Extract equivalent RGB without transparency from PNG with alpha channel
13 views (last 30 days)
Show older comments
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).
[im2png,cmap,xparncy]=imread('image2.png');
hm=heatmap(xparncy,'GridVisible','off');
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.
2 Comments
Accepted Answer
Yongjian Feng
on 5 Jul 2021
For each color channel, try
re = (1-alpha)*foreground + alpha*background.
Since here background is white, so each channel, background is just 255.
More Answers (1)
Walter Roberson
on 6 Jul 2021
[im2png,cmap,xparncy]=imread('image2.png');
if ~isempty(cmap)
im2png = ind2rgb(im2png, cmap);
end
alpha = repmat(double(xparncy)/255, 1, 1, 3);
white = 255;
im_corrected = uint8(double(im2png) .* alpha + (1-alpha) .* white);
imwrite(im_corrected, 'image2_corrected.png');
See Also
Categories
Find more on Lighting, Transparency, and Shading in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!