How to write a Image with Alpha channel ?using imwrite matlab command?

How to write a Image with Alpha channel ?using imwrite matlab command?

Example:

RGB=imread('football.jpg');

Alpha=?

imwrite(X,Y,X,X,)

Answer 1

[Voss]

% original image
RGB=imread('football.jpg');
imshow(RGB)

Use .png format with Alpha parameter in imwrite

% Attempt to make the blue background green, using transparency/opacity
% parameter 'Alpha' and the 'Background' parameter, which is the color 
% that shows "under" the places where the image is transparent.

% alpha is a matrix of uint8, based on a logical condition
alpha = uint8(255*(RGB(:,:,1) >= 100 | RGB(:,:,3) <= 50));

% the first pixel is fully transparent (no opacity)
alpha(1,1)

ans = uint8 0

% write the png file, with alpha and background
imwrite(RGB,'football.png','Alpha',alpha,'Background',[0 0.6 0])

% check the outcome
imshow(imread('football.png'))

% attempt to make the football transparent by 
% using the opposite condition
alpha = uint8(255*(RGB(:,:,1) < 100 & RGB(:,:,3) > 50));

% the first pixel has no transparency (fully opaque)
alpha(1,1)

ans = uint8 255

% write the png file, with alpha and background
imwrite(RGB,'football.png','Alpha',alpha,'Background',[0 0.6 0])

% check the outcome
imshow(imread('football.png'))

[Selva Karna]

Thank you for answer. but I want to save the Alpha channel. if I read the Image it should show 4 channel but your code show 3 channel only.

your code result:

[Voss]

You're welcome!

"I want to save the Alpha channel"

The transparency information is saved in the image.

"if I read the Image it should show 4 channel"

The transparency matrix is returned as the third output from imread, separate from the m-by-n-by-3 RGB array of colors.

% read the original image, and get its alpha (which is empty)
[RGB,~,original_alpha] = imread('football.jpg');

whos RGB original_alpha

  Name                  Size                Bytes  Class     Attributes

  RGB                 256x320x3            245760  uint8               
  original_alpha        0x0                     0  double              

% make up a new alpha
alpha = uint8(255*(RGB(:,:,1) >= 100 | RGB(:,:,3) <= 50));

% write the image (as png) with new alpha
imwrite(RGB,'football.png','Alpha',alpha,'Background',[0 0.6 0])

% get the RGB and alpha of the new image
[new_RGB,~,new_alpha] = imread('football.png');

whos new_*

  Name             Size                Bytes  Class    Attributes

  new_RGB        256x320x3            245760  uint8              
  new_alpha      256x320               81920  uint8              

% RGB didn't change
isequal(new_RGB,RGB)

ans = logical
   1

% alpha from the new file is what I made up
isequal(new_alpha,alpha)

ans = logical
   1

% alpha from the new file is different than alpha from the original file
isequal(new_alpha,original_alpha)

ans = logical
   0

[DGM]

To pre-empt any confusion about what gets preserved, specifying both output arguments like this:

% get the RGB and alpha of the new image
[new_RGB,~,new_alpha] = imread('football.png');

will give you the original RGB content (of both the ball and blanket) and the new alpha as Voss demonstrates in the second example.

However, let's say you just want to discard the new alpha and get the original RGB content. Doing this:

% get the RGB and alpha of the new image
new_RGB = imread('football.png');

will not give you the expected RGB content anymore. As the first examples demonstrate, if no alpha is requested when reading an image which has alpha content, the image will interally be composited with a solid color field defined by the "BackgroundColor" field specified in the PNG file, or by the "BackgroundColor" option specified when calling imwrite() (default is black).

The lesson here is that if you want the actual content of the RGB channels, you need to request alpha if there's a possibility that the image may have alpha content -- even if you don't need it.