I have an image already at 100% opacity but the whole image is partially transparent (with a transparent background).
Is there some way to uniformly increase the opacity (effectively going beyond the current 100%) ?
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Easily done by "stretching" the alpha channel so that the maximum value becomes 100%. The "Levels" tool (Colors>Levels) conveniently shows the histogram:
Now and then converting an image to PNG really makes it partially transparent - maybe an error somewhere or a software bug (not in GIMP, I use some old software). But typically one has a fully opaque layer, the transparency is in the colors - in the alpha channel of the image layer.
One way to fix it is to edit the alpha channel. Unfortunately easy methods also destroy the anti-aliasing i.e. the partial transparency of the edges to make sharp curved and sloped edges less jaggy. There's another answer about it. The introduced jagginess is at least partially fixed with feathering the edges.
Another way is to make layer duplicates and merge them. Each new copy reduces the transparency. An example of image with unwanted transparency:
Stacking layer duplicates brings the opacity back:
But the antialiasing is still there after inserting a few duplicates (see only the edges on the checkerboard)