Film frame

• 1. Introduction

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  A film frame is a single still image, one of the many that make up a complete moving picture. At 24 frames per second, a two-hour film contains more than 170,000 of them. Each one flashes on screen for a fraction of a second before being replaced by the next, and a phenomenon called persistence of vision does the rest, blending those discrete images into the illusion of fluid motion.

  The word "frame" itself comes from the physical look of early film stock. Hold a strip of movie film up to the light and each image sits inside a bordered rectangle that looks exactly like a picture in a frame. From that practical observation, the term spread outward to cover nearly every discipline in the moving image world: filmmaking, video production, animation, broadcasting.

  What does it take to persuade the human eye that a series of photographs is a river of continuous movement? How do the engineers who set frame rate standards balance image quality against the hard limits of transmission technology? And what happens when animators deliberately break the invisible boundary at the edge of the frame itself? The answers stretch from the physics of human vision to the cue marks at the corner of a reel in a film like Inglourious Basterds.

• 2. The Physics of Illusion

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  Persistence of vision is the mechanism that makes cinema possible. When each frame is replaced by the next fast enough, the eye does not perceive individual images but reads them as unbroken movement. The brain fills the gap between one still photograph and the next.

  The frame rate needed to achieve this illusion is not fixed. Silent films and 8mm amateur movies ran at 16 or 18 frames per second. Modern cinema projection settled at 24 frames per second as the standard. Special venue systems pushed further: IMAX, Showscan, and Iwerks 70 have used 30, 48, or even 60 frames per second to sharpen the sense of motion and reduce flicker on their unusually large screens.

  Broadcasting introduced a different set of compromises. In North America and Japan, 30 frames per second became the broadcast standard, while much of the rest of the world standardized on 25. Systems rooted in the NTSC standard operate at a frame rate of exactly 29.97 frames per second, a figure that traces back to the design of the Chromilog NTSC television system. That fractional difference from a round 30 creates synchronization problems unknown in other broadcasting environments. Engineers developed drop-frame timecode as a workaround, a technique specific to the NTSC world and the peculiarities its history imposed.

• 3. Size and Material

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  Not all film frames are the same size. In the smallest 8mm amateur format, a single frame measures roughly 4.8 by 3.5 millimeters. At the other end of the scale, an IMAX frame reaches 69.6 by 48.5 millimeters. The relationship matters because the larger the frame relative to the projection screen, the sharper the resulting image.

  The most widely used format, 35mm film, carries its own internal complexity. In a still camera, where the film moves horizontally, the frame measures 36 by 24 millimeters. When used for motion picture work, the film moves vertically instead, and the frame size changes accordingly. Two exceptions exist: VistaVision and Technirama both kept the film moving horizontally even in motion picture production. In the standard 4-perf pulldown configuration, exactly 16 frames fit into one foot of 35mm film, which is why crew members and editors have historically counted footage in "feet and frames."

  The silent-era maximum frame size on 35mm is 18 by 24 millimeters, a figure that shrinks once a sound track is added to the strip. Individual frames within a production are tracked using a system called KeyKode, which lets editors and post-production teams locate a specific physical image in a reel without ambiguity. Each frame is also separated from its neighbors on the strip by a thin line called a frame line, a detail that would later give animators and filmmakers material for comedy.

• 4. From Analog Waves to Digital Pixels

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  Video frames began their life as analog waveforms. Varying voltages represented the intensity of light scanned across a screen in horizontal bands, with blanking intervals between frames playing the same role that frame lines played on physical film. Most early systems used interlaced scanning: each frame was divided into two fields, with odd-numbered lines captured in the first field and even-numbered lines in the second. The two fields were sampled at slightly different moments in time, which meant a single paused video frame was rarely a clean still image of anything in motion.

  The shift to digital changed the fundamental representation. A modern video frame is a rectangular grid of pixels, described in a color space such as RGB or YCbCr. Standards bodies codified these grids: Rec.601 governs standard-definition television frames, and Rec.709 governs high-definition frames. Video frames are identified in post-production using SMPTE timecode, a standardized addressing system that gives every frame a unique label.

  Resolution in a video frame comes from two intersecting dimensions. Each horizontal band of pixels is called a line, and the total resolution is the product of the number of lines and the maximum number of sine signals that can be transmitted per line. Doubling the resolution produces better image fidelity but raises costs and technical demands, so every system design reaches a compromise. In the example of System B, which runs at 25 frames per second with 625 lines and a maximum video bandwidth of 5 MHz, roughly 19 percent of each line is reserved for auxiliary services, leaving about 260 useful sine signals per line rather than the theoretical maximum of 320.

• 5. The Still Within the Moving

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  Freeze frames carry several names. A single static image extracted from film or video may be called a still frame, a freeze frame, a video prompt, a preview, a keyframe, a poster frame, or a screen grab, among other terms. Video platforms widely use them as previews, with many services defaulting to a frame taken from the midpoint of the video. Some platforms let creators select a different frame individually.

  Filmmakers and video artists have long used deliberately frozen frames within a moving work to create specific effects. A narrative film can hold a single image to punctuate a moment or to signal the end of action.

  Beyond artistic use, still frames from surveillance footage have become a standard tool in criminal investigations, published to identify suspects and invite witnesses to come forward. The frames of the film recording of the Kennedy assassination have been analyzed repeatedly, with different investigators reading different things into the sequence when examined image by image. Medical diagnostics also rely on still frames extracted from Magnetic Resonance Imaging videos, where freezing the image allows clinicians to examine a specific moment in a scan without the distraction of movement.

• 6. Breaking the Frame

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  Some of the most inventive humor in animation comes from treating the physical border of the film frame as a surface characters can interact with. Early cartoons established a tradition of characters who are aware they exist inside a cartoon, able to look at credits and acknowledge things outside the story being told.

  Split frames take the joke to a structural level. When a projector does not advance the film quite right, the lower half of one frame and the upper portion of the next can appear on screen simultaneously, dividing the image with a visible frame line. Animators turned this malfunction into a punchline.

  Film break gags exploit a different interruption. The film appears to snap or burn through, leaving the screen dark or white at the moment when, technically, nothing should be visible. Characters sometimes appear in that blank space to comment on their situation.

  Gate hair jokes are more subtle. The gate of a film camera can trap a hair or piece of dust that then appears in every subsequent frame. In these animated gags, a character reaches up and plucks the apparent hair from the image itself. Editorial marks, the symbols an editor draws on a work print to indicate a fade or a dissolve for the effects department, become something characters can respond to or ignore.

  Cue marks sit in the corners of film frames to signal a reel change to the projectionist. Normally, these marks are small and unobtrusive. In Inglourious Basterds, the cue marks for the reel changes of the Nation's Pride pseudo-documentary used exceptionally large scribed circles each containing a large X, marks that would never appear in standard editorial practice and that served the film's comedic and meta-cinematic purposes. The convention of cue marks has also been borrowed for the "false ending" effect, used in popular music as well as film.