Axis 2400+ Manuale Utente Scaricare il pdf (Pagina 34)

Many compression standards

to choose from

Cameras using still image standards send single images

over the network. Cameras using video standards send

still images mixed with data containing the changes.

This way, non-changing data, like the background, is

not sent in every image. Video standards also include

audio in the data stream. The image refresh rate is

referred to as frames per second, or fps for short.

Main differences between still image

and video compression

• Still image compression is simple and easy to work with

• It is difﬁ cult to obtain a single image from a video

stream using video compression

• Video compressions use less data to store/transmit a

video sequence

• It is not possible to reduce the frame rate when using

video compression

• Still image compression is more suitable when using

a modem, or other media that can only offer narrow

bandwidth

With the exception of Motion-JPEG, all video com-

pression standards mix still images with partially

complete images. By storing only the changes from

one full image to another, these ‘partially complete’

images reduce the ﬁ le size of the compressed video

sequence. Scenes containing little or no variation can

be compressed quite dramatically.

NB. When using Pan, Tilt and/or Zoom camera

functions large image variations can occur as the result

of sudden camera movement.

Compression standards for still images

JPEG

This is short for Joint Photographic Experts Group

international – a good and very popular standard for

still images that is supported by many modern programs.

Conforming to ISO/IEC 10918, this is the preferred

compression standard for many network cameras.

Each image is divided into 8x8 pixels. Each block is

then individually compressed using the Digital Cosine

Transform (DCT). If a very high compression ratio is used,

the 8x8 pixel blocks can actually be seen in the image.

Wavelet

This standard is optimized for images containing low

amounts of data. Consequently, the images are not of

the highest quality. Wavelet is not standardized and

requires special software for viewing.

JPEG 2000

Based on Wavelet (and not JPEG) technology, this is a

new and still relatively little-used standard.

GIF

This is short for Graphics Interchange Format - a bit-

mapped graphics ﬁ le format used widely on the Web.

Limited to 256 colors, it is a good standard for images

that are not too complex, e.g. scanned images and logo-

types. It is not recommended for network cameras as

the compression ratio is too limited.

Compression standards

Digital images and digital video are always compressed in order to

save space on hard disks and to speed up transmission. Usually the

compression ratio is 10-100. An uncompressed image with a resolution

of 640x480 pixels is approximately 600 kB (2 bytes per pixel).

Compressed 25 times the image ﬁ le size will be approximately 25 kB.

I MAGE QUALITY

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 51 52

Commenti su questo manuale

Nessun commento

Axis 2400+ Manuale Utente Pagina 34

Commenti su questo manuale

Prodotti e manuali riguardandi Networking Axis 2400+