问题
In previous question I made false assumption that I need to remove pixels with alpha, but I actually need to use alpha percentage as shade of white.
Because just removing alpha-pixel changes lines. But as background is to be white.
Image btn_bg_common_press.9.png
becomes
I also looked at ImageMagick, replace semi-transparent white with opaque white and found that I can do with
convert icon.png -channel A -fx "(a>0.5) ? 1.0 : 0" output.png
Though that is still rough.
来源：https://stackoverflow.com/questions/28939645/imagemagick-replace-alpha-pixel-with-color-shade-pixel