On-set markers — reflective dots or objects in frame — that VFX uses later for tracking and spatial reference. Critical for rotoscoping, tracking, and motion matching accuracy.
On set, you place small, reflective markers—gaffer tape, reflectors, or special witness points—within your frame to give the VFX team precise spatial anchor points later. These markers are not decoration; they are measuring instruments. The compositor needs them to know how the camera moved, the depth of the space, and where objects actually stood before you digitally alter or insert them.
In practice, this works as follows: During the shoot—whether it's a greenscreen, compositing shot, or motion capture session—you place the markers in positions that will remain visible later but do not interfere with the performance. These can be small, highly reflective dots on stage edges, on the floor, or on set furniture. The VFX supervisor also photographs reference cards: a white sheet of paper or a special color reference chart with known dimensions, shot under the same lighting as the scene. This combination—witness points plus reference card—provides the tracking software with three essential pieces of information later: spatial position (X, Y, Z), lighting situation, and true scale.
For rotoscoping, for example, you need these points for calibration: the artist can calculate the camera's movement and precisely fit your element to be matted into the space. For motion matching for match cuts—when you want to replace a location with a digital environment in real-time—witness points are the only reliable orientation. Without them, the VFX supervisor works blind; they have to derive everything manually based on perspective lines and shadows.
A practical tip: Mark the reference points so they are in the background or easily retouched, but remain clearly visible. Document their position—photograph them along with the scene, note distances to the camera position. A Polaroid or a quick stills reference card from the focus puller is worth its weight in gold. Some sets today also use digital markers (AR-based tracking systems), but physical witness points are still the standard—they work regardless of the technology used later in post-production.