resize and save transparent PNG file in matlab

I have an transparent png file(colored image not black and white).I want to resize this png file and save that with transparent background.but i cant save that with transparent background.file saved with dark background.

     [im,map]=imread('image.png');
     im=ind2rgb(im,map);
     im2=imresize(im,[200,200]);
     imwrite(im2,'image2.png');

in above code image2.png saved with black background and not transparent

[Walter Roberson]

You are not reading the alpha correctly

https://www.mathworks.com/matlabcentral/answers/57664-how-to-add-alpha-channel-to-the-image-and-convert-that-image-into-png-format#answer_69795

[Mohsen Farjami]

thanks for reply but when i resize and save png file the file give dark background. this image is my png file

Answer 1

[Guillaume]

For a start you never retrieve the transparency information so it's no wonder it's not preserved.

PNG images have two ways to specify transparency. For grayscale and true colour images, the transparency can be specified as an additional alpha channel the same size as the image. You get that in matlab from the 3rd output of imread (not the 2nd). Indexed images in PNG cannot have an alpha channel

For all types of images (so grayscale, true colour and indexed), PNG also allows you to specify a specific colour as transparent. To get that transparent colour in matlab you have to use at the SimpleTransparencyData field of imfinfo.

Assuming your image is actually true colour and that your code is a mistake, then:

[im, ~, alpha] = imread('image.png');  %transparency not valid for index images, so no point in getting map
im2 = imresize(im, [200, 200]);
alpha2 = imresize(alpha, [200 200]);
imwrite(im2, 'image2.png');

If the transparency is actually the second type ( imfinfo will have 'simple' for the Transparency field, then:

fileinfo = imfinfo('image.png');
assert(strcmp(fileinfo.Transparency, 'simple'), 'This file does not have simple transparency')
im = imread('image.png');  %if image is actually indexed, you also need to retrieve the colour map
im2 = imresize(im, [200 200]);
imwrite(im2, 'image2.png', 'Transparency', fileinfo.SimpleTransparencyData);

However, in that second case, note that because of the default interpolation, some transparent pixel may get assigned a colour that is no longer transparent. Using the non-default 'nearest' interpolation option in imresize would solve that problem.

[Mohsen Farjami]

thanks.but i get this errors from your codes

[Guillaume]

Yes, I made a mistake the field is SimpleTransparencyData not SimpleTransparency.

I also forgot that for indexed image, the simple transparency can be an array of alpha value for each index. Your image is actually indexed with simple transparency. You have two choices:

• keep it indexed with simple transparency. To avoid the resizing interpolation creating indices out of the colour and transparency map, the easiest is to use 'nearest' interpolation:

fileinfo = imfinfo('image.png');
[im, map] = imread('image.png');
im2 = imresize(im, [200 200], 'Method', 'nearest');
imwrite(im2, map, 'image2.png', 'Transparency', fileinfo.SimpleTransparencyData);

• convert the image to RGB and the simple transparency to an alpha map. Interpolation can then be the default 'bicubic' (better aliasing):

fileinfo = imfinfo('image.png');
[im, map] = imread('image.png');
transmap = [0; fileinfo.SimpleTransparencyData];
alpha = transmap(im + 1);  %convert simple transparency into alpha map
im = ind2rgb(im, map);
im2 = imresize(im, [200 200]);
alpha2 = imresize(alpha, [200 200]);
imwrite(im2, 'image2.png', 'Alpha', alpha2);

However, note that if you have the original vectorial inkscape image you would be much better off resizing that before converting to a png of the correct size.

[Mohsen Farjami]

thanks very much.both of your solutions was fine.

[Guillaume]

Note that following this answer, I asked Mathworks to make it easier to access transparency information for png images with 'SimpleTransparencyData'. This was implemented and released, I can't remember in which version. You can now get the transparency directly from the 3rd output of imread. The above code can be simplified to:

[im, map, alpha] = imread('image, png');  %get simple transparency directly as alpha map
%rest of code as before

[Ronan da Silva]

In the sample code above, you do not pass the resized alpha2 to imwrite .

I don't assume MATLAB assigns it automatically?

Ronan

[Walter Roberson]

In https://www.mathworks.com/matlabcentral/answers/423418-resize-and-save-transparent-png-file-in-matlab#comment_620927 Guillaume does pass the resized alpha2 to imwrite, on the line

imwrite(im2, 'image2.png', 'Alpha', alpha2);
                                    ^^^^^^^