PNGOUT
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pngquant is a command-line utility and a library for lossy compression of PNG images.
The conversion reduces file sizes significantly (often as much as 70%) and preserves 
full alpha transparency. Generated images are compatible with all web browsers and 
operating systems. https://pngquant.org/

OptiPNG
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Copyright (C) 2001-2017 Cosmin Truta and the Contributing Authors.
For the purpose of copyright and licensing, the list of Contributing
Authors is available in the accompanying AUTHORS file.

This software is provided 'as-is', without any express or implied
warranty.  In no event will the author(s) be held liable for any damages
arising from the use of this software.

Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must not
   claim that you wrote the original software.  If you use this software
   in a product, an acknowledgment in the product documentation would be
   appreciated but is not required.

2. Altered source versions must be plainly marked as such, and must not
   be misrepresented as being the original software.

3. This notice may not be removed or altered from any source distribution.