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

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 :)

Answer 1

[Walter Roberson]

It appears to me that the PNG alpha parameter should be floating point in the range 0 to 1, not uint8.

Answer 2

[Ruben Costa]

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

[Malini Bakthavatchalam]

Hi I used this transparent mask to hide my background, and used imwrite to save my image, now when i open the new saved image to futher analysis like mean and median of the histogram the transparent background also comes inside the analysis, so wht I should do to only include the foreground without alpha background.. ?

Thanks

Malini

[Ruben Costa]

Hello Malini,

When you open the new image what are it’s dimensions?

[Walter Roberson]

PNG files can store alpha in one of two different ways.

1. PNG can store RGBA files, which read back as height x width x 4 images when you imread();
2. PNG can store alpha separately, in which case you need the three-output form of imread() to get the Alpha values.

[Malini Bakthavatchalam]

@ruben my dimensions are 502*480*3*uint8.

[Walter Roberson]

In my experience, when you imwrite() a .png with Alpha channel, then the stored file never "pre-blends" the alpha.

[Malini Bakthavatchalam]

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?