We strongly recommend using the After Effects SDK to develop effects plugins.
Almost all of the effects included in Premiere Pro are After Effects plugins, and future development will be based on the After Effects API.
Video filters process a video frame into a destination frame. Filter parameters can vary with time.
Premiere provides basic user interface in the Effect Controls panel, drawing sliders, color pickers, angle dials, and checkboxes based on the parameter definitions in the PiPL resource. Video filters can have their own custom modal setup dialog for additional settings.