Problem obtaining alpha channel of pngquant/tinypng files.

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 (256x3 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?

[Saurabh Gupta]

The imread documentation says "For PNG files, transparency is the alpha channel, if present. If no alpha channel is present, ... then transparency is []."

Are you sure your image file has an alpha channel?

[Ernest Woei]

Yes, unless the image viewer is creating a transparency layer on its own, which I doubt.

Upon opening the pngquant'ed file in Photoshop. I can view the transparency layer, i.e., select layer, Menu: Layer -> Layer Mask -> From Transparency, then viewing the mask channel through the 'Channels' tab.

Answer 1

[Walter Roberson]

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);

[Ernest Woei]

Minor change from

ACHAN = imfinfo.SimpleTransparencyData;

to

ACHAN = minfo.SimpleTransparencyData;

and everything works fine.

[Ernest Woei]

Web Logo:

Web Logo pngquant

[Walter Roberson]

Thanks, I have corrected my typo for future reference.