MATLAB: Problem obtaining alpha channel of pngquant/tinypng files.

Image Acquisition Toolboximage processing

I am having trouble obtaining the alpha channel of a pngquant/tinypng file using imread. Is there another function that handles this properly?

Used:

[img,imgMap,imgAlpha] = imread('some_pngquant_file.png');

img (mxn uint8 matrix), imgMap (256×3 double matrix with values in [0,1]), imgAlpha is empty

img entries index into imgMap rows (RGB values in [0,1]). Could it be that I'm interpreting this improperly?

Best Answer

The sample "Firefox logo" image I found that had been created by the open source library did not have an Alpha channel. Instead, imfinfo says that 'Transparency' is 'simple' and there is a SimpleTransparencyData property that appears to give some kind of alpha mapping. The PNG specification does not permit true Alpha channels for indexed (pseudocolor) images.

Looking at http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2 the implication is that all entries past the end of the provided SimpleTransparencyData are the maximum (fully opaque.)

This leads to:

filename = 'NameOftinyPNG.png';
minfo = imfinfo(filename);
ACHAN = mfinfo.SimpleTransparencyData;
ACHAN(end+1:256) = 1;
[img, imgMap] = imread(filename);
AMAP = ACHAN(double(img)+1);
image(img, 'AlphaData', AMAP);
colormap(imgMap);

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MATLAB: Resize and save transparent PNG file in matlab

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.

MATLAB: How do you read the alpha channel of an indexed image

Hum, yes it does appear that you cannot retrieve the alpha value associated with the palette of a png image with imread. I think that is worthy of a bug report to mathworks.

You can still get the transparency info with imfinfo:

pnginfo = imfinfo('Dallas_Fuel_logo_std.png');
pnginfo.SimpleTransparencyData

Note:

The transparency for png indexed image is stored in a tRNS chunk. The length of the transparency array is at most the length of the colour map but can be shorter (as is the case in your image). In that case, the remaining colours all have 0 transparency. Consequently, I would read the transparency as follow:

[img, map] = imread(pngpath);
pnginfo = imfinfo(pngpath);
maptrans = zeros(size(map, 1), 1);
maptrans(1:numel(pnginfo.SimpleTransparencyData)) = pnginfo.SimpleTransparencyData;

Best Answer

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MATLAB: Resize and save transparent PNG file in matlab

MATLAB: How do you read the alpha channel of an indexed image

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