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I'm having a really hard time working with some source images in PNG format that have premultiplied alpha because most tools simply do not support it correctly.
Is there anything out there that can do a "best guess" conversion to a more conventional PNG?
If by "premultiplied alpha" you mean iOS's PNG derivative, then you can convert them back to PNGs with pngdefry.
Standard PNGs never use premultiplied alpha.