问题
What command or series of commands could I execute from the CLI to recursively traverse a directory tree and reduce the bit-depth of all PNG files within that tree from 24bpp to 16bpp? Commands should preserve the alpha layer and should not increase the file size of the PNGs - in fact a decrease would be preferable.
I have an OSX based system at my disposal and am familiar with the find command so am really more keen to to locate a suitable PNG utility command.
回答1:
AFAIK the only PNG format that supports the alpha layer is PNG-24; Reducing the PNG to another format may require specifying a transparent color in a CLUT, which will not give you the output you want.
From the feature list on PNG's website:
- 8- and 16-bit-per-sample (that is, 24- and 48-bit) truecolor support
- full alpha transparency in 8- and 16-bit modes, not just simple on-off transparency like GIF
... which I read to mean that anything other than PNG-24 or PNG-48 does not support full alpha transparency.
回答2:
Install fink
Say "fink install imagemagick" (might be "ImageMagick")
"convert -depth 16 old/foo.png new/foo.png"
If that did what you want, wrap it in a find call and be happy. If not, say "convert -help" and RTF-ImageMagick-M. :)
Optional: "fink install pngcrush" and run that as a second pass after the convert pass.
来源：https://stackoverflow.com/questions/1329873/reduce-bit-depth-of-png-files-from-the-command-line