Video special effects apparatus comprises a core program object; and a plurality of render processor plug-in program objects responsive to the core program object, the render processors being arranged in directed acyclic graph, each operable to render an output result relating to successive images of a video signal from input data relating to that and/or other images received from a preceding render processor in the directed acyclic graph and in response to operational parameters relating to that render processor; in which each operational parameter is defined by a parameter plug-in object arranged to communicate a parameter value to the render processor plug-in object.