file types and options. For PNG, the alpha channel has to be separated and passed as an optional argument.

dd = load('Opaque_Image.mat');
inpict = dd.opaque_image;

th = 253; % pick some upper threshold
whmask = ~all(inpict>th,3);

imwrite(inpict,'circlething.png','alpha',double(whmask))

Of course, now you can't really tell because the website bg is white ...

[Ahmet Usta]

Thank you so much for your quick answer! It worked :)