Canon XL 1 Manual do utilizador - Página 21

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There are other techniques which can have as much effect on compressibility as the

above mentioned techniques. Because these techniques occur later in the process, it is

often possible to adjust them based on the results of test compressions, making fine-

tuning much easier.

• Static versus moving images As in production, superfluous moving images aren't

appropriate for content that you plan to aggressively compress. They are not that

important for DVD, but they are for web and CD-ROM.

• Motion blur In the same way that a real-world shutter speed introduces motion blur

that helps compression, you can use the virtual shutter in rendering applications. For

effects that support motion blur, render them with a shutter speed that matches that

of the source. This speed might be expressed as a duration, or, as in After Effects, as

the angle of a virtual shutter. The default is 180°, which corresponds to a shutter

speed half that of the frame rate.

• Anti-aliasing This rendering technique softens the edges of rendered elements,

letting a pixel at the edge of an area be an intermediate value. Smoothing out

transitions among parts of the image improves compressibility and appearance.

Capturing video: choosing formats and connections

• Analogue formats Analogue video uses fluctuating voltage and frequency signals to

transmit image data. It ranges greatly in quality but is inherently susceptible to

visual interference from electronic noise, like power lines, and suffers attenuation,

or the loss of signal power and quality over long cable runs (usually 300 meters or

more). Equipment that supports analogue signals is generally more affordable, and

analogue video formats are still the most commonly used because of the long-term

use of legacy equipment that is still active today.

Because computers only use digital data, the analogue source must be converted to a

digital copy to be captured. When it is sent back to an analogue medium like

videotape, it is converted back to an analogue signal. This conversion can lead to

minor errors (quantization errors) being introduced because of the imprecise

relationship between analogue and digital signals. You can minimize these

quantization errors by using the right format, good equipment, cables, and

connections.

• Composite A composite video signal carries the luminance and the chrominance

channels in a single signal. The biggest problem with composite video is that luma

information can leak into chroma, and vice versa. This leakage can lead to noise,

known as chroma crawl, which can be amplified by the video capture process and

reduce the overall quality. Composite video is generally understood to be the lowest

quality video signal that you can use and is more appropriate for video delivery

(broadcast) than as a source for content creation (video editing, DVD authoring, or

encoding for the web). Common sources of composite video are VHS (1/2 inch

tape) VCRs and camcorders, broadcast television signals, and older U-Matic 3/4-

inch professional VTRs.

The connectors common to composite video are RCA or Cinch for consumer

equipment and BNC (Bayonet Neill-Concelman), a twist-locking connector