Extract equivalent RGB without transparency from PNG with alpha channel
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I used the "imread" (https://www.mathworks.com/help/matlab/ref/imread.html#btnczv9-1-transparency) function to read a PNG image. I found that the RGB did not all approach 255 in white regions of the image. It turns out that I also needed to read the per-pixel transparency (https://www.mathworks.com/help/matlab/ref/imread.html#btnczv9-1-transparency) data from the PNG file. From a heatmap of the transparency data, I found that it was zero in the white parts, which means full transparency (https://www.w3.org/TR/PNG-DataRep.html).
The fact that this shows as white means that the assumed backdrop for the image is pure white.
How do I save this image as PNG *without* alpha channel? I want the pixels where transparency/alpha was 255 as completely determined by the RGB planes. For pixels with less than 225 transparency/alpha, I want the RGB values to be modified to as to mimic the corresponding transparency, *assuming* a white background.
More Answers (1)
Walter Roberson on 6 Jul 2021
im2png = ind2rgb(im2png, cmap);
alpha = repmat(double(xparncy)/255, 1, 1, 3);
white = 255;
im_corrected = uint8(double(im2png) .* alpha + (1-alpha) .* white);