The FXScript Reference

RenderEachFrameWhenStill tells FXScript that each frame of the effect should be processed regardless of whether or not the source changes over time (ie. is a still image or generator). Anytime an effect uses an input clip (user added additional video sources), it's a good idea to add this flag. Without RenderEachFrameWhenStill, the value of Frame is always 0 (or 0 and 1, depending on whether the effect is fullframe or not), and it's impossible to tell GetVideo to pull other frames from the additional clip.