Page 208 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes The JPEG Baseline is the defect standard that is used, and is the version for which this project
is based. The reasons for this are that is the simplest algorithm as well as being the most widely
supported because the JPEG standard specifies that all JPEG decoders must support the baseline
version, even if it uses some of the other features within the JPEG standard.

JPEGs are cross platform, i.e. the software that can run on multiple types of
computer systems.

12.4.1 JpeG fundamentals
A number of concepts need to be developed before JPEG encoding can be discussed in depth.
The JPEG standard is applicable to both grey scale and colour images. This is done by breaking
a picture down in to a number of components. When each component is put together, the end
image can be viewed. For example, a black and white image would have only one component
which would depict the grey scale of the image, whereas a colour image generally has three
components (one for each primary colour, red green and blue or equivalent).
Each component is made up of x columns and y rows of samples. These samples are simply
the actual “pixel” data of the image. In JPEG Baseline the number of samples is the same as the
resolution of the image. A single pass through a component and its samples is known as a scan.
In JPEG Baseline this does not mean a great deal as only one scan of the image is used.
In JPEG encoding the data is broken into a number of blocks called Minimum Coded Units (MCU’s).
These MCU’s are simply made by taking a number of 8 × 8 pixel sections of the source image.
The MCU’s are used to break down the image into workable blocks of data as well as to allow
manipulation of local image correlation at a given part of the image by the encoding algorithm.
When processing each MCU, the algorithm always moves through the component from the left
to right, then top to bottom order.

There are two methods of processing MCU’s and components. The first method is whereby each
MCU contains data from a single component only, and hence is comprised of only one unit of
data. The second method is where the data from each component is interleaved within a single
MCU. This means that each MCU contains all the data for a particular physical section of an
image, rather than a number of MCUs containing each component of that section. This project
uses the latter option
Further to this, there are horizontal and vertical sampling factors. These sampling factors dictate
how many 8 × 8 pixel sections are to be placed within an MCU when the component data is
interleaved.

12.4.2 Human visual system
A number of inherent properties of images and the human visual system can be manipulated to
allow for greater compression through the JPEG algorithm.
One of these is the tendency for the human eye to notice variations of brightness intensity much
more than variations of colour in an image. The JPEG algorithm can take advantage of this by
applying different rules for brightness and colour variations.
Another property is that real world images generally do not have sharp boundaries for brightness
intensity changes and colour changes. This means that spatial frequencies detected within an
image are generally low order, i.e. gradual change rather than quick changes over a localized
area of an image.
12.4.3 Colour system

The JPEG algorithm generally does not use the RGB (Red, Green, and Blue) colour format. Instead
it uses a derivative of this called YCbCr (Luminance and Chrominance).

202 LoveLy professionaL University