Can you help me with this code? It's about setting the 'Alpha' of an image with an Alpha channel.

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Kazi
Kazi 2016-11-25
评论： Malini Bakthavatchalam 2020-6-26
Hi, I am trying to convert the background of an image into transparent.
I took an image as input, then created an alpha channel. The alpha channel is supposed to be a matrix with 1 where there is white color. Then the output image will be transparent where there were initially white color pixels.
This is the code I wrote ...
clc
clear
img=imread('practice.jpg'); % Taking the truecolor image as input.
alphachannel = all(img == 255, 3);% convert the white colors in each pixel
% into 1, and all other color into
% zero. The type of the output image is
% logical.
alphachannel_2=uint8(alphachannel); % convert the logical array into uint8 type
imwrite(img, 'practice_2.png', 'Alpha', alphachannel_2); % output image
I practiced with the following image ...
The white color is supposed to be transparent in the output image. But as it turns out the whole image is being transparent in the output image. The red,blue and black boxes aren't appearing in the output image.
But instead, if I use the ' Transparency' attribute of imwrite() function, then it works.
clc
clear
img=imread('practice.jpg'); % Taking the truecolor image as input.
imwrite(img, 'practice_2.png', 'Transparency',[1 1 1]); % output image
But, using 'Transparency' lets me to convert the image into transparent background based on only a single color, not on a certain Alpha Channel. What am I doing wrong with the first code ?
I am including the image and the matlab file, if you need.
Thanks in advance :)

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回答（2 个）

Walter Roberson
Walter Roberson 2016-11-25
It appears to me that the PNG alpha parameter should be floating point in the range 0 to 1, not uint8.
Ruben Costa
Ruben Costa 2019-7-5
编辑：Ruben Costa 2019-7-8
I picked up your code and try to do some changes and for me this works! Probably you don´t need it already but is here for people like me, who needs to solve the same problem and are searching for a solution!
You can change the condition to the colour that you want to remove but i used this condition to solve the problem of Kazi!
clc
clear
img=imread('practice.jpg'); % Taking the truecolor image as input.
lin=size(img,1);
col=size(img,2);
M=ones(lin,col); % set a matrix of ones with the same dimensions as original image
for lines=1:lin
for columns=1:col
if (img(lines,columns,1)>=250) & (img(lines,columns,2)>=250) & (img(lines,columns,3)>=250)
M(lines,columns)=0; %setting the value on the matrix to 0 when the condition is true
end
end
end
%the M is a mask and when M is 0 the pixel will be transparent
imwrite(img, 'practice_final.png', 'Alpha', M); % output image
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Walter Roberson
Walter Roberson 2020-6-25
In my experience, when you imwrite() a .png with Alpha channel, then the stored file never "pre-blends" the alpha.
Malini Bakthavatchalam
Malini Bakthavatchalam 2020-6-26
So, now how can I use only the foreground image alone for the analysis? my only concern is I dont want the background to be involved in my analysis, I did color threshold, img segmentation, evn i have tried the imfreehand roi, nothing worked. So finally i decided to make the background transparent and take the foreground for further but I cannot make that happen . Is there any other way possible?

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