Higher Computing
Multimedia Technology
Graphics
I Power
Higher
Int. 2
Digital Cameras: CCD Array
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In digital still cameras and video cameras,
a CCD array is used to capture the image.
CCD is short for charge-coupled device.
A CCD is basically a series of photo
sensors that are connected together.
The sensors turn the light levels into
analogue signals.
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Scanners: linear CCD
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Uses a CCD, a row of photo sensors linked
together, in a similar way to a digital camera but
they are not in arrays but set in a row.
This is known as linear CCD.
The scanner uses a moving scan head, with a row
of photo sensors, taking in the width of document
at a single pass.
Some scanners will have more than one row.
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Analogue to Digital Convertors
(ADCs)
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The analogue signals from the array of
CCDs is fed to analogue to digital
convertors (ADCs).
These ADCs receive continuous streams of
analogue current which they convert to
digital data representing the image.
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Digital to Analogue Converter
(DAC)
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The digital to analogue converter (DAC) reverses
the process which is carried out by the ADC.
The DAC takes a series of discrete digital values
and converts them to a signal whose amplitude
varies according to the digital data.
An example of a DAC in action is turning a
digital music file into sound output to speakers.
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Colour LookUp Table (CLUT)
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A colour lookup table is a facility available in
graphics applications which enables the user to
specify a subset of colours to be used in the
creation of graphics.
A user may specify a CLUT which meets the
need of printers or of displaying web pages.
For example, in web design a CLUT would be
used to specify a set of colours that would be
easily handled by different systems and browsers.
Higher
Int. 2
24-bit Bitmap
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24-bit bitmap will give true colour: 224
colours. Each pixel will need 3 bytes to
encode. File sizes get very big and
storage/transmission can be a problem.
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Compressed Bitmap (RLE)
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Because of the large file sizes involved in
bitmaps, some image formats have built in
compression called run length encoding (RLE).
RLE takes advantage of the fact that, in many
images, large stretches of pixels are exactly the
same uses a keybyte which tells the software
whether the next byte represents several pixels or
only one.
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RLE (Cont.)
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Let us take a single line in a graphic with B
representing a black pixel and W representing white:
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If we apply a simple Run-Length Code to the above
graphic line, we get the following:
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WWWWWWWWWWWWBWWWWWWWWWWWW
BBBWWWWWWWWWWWWWWWWWWWWWW
WWBWWWWWWWWWWWWWW
12WB12W3B24WB14W
Interpret this as twelve W's, one B, twelve W's, three
B's, etc. The run-length code represents the original
67 characters in only 16.
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RLE (Cont.)
•Ideal for RLE
Compression
•Poor for RLE
Compression
Higher
Int. 2
GIF
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A graphics file format which uses lossless
compression techniques. The graphics
interchange format supports 256 colours
and needs 1 byte per pixel to encode a
graphic.
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GIF Animation
An
animation is a series of graphics that are displayed in
rapid sequence in a web browser to appear as a moving
picture.
Frames
In
animation software each still image is called a frame.
GIF animation is a common way of creating animated
graphics.
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GIF Animation (Cont.)
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To get a good realistic level of animation the system has
to process around 24 frames per second. This means that
the files storing the animations can be very large.
To compress the files a technique known as LZW is
commonly used.
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LZW
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A method of compression which makes use
of repeated strings of data.
Lempel-Ziv Welch compression stores the
repeated patterns of data in a dictionary and
then uses pointers to point to the dictionary.
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LZW (Cont.)
•This pattern is repeated throughout
the picture.
•The pattern is stored in a dictionary.
•When it is used again we can
just tell the computer that it
was the same pattern as
before.
Higher
Int. 2
Joint Photographic Experts
Groups (JPEG)
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This is a graphics file format which uses a lossy
technique.
That means it cuts out aspects of the graphic that won't be
noticed by the human eye. The level of compression in a
JPEG file can be varied. The higher the degree of
compression, the lower the graphic quality.
At a compression ratio of 100:1 a file will be considerably
smaller, for example a 100 MB file would be reduced to 1
MB in size, but the quality of the graphic would be
noticeably downgraded.
A compression rate of 20:1 produces little noticeable loss.
Higher
Int. 2
JPEG (Cont.)
• Once an image is compressed using JPEG compression,
data is lost and you cannot recover it from that image file.
Always save an uncompressed original file of your
graphics or photographs as backup.
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PNG
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This is short for Portable Network Graphics: a
file format for bit-mapped graphic images,
designed to replace GIF. Like GIF it uses a
lossless compression technique.
PNG has the following features that are available
in GIF format:
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a range of 256 colours;
Interlaced images;
supports data streaming;
has a transparency feature;
parts of an image can be marked as transparent;
allows text comments to be stored within the image
file.
