Rescan
Also called: camera at TV, screen rescan, filming a monitor, TV recording
Rescan is the look of a camera filming a CRT screen: refresh bars, moire and soft glow. Learn what it does to a picture and how to build it with ECHO PARASITE.
WHAT CAUSES IT
Rescan is the process of pointing a video camera at a display and capturing what it shows, so the picture is scanned twice, once by the electron beam and once by the camera. The first scan paints the image one line at a time with a beam that fades quickly, so at any instant only a few dozen lines are lit. A camera whose shutter stays open for a different time than the display refresh catches the screen at various points of that sweep, which prints dark rolling bars across the frame. Camera sensors sample on a regular grid, and the display has its own grid of phosphor triads and scanlines, so the two grids interfere and produce moire patterns. Every pass adds optical blur, glare from the glass, gamma changes, color shifts, perspective distortion from the angle, and curvature from the tube face. The glow of the phosphor adds a slow trail of afterimage. Early video synthesizers, such as scan converters, used rescan deliberately to change line standards and to build feedback.
WHAT IT LOOKS LIKE
A picture that looks like a photograph of a television: softened, slightly curved at the corners, with a gentle vignette and a glow around bright areas. Dark horizontal bars roll slowly up or down the frame when the shutter and refresh drift, and a fine moire shimmers over fine detail, especially against the scanline structure. Colors shift toward a warm or green cast and contrast rises. The image can show a faint double from glass reflections and a visible grid of lines. Motion leaves faint trails because the phosphor keeps glowing after the beam moves on.
WHERE YOU SEE IT
Broadcast footage of a television shown in another television, archival films of early computer displays, 1990s documentaries with a monitor in the shot, VHS copies made by pointing a camcorder at a screen, and music videos that film monitors on purpose. Rescan is a standard ingredient of analog horror and found-footage, where a screen being filmed signals that the picture has already been through a second device. Video artists also loop rescan to get feedback.
RECREATE IT
ECHO PARASITE
CONTROLS RESCAN, ECHO DECAY, PARASITE WARP
RESCAN recreates the camera-pointed-at-a-TV look and is the main control. Raise it gradually and judge the glow and softness on a bright shot. Use ECHO DECAY to set how long the phosphor-style afterglow lingers, and add a little PARASITE WARP for the slight geometric distortion from the screen. Avoid very high settings, which erase the picture and leave only a tunnel.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Pre-compose the footage and apply ECHO PARASITE. 2. Raise RESCAN slowly until the picture takes on the soft, glowing look of a filmed screen. 3. Add ECHO DECAY to lengthen the afterglow; watch bright objects crossing the frame to judge it, since the trail is most visible there. 4. Add a small amount of PARASITE WARP to introduce curvature and slight drift. 5. Play the shot from the start of the composition, because the effect depends on previous frames and may need a few frames to build. 6. If you want the rolling dark bars of a shutter mismatch, add them as a separate layer with a mask, or use STATIC DRIFT with a slow DRIFT ROLL for a moving band. 7. Compare against a photograph of a real filmed screen. 8. For a repeated rescan, such as a screen filming a screen, apply the effect twice at lower settings instead of one extreme pass. 9. Render a test sequence and check the first few seconds for startup behavior.
QUESTIONS
- Why do dark bars appear when you film a TV?
- The camera's shutter opens for a time that does not match the display's refresh, so each frame catches the screen at a different point of the scan. Dark regions are areas the beam has not yet redrawn when the shutter was open.
- How do you avoid moire when filming a screen?
- Change the angle or distance, defocus slightly, or use a different resolution camera so the sampling grids do not line up. Matching the shutter to the refresh rate removes the bars. Moire is a property of two grids and cannot be removed afterward.
- Is rescan the same as video feedback?
- Rescan is one pass of a camera filming a screen. Feedback happens when that camera sees its own output, so the picture repeats and builds a tunnel. Rescan is the first step in the loop, so the two share many visual traits.
- Why does a filmed screen look so soft and warm?
- The glass adds reflections, the camera lens blurs, the phosphor spreads light around each line, and the color balance of the tube differs from the camera. The result is a lower contrast, glowing picture with a color cast. Each of those changes is small alone, but together they are why a filmed screen is instantly recognizable.