The Moving Image
A film or video camera has a lens that focuses an image of the world onto a light-sensitive piece of film or a light-sensitive electronic chip. This part is much like the still camera. But how do we capture movement? The impression of continuous movement in a motion picture is an illusion. A film or video records a sequence of still images (frames) in rapid succession. In film, the standard frame rate is 24 frames per second, written 24 fps. When the images are then displayed one after another on a screen in a theater, TV, or a device, if the frames in the sequence change from one to the next quickly enough and the differences between them are not too great, the brain perceives smooth, realistic motion. This effect brings the magic of motion to film, video, or flipbooks.
The full picture of how the brain and eye perceive motion is still under debate. We know that for a realistic viewing experience, we need to create the illusion of smooth motion and consistent illumination. If the images change too slowly from one to the next, the illusion falls apart. Instead of smooth motion and consistent illumination, the screen may appear to flicker and get brighter and darker as the image changes.
Traditionally, this illusion has been explained by the persistence of vision, which is based on the idea that the eye retains an impression of each frame slightly longer than it is exposed to. According to this theory, when e a new frame is displayed, the eye blends it with the afterimage of the previous frame, creating a smooth transition between them. It is important to understand that this explanation is imperfect, as the aftermath image moves with your eyes. They don’t stay in one place if you look left or right.
A perceptual illusion called beta movement describes one situation in which viewers interpret successive still images as motion. A static shot of a ball flashes on the left side of the screen, then on the right side, and viewers see it moving from left to right. Think of a lightened ticker tape-style sign in a store, which seems to scroll from right to left across the display.
Video Systems
Camera and Recorder Basics
The video camera focuses its image on the flat surface of a solid-state electronic chip sensitive to light. This chip is the camera’s sensor or imager. There are two types of sensor chips: CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor). CCD is the most common chip in video cameras, although the CMOS is catching up. CCD is used interchangeably to mean both sensors or refer to it as the chip and not make a distinction between the two.
The surface of a CCD or CMOS sensor is divided into a very fine grid of spots or sites called pixels (picture elements). Each pixel in the chip is, in some ways, like a tiny light meter that reads the brightness of the light at that spot. When a pixel is struck by light, it creates and stores an electric charge. The more light that strikes it, the more charge builds up (this process can be compared to a bucket filling with rainwater).
A given sensor may have millions of pixels in a chip less than an inch across. To capture the whole picture, the charge of each pixel in the grid is read at an instant in time, and the output from all the pixels together is processed into the video signal, which puts the picture information into a format that can be recorded and displayed.
A video frame is displayed as a series of thin horizontal lines called scan lines.This pattern of scan lines is called a raster from the Latin word for rake: a video frame is a raked image. Different from video systems, use different numbers of scan lines.
NTSC (National Television System Committee) video is the traditional analog broadcasting system used in North America and Asia, including Japan, South Korea, and Taiwan. NTSC video has 525 lines per frame and a frame rate of about 30fps. (29.97 fps, tp be precise).
The PAL (Phase Alternative Line) standard is used in the UK, Western Europe, Australia, Asia, and South America. PAL has 625 lines, and the frame rate is exactly 25 fps.
With their pixel structure, today’s CCDs and CMOS sensors capture an entire frame simultaneously, like a film camera. Still, many also borrow a technique from tube cameras called interlace scanning, in which only half of the scan lines are recorded at a time.
Progressive and Interlacing Scanning
When a modern CCD and CMOS camera uses progressive scanning, all the pixels of each frame are captured simultaneously. Progressive scanning is similar to the way a film camera exposes an entire frame at once and then moves on to the next frame. Processing one whole frame at a time is very simple, very clean.