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PNG (Cont.)
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PNG also has:
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compression between 5-25% more than the
equivalent GIF file;
bit depth of 48 bits per pixel, giving a range of 248
colours;
greyscale encoding up to 16 bits per pixel;
Opacity, giving control of the degree of transparency
of a graphic;
support for automatic display of images with correct
brightness/contrast despite variations in hardware
between the systems creating and those displaying
the graphic;
a file corruption detection mechanism
Here is a comparison of the files
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RGB colour codes
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The RGB colour model defines the different
amounts of red, green and blue (the primary
colours) present in an image and is ideal for use
in display.
This colour code can be used to produce a palette
of 224 colours (true colour): the amount of
colours which are recognised by the human eye.
Here is an animation
Higher
Int. 2
Calculating File Sizes
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You need to be able to perform calculations using
the relationship:
File Size =
Resolution
X
(Dots per inch)
E.g.
1600 x 1200
Size
X
(no of Sq inches)
X
8 x 10
=1228800000bits
=153600000 bytes
=146.4844 Megabytes
X
Colour Depth
(no of bits)
8
Divide by 8 to get answer in
Bytes by 1024 twice to
Divide
get answer in Mb.
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Dithering
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Dithering is a technique used by GIF graphics.
It is used to soften jagged edges in lines and curves at low
resolution. The human eye tends to blur spots of different
colours. Dithering takes advantage of this by identifying
groups of dots or pixels that have a pattern then merging
them into a single shade or colour.
Using this technique it can create an illusion of varying
shades of grey or colour based on its own 256 colour
palette.
Dithering works well in graphics where there are few
colours. It is not so successful when dealing with photos
with, for example, thousands of colours.
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Dithering (Cont.)
When
there aren't enough colours in a display system
to render an image properly, an infinite palette can be
created by dithering. Quite often, a 24-bit colour image
is dithered to 256 colours (8bits).
This is a 24-bit colour picture which has
been dithered to 2 colours (1 Bit).
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Anti-aliasing
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Aliasing refers to the jagged appearance of curves or
diagonal lines on low resolution displays.
Anti-aliasing is a software technique for smoothing these
edges. Techniques used include surrounding pixels with
intermediate shades and manipulating the size and
horizontal alignment of pixels.
If the image you have scanned in is of poor quality AntiAliased then one solution is to rescan the image at a
higher resolution. This will increase the density of pixels,
give a clearer image but increase the file size.
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Anti-aliasing Example
Anti-Aliasied
Aliased
The higher resolution will get rid of the problem of
aliasing since this occurs only at low resolutions.
Higher
Int. 2
Vector Graphics
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Vector graphics (object-oriented graphics)
contain a number of individual objects grouped
together to make an image.
These objects are described mathematically in the
computer.
These are resolution independent. The images
are drawn to the best possible resolution of the
output device.
This also means the objects can be rescaled
without loss of quality.
Higher
Int. 2
Vector Graphics (Cont.)
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Each object is independent of the others.
They all exist within their own layer of the
drawing and can be individually edited,
moved, overlapped or deleted.
Higher
Int. 2
Vector Graphics (Cont.)
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The files are merely a list of the objects on
the screen and the blank area takes up no
space at all, unlike a bitmap!
The scale of the drawing will have no
effect on the size of the file.
E.g. the Japanese flag will be the same file
size for a flag the size of a stamp or a flag
the size of a football pitch.
Higher
Int. 2
Vector Graphics (Cont.)
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The size of a vector graphic file increases the
amount of objects in the image, not the physical
size.
Therefore a very complicated drawing the size of
an A4 page may be converted to a bitmap before
being stored or transferred.
This is why many graphics are converted to GIF
or JPEG before being put into web pages.
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Vector Graphics File Formats
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Most vector graphics packages have their
own way of storing objects and their
attributes, but a few standard formats exist.
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SVG (Scalable Vector Graphics) is a
language for 2D vector graphics.
VRML (Video Reality Modelling Language)
is used for animations and 3D imaging by
describing a scene by attributes.
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Graphics Processing Unit (GPU)
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The GPU is a processor specifically aimed
at supporting 3-D graphics.
When processing complex graphics a
whole range of operations have to be
performed by the system such as
transferring bitmaps, resizing and
repositioning graphic objects, drawing
lines, scaling and rescaling objects such as
polygons.
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GPU (Cont.)
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This involves
performing millions of
calculations that have to
be processed each time a
complex 3-D graphic is
drawn.
The GPU deals with all of this processing, freeing
the main processor from this demanding task.
Navidia and Geforce are two companies which
make this type of microprocessor.
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Digital signal processor (DSP)
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The DSP is an integrated circuit designed
for high speed data manipulation used in
image manipulation (as well as audio and
other applications, for example
communications).