Computer Stores - This Part of Computer Animation Deals With Frame by Frame a Videotape, Anim

Frame-by-frame computer animation is a foundational technique in digital media, where individual images are created and then played in sequence to produce the illusion of movement. Historically, this often involved recording these animated frames directly onto videotape or film, a process that required specialized hardware and careful synchronization. Understanding the technical aspects of video signals and recording devices was crucial for animators working with early computer graphics systems, even those using desktop computers.

What is Frame-by-Frame Computer Animation?

This animation method involves generating each frame individually, often using graphics hardware. To record these frames onto a physical medium like videotape or film, a video recorder or a movie camera with single-frame exposure capabilities, along with a dark room, were essential prerequisites.

This technique allowed for the production of sophisticated graphics. Each animated picture, once completed in a frame buffer, could be recorded as a single frame. However, this process presented challenges:

Directly recording the frame buffer's content onto a magnetic track on videotape avoided the intermediate optical picture but introduced its own set of difficulties.

Understanding NTSC Composite Video Signals

The signal stored on videotape and the color TV broadcast signal are identical, known as the NTSC Composite video signal. This signal is complex, composed of various types of information:

In the NTSC standard, one TV frame consists of two interlaced fields. Each field contains 262.5 horizontal scan lines, totaling 525 interlaced scan lines for the entire TV frame. The composite TV signal typically carries 60 fields per second, which corresponds to 30 frames per second.

How Do Videotape Recorders Work?

Present-day videotape recorders are primarily of the helical type. In these devices, magnetic reading and writing heads are mounted on a spinning cylinder. The magnetic tape is pulled around this cylinder in a slanted track. The rotation speed of the cylinder and the tape speed vary depending on the specific machine.

Most modern video recorders record one field per track. The cylinder's read and write heads typically rotate at 60 revolutions per minute (rpm). The magnetic tape is wrapped almost completely around the cylinder in a slanted position. As the cylinder rotates, the video track is written or read along a slanted path across the tape.

Separate heads are required for recording audio signals onto a dedicated audio track. A pulse for each frame is also recorded. Later, a binary time code for each field can be added using specific devices, which is crucial for finding and identifying individual fields during videotape editing.

Types of Videotape Recorders for Quality Output

For the best quality video recording, C-type recorders, which use one-inch wide magnetic tape wrapped around spinning cylinders, were considered top-tier. In contrast, video cassette recorders (VCRs) generally produced lower picture quality. This was because their magnetic tape was enclosed in a cassette, not operated on an open reel, and was typically narrower (either 1/4 inch or 1/2 inch wide).

Connecting Graphics Systems to Video Recorders

When recording a single frame stored in a graphics system's frame buffer, the output from the frame buffer to the monitor can be handled in several ways. If the output is an RGB signal (three separate signals for red, green, and blue colors, with synchronous pulses on the green channel), it must be transformed into the NTSC standard to be recordable on videotape.

Mid-range graphics cards can produce an NTSC composite video output directly. A "genlock" feature is often necessary to synchronize the frame buffer's scanning rate with the recording device's rate. It's also important to note that while frame buffers may have varying numbers of scan lines, NTSC frames consistently have 525.

The Rise of Digital Video Recording

With evolving technology, many digital video recording devices have emerged, simplifying these tasks considerably. Early precursors to these were RGB video cameras, which stored information in RGB code rather than NTSC composite. RGB video cameras recorded the red, blue, and green intensities, along with horizontal synchronous signals, for each scan line. To play back such a tape, an RGB monitor or a converter to produce an NTSC signal for a TV screen was required.

Storing Animation Frames and Real-Time Graphics

The question of storing animation frames in memory is central to computer graphics. Even low-priced desktop computers can display complex animated scenes. For example, a juggling robot animation on an Amiga computer, where scenes were created by ray tracing, showcased this capability. The robot, composed of partially overlapping, non-reflective spheres, juggled reflective spheres that cast shadows on the ground. This frame-by-frame animation typically consisted of 20 frames per second.

In applications like CAD/CAM or paint and draw programs, real-time animation is present on the screen in the form of "rubber band" lines or growing rectangles that respond to input device movements. In these cases, animation is created by rapidly drawing, erasing, and then redrawing figures as needed.