One brake on the GIMP's popularity is that, while it boasts dozens of filters, a rival like Photoshop boasts thousands. You may only occasionally need a special effect that imitates a pencil sketch or a famous style of painting such as Impressionism or Cubism, but, when you do, having a filter to create the effect instantly saves serious amount of time. To help bridge this divide, the GIMP is reviving the User Filter from its 1.x releases. This filter is a kind of meta-plugin that allows users to import and manage Photoship filters or, if they have the knowledge, to write their own.
The GIMP User Filter is available from the project's SourceForge.net site as source code or as a Debian package that may or may not work on Ubuntu, to judge from mailing list chatter. Once you install it, you will find it under Filter - > Generic -> User Filter.
You can find links to Photoshop filters through the portal site Photoshop Filters.com.
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