I ask the flash viewer creator about the license, and if I can redistribute it with my plugin.
Here is his anwser : 
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Hello,

You can use my stereoscopic flash viewer freely.

Masuji
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so I can redistribute it with my plugin but it's not a free (like free speech) software, it's not under GPL (the plugin is under gpl Not the flash player), you can use it on your website with or without the piwigo plugin. I don't know if you can redistribute it or if the authorisation is only for my plugin, so in that case, ask the author :  http://stereo.jpn.org/eng/index.html