The Lightroom Journal posted an article on Adobe's current feelings about Plugins and External Editors. During the beta testing of Adobe's Lightroom 2.0, there were many posts about the ability for Lightroom to perform certain tasks. This was especially true regarding functions available in Adobe Bridge, Adobe Photoshop and other programs. This article has a great insight into the purpose of non-destructive editing, which is new in the Adobe lineup with the creation of Lightroom 1.0.
While leaps were made in external editing - smart objects, multiple external editor links - the population of true Plugins for Lightroom will be up to the programmers of the world to create non-destructive products.
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