Create PNG image with alpha transparency

The sample shows how to create an RGBA image and store it as a PNG file. It also demonstrates how to set custom PNG compression parameters.

Sources:

create image with alpha channel

sz = [480 640];
img = create_rgba_image(sz);
whos img

write to PNG file with alpha data

fname = fullfile(tempdir(), 'alpha.png');
if true
    cv.imwrite(fname, img, 'PngCompression',9, 'PngStrategy','RLE');
else
    imwrite(img(:,:,1:3), fname, 'Alpha',img(:,:,4))
end
imfinfo(fname)

ans = 
  struct with fields:

                  Filename: 'C:\Users\Amro\AppData\Local\Temp\alpha.png'
               FileModDate: '05-Oct-2018 00:59:56'
                  FileSize: 41853
                    Format: 'png'
             FormatVersion: []
                     Width: 640
                    Height: 480
                  BitDepth: 24
                 ColorType: 'truecolor'
           FormatSignature: [137 80 78 71 13 10 26 10]
                  Colormap: []
                 Histogram: []
             InterlaceType: 'none'
              Transparency: 'alpha'
    SimpleTransparencyData: []
           BackgroundColor: []
           RenderingIntent: []
            Chromaticities: []
                     Gamma: []
               XResolution: []
               YResolution: []
            ResolutionUnit: []
                   XOffset: []
                   YOffset: []
                OffsetUnit: []
           SignificantBits: []
              ImageModTime: []
                     Title: []
                    Author: []
               Description: []
                 Copyright: []
              CreationTime: []
                  Software: []
                Disclaimer: []
                   Warning: []
                    Source: []
                   Comment: []
                 OtherText: []

show image (over checkerboard image as background to better see transparency)

bg = create_checkerboard_image(sz);
figure('Menubar','none', 'Color','k', 'Name','RGBA')
if ~mexopencv.isOctave()
    imshow(bg, 'InitialMagnification',100, 'Border','tight')
else
    image(bg), axis image off
    set(gca, 'Units','normalized', 'Position',[0 0 1 1])
end
image('CData',img(:,:,1:3), 'AlphaData',img(:,:,4))

Helper functions

function img = create_rgba_image(sz)
    %CREATE_RGBA_IMAGE  Create image with transparency, with a gradient effect

    nrows = sz(1);
    ncols = sz(2);
    img = zeros(nrows, ncols, 4, 'uint8');
    [I,J] = ndgrid(1:nrows, 1:ncols);
    img(:,:,1) = uint8(255 * (nrows-I+1)/nrows);     % red
    img(:,:,2) = uint8(255 * (ncols-J+1)/ncols);     % green
    img(:,:,3) = uint8(255);                         % blue
    img(:,:,4) = uint8(0.8 * sum(img(:,:,[1 2]),3)); % alpha
end

function img = create_checkerboard_image(sz)
    %CREATE_CHECKERBOARD_IMAGE  Create checkerboard image

    % n should evenly divide both width and height
    if true
        n = 40;  %HACK: hardcoded for 640x480
    else
        n = gcd(sz(1), sz(2));
    end
    s = sz / n;
    assert(isequal(s, fix(s)));
    img = kron(bsxfun(@xor, mod(1:s(1),2).', mod(1:s(2),2)), ones(n));
    img = repmat(max(img, 0.75), [1 1 3]);
    img = uint8(255 * img);
end

  Name        Size                 Bytes  Class    Attributes

  img       480x640x4            1228800  uint8