Apply transparent background to a figure

I want to apply only to the background (white) of the figure "background.png" a certain level of transparency (thus keeping the pixels black).

This figure then needs to overlap with the "background_pink.png" figure.

I am using the following code:

figure();

folder_immagine_png = 'C:\...';
filename_png = 'background_pink.png';
ful_file_png = fullfile(folder_immagine_png,filename_png);
imshow(ful_file_png)

hold on
figure();
folder_immagine = 'C:\....';
filename = 'background.png';
ful_file = fullfile(folder_immagine,filename);
imshow(ful_file)
hold off

I have tried using the AlphaData command but to no avail.

[Jan]

It is not getting clear to me, what this means: "I want to apply only to the background (white) of the figure "background.png" a certain level of transparency (thus keeping the pixels black)."

What is the wanted output? A displayed image on the screen or another file? What do you want to see through the transparent parts?

[Alberto Acri]

Dear @Jan, Let me explain in more detail.

1) First of all, the figure "background.png" must be positioned above the figure "background_pink.png".

2) I want to figure out how to make the figure "background.png" transparent (with variable transparency values) so that I can see the figure "background_pink.png".

3) Since transparency is applied to the whole "background.png" figure, I would like (always if it is possible!) that only the white background of the "background.png" figure is transparent (with varying transparency values) while keeping the color of the black curve.

[Matt J]

"What is the wanted output? A displayed image on the screen or another file?"

[Alberto Acri]

The output is to plot a figure to be displayed on the screen.

Answer 1

[Voss]

im_bw = imread('background.png');
im_pink = imread('background_pink.png');

% make an image of the pink background as-is:
image(im_pink)
hold on

% varying (in this case, random) opacity values:
alpha = rand(size(im_bw));

% set alpha to 1 (fully opaque) where im_bw is 0 (black):
alpha(im_bw == 0) = 1;

% add the new black-and-white image with opacity:
image(repmat(255*im_bw,[1 1 3]),'AlphaData',alpha)

[Alberto Acri]

OK, thank you!

How can I save this created image?

[Alberto Acri]

I also wanted to retrieve images from appropriate folders, but the code doesn't work. Any recommendations?

folder_IMMAGINE = 'C:\Users\......';
filename_IMMAGINE = 'background.png';
ful_file_IMMAGINE = fullfile(folder_IMMAGINE, filename_IMMAGINE);
% imshow(ful_file_IMMAGINE)

folder_pink = 'C:\Users\.....';
pink = 'background_pink.jpg';
ful_file_pink = fullfile(folder_pink, pink);
imshow(ful_file_pink)

hold on
% varying (in this case, random) opacity values:
alpha = rand(size(im_bw));
% alpha = rand(size(im_bw));
% set alpha to 1 (fully opaque) where im_bw is 0 (black):
alpha(filename_IMMAGINE == 0) = 1;
% add the new black-and-white image with opacity:
image(repmat(255*filename_IMMAGINE,[1 1 3]),'AlphaData',alpha)

[Walter Roberson]

alpha(filename_IMMAGINE == 0) = 1;

You are comparing the file name characters to 0. You never read the content of the file.

[Alberto Acri]

Could you tell me how to save the created image?

[DGM]

Again, you compose images by working on the data itself, not trying to capture a figure.

im_bw = imread('background.png');
im_pink = imread('background_pink.png');

% varying (in this case, random) opacity values:
alpha = rand(size(im_bw));
% set alpha to 1 (fully opaque) where im_bw is 0 (black):
alpha(im_bw == 0) = 1;

% compose the image directly
outpict = alpha.*im2double(im_bw) + (1-alpha).*im2double(im_pink);

imwrite(outpict,'op1.png')

The above example assumes that the alpha array is of floating-point class.