Re:Photoshop 1.0 Source Code
by nitrogen on Thursday February 14, 2013 @04:16AM
(#5218605)
Photoshop 1 is old enough that any such patents should have expired.
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If that is true you could reimplement Photoshop 1.0 with the clean room method [1] and have a perfectly legal clone, which you would then be free to expand.