With images, most of the data is already in a digital form before
the PC receives it. Cameras and scanners have already taken
the analog image and converted it to digital data during the
process of making an image. There are also digital video cameras
that use the Fire-Wire/i.Link or USB connection to
input digital information directly to your PC.
Analog videos also need to be converted to digital format. These
images include most camcorder images, television video, and
filmed video. As discussed earlier in this study unit, converting
analog input to digital data requires sampling the input and
then digitizing it. The sampling and digitizing is done using
the same method as for sound input, pulse code modulation
(PCM). PCM turns the analog input into a series of 0s and 1s
by sampling the input at set time periods. So in simple terms,
digital video is a sampled form of analog video.
The most common sampling schemes in use today are listed:
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VIDEO SAMPLING
RATE
|
|
|
|
Clock Rate
|
Horizontal Pixels
|
Vertical Pixels
|
NTSC square pixel
|
12.27 MHz
|
780
|
525
|
NTSC CCIR-601
|
13.5 MHz
|
858
|
525
|
NTSC 4FSc
|
14.32
|
910
|
525
|
PAL square pixel
|
14.75 MHz
|
944
|
625
|
PAL CCIR-601
|
13.5 MHz
|
864
|
625
|
PAL
4FSc
|
17.72 MHz
|
1135
|
625
|
Technology has progressed to the point that high-quality
digital video is available and affordable. An important innovation enabling
this growth has been the emergence of high-definition
multimedia interface (HDMI), a compact audio/video interface for transmitting
uncompressed digital data. A single HDMI link can transfer up to 24 bits of
user data at 165 megapixels per second, resulting in a massive bandwidth of
nearly 4 Gbps. HDMI can support all existing and planned PC or TV video
formats, including:
•
SDTV: 720 480i (NTSC), 720 576i
(PAL)
•
EDTV: 640 480p (VGA), 720 480p (NTSC progressive),
720 576p (PAL progressive)
•
HDTV: 1280 720p, 1920 1080i
MPEG-2 and MPEG-4 Files
MPEG files for video are supported by the MPEG-2 and MPEG-4
formats. These formats are commonly used for commercially available movies.
Creating videos in these formats and transferring the data onto a DVD can often
produce videos playable on your home DVD player. These formats displays a
relatively high-quality image in high-end computers, but a jerky image on
lower-end computers.
The MPEG video files are of
higher resolution (at least 352 240 pixels) and better quality (150 KB/s, 30 fps) than many equivalent
files of other formats. The .mpg files require either an MPEG playback video
board (hardware assisted playback) or a Pentium MMX CPU or better with an AGP
or PCI graphics card and .mpg playback software. With this combination of
hardware and software, it’s possible to play commercially available movies on
your PCs DVD player if it supports the DVD- or DVD+ standards.
Under the category of MPEG-2 and
MPEG-4 videos is DivX™. This is a proprietary, MPEG-4 based video compression
technology, using its own CODEC, which can shrink digital video to sizes small
enough to be transported over the Internet, while maintaining high visual
quality. This CODEC can reduce an MPEG-2 video (the same format used for DVD or
Pay-Per-View) to ten percent of its original size. Video on regular VHS tapes
can be reduced to about one hundredth of their original size. To view a DivX
compressed version of the video, you need a media player designed to play DivX
encoded content. This player can be downloaded free from www.divx.com/divx.
The DivX CODEC is included in this DivX software bundle.
DivX files aren’t compatible with the DVD– or DVD+ file
types, and won’t play on stand-alone video players. As with all technologies, the
advantages of the DivX CODEC (small file size) need to be weighed against its
disadvantages (incompatibility issues).
QuickTime
QuickTime is a proprietary multimedia framework developed by
Apple, capable of handling various formats of digital video, picture, sound,
panoramic images, and interactivity. It’s available at no cost for Windows XP,
Vista, and Windows 7, etc. QuickTime handles popular audio, graphic, and video
formats, including MPEG-2, H.264, and MPEG-4. The Pro version adds features such as full-screen playback, easy editing, production, and the ability to save
movies from the Web.
Compress the Data (Still Image Files)
Still images are usually
sent to the PC using the TWAIN API. TWAIN was designed to provide mechanisms
for transferring images from the source device to the image application (such
as Photoshop). Multimedia applications don’t accept TWAIN files as one of the
image file types used in presentations. The image files need to be converted
into a compatible format.
BMP
This file type is reserved for bit map
(often spelled “bitmap”) files. Bit maps define the display space and color for
each
pixel or “bit” on the display space. These files aren’t
compressed and have a tendency to be quite large. One good thing about .bmp files
is that they don’t need to contain a bit of color-coded information for each
pixel on every row; they need to contain information indicating a new color
only as the display scans along a row. This way an image with large areas of
solid color will require a smaller bit map.
Bit maps use the raster graphics
method of specifying an image. A raster is
a grid that defines how an array of pixels (in x- and y-coordinates) is
illuminated, be it monochrome or color. Changing values for one pixel is rather
difficult. BMP, GIF, JPEG, and TIFF files are examples of raster graphic files.
For this reason, you can’t immediately re-scale a bit map image without losing
image quality. In direct contrast to this, vector graphics images are designed to be quickly re-scaled. A vector file (also known as a geometric file) uses commands and mathematical calculations to render an image. Brightness, density, tone, and color are easy to modify. Typically, an image is created using vector graphics editor and then converted to a raster graphic file or bit map for use in a multimedia application.
EMF
Enhanced Meta-File
(EMF) is a term for spool file formats used in printing by the Windows OS.
When spooled (which stands for “simultaneous peripheral operations online”), a computer document is read and stored (usually
on a hard disk) so that it can be printed/processed later.
When
a print job is sent to the printer and it’s already printing another file,
the computer reads the new file and stores it. Spooling allows multiple print
jobs to be sent to the printer at one time.
Vector graphics are digital images created through a sequence of
commands that place lines and shapes in a given space. In physics, a vector is
a representation of both a quantity and a direction at the same time. In vector
graphics, the file is created and saved as a sequence of vector statements.
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