A holofilter was a device used to hide a person's real identity from visual contact during communications. The holofilter took the image from one end and changed it to a new appropriate setting so that when the person moved, the holographic image moves the same way.
The USS Defiant was equipped with a holofilter. In 2371, to enter Cardassian space, Commander Benjamin Sisko used one to make himself appear to be a Kobheerian captain to Cardassian patrols. (DS9: "Second Skin")
Gul Dukat's Klingon Bird-of-Prey was also equipped with one. In 2373, when he transported Worf, Sisko, O'Brien, and Odo to Ty'Gokor, he replaced his image with that of a rather imposing Klingon. (DS9: "Apocalypse Rising")