Transcript Slide 1

Topic 2 – Bit mapped & Vector graphics
Still images – the general name for any graphic that doesn’t change (unless
edited). It is static compared to a video clip in that it does alter with the
passage of time.
Hardware required to capture an image.
Digital Camera
Digital cameras store images as bit map ie. a grid of pixels.
Inside at the back of the camera is a grid/array of small circuits called
Charge Coupled Devices (CCDs). This is basically a grid of light sensitive
cells. Each cell measures how much red, green and blue light there was when
a photograph is taken. Each CCD stores information about one pixel.
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An analogue to digital converter (ADC) changes the amount of light into
electrical format which is then stored in binary.
The image can be viewed on the camera’s screen and can be deleted if required.
The image is stored on a memory card – they can store a lot of
information, are small and portable. The card can be taken to a shop
where a special reader can be used to produce the photographs.
Special effects or changes can be applied to the image using photo
editing software on a computer. The images can be printed.
Disadvantages – special paper required and a lot of ink is used.
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Scanner (Flatbed or handheld)
Flatbed Scanner
The document to be scanned is placed on a glass plate.
Below the plate a lamp is used to illuminate the page.
A scan head moves slowly across the page.
The amount of light reflected is focused through a lens onto a line of
CCDs (linear CCDs). The ADC converts the amount of reflected light into
electrical format then this is stored as binary.
Once one line has been scanned, it is sent to the computer’s RAM and
displayed on the monitor.
The scan head moves down and scans the next line.
The process is repeated until the whole document has been scanned.
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Bit Mapped Graphic – one that is stored dot by dot called pixels
(short for picture element)
To be able to represent colours, a computer must use more than
one bit per pixel. The number of bits in each pixel is called the
colour depth or bit depth.
Colour depth of three – 8 colours
Binary Code
Colour
000
white
001
cream
010
yellow
011
brown
100
blue
101
red
110
grey
111
black
Rule To Learn
The number of colours = 2 bit depth
Eg a bit depth of 5 would give
5
2 colours
= 2x2x2x2x2
= 32 colours
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True Colour – when a bit mapped image uses 24 bits (3 bytes) for each pixel
(approx 16 million colours). This is reckoned to be the number of colours
perceived by the human eye.
All displayed colours are made up of the three primary colours – Red, Green
and Blue (RGB). The first byte in true colour contains details about the
amount of red, the second byte stores the amount of green and the third
byte the amount of blue.
Each pixel contains a binary code but how
can this be translated into a colour?
Colour Look Up Table (CLUT) or Indexed Colour
A CLUT is a table of colours. Each colour has an index position eg. Red might
be in position 5. The binary code in each pixel represents a numbered entry in
the table. The computer looks at each pixel, looks at what index entry is
contained in that pixel then the colour in that position.
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Compression
Bit mapped files can be very large because of the number of pixels and large
colour depth.
Compression – to reduce the size of a file either by;
using an algorithm (mathematical calculation) or
using a special piece of software designed to make files smaller or
by saving a file in a format that compresses data.
Two types of compression;
Lossy – when a file is reduced some data is lost in the process. The
compressed file is of poorer quality.
Lossless – when a file is reduced no data is lost. The data is simply stored
more efficiently.
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Compression cont.
Run Length Encoding (RLE)
Built in form of compression.
In many images there are larges stretches of adjacent pixels that are exactly
the same colour. In RLE the number of pixels a colour stretches for is stored
in a special byte called a keybyte instead of storing each of these pixels
separately.
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GIF reduces the file size in two
ways;
1. Bit depth is reduced to 8
(256 colours). GIF not used for
photographs.
2. LZW technique
used. Looks for
repeating data.
Continuous blocks of
identical colours are
stored more
efficiently.
Eg. Instead of
storing ‘red, red, red,
red, red’ - ‘5 red’ is
stored in a dictionary
of patterns then
pointers are used to
point to the
dictionary.
Lossless – no data
is lost in
compression.
GIF (Graphics
Interchange
Format) for storing
compressed
graphics
Advantages;
1. Each colour only needs
to be stored once
2. The pointers to the
dictionary entries take
up less memory than
binary codes for colours.
3. The dictionary is not
stored with the file, but
is generated by the
piece of software being
used to open the image.
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GIF – additional
features
Transparency – areas of the
image are transparent.
Animation – GIF images can be played
one after the other as an animation.
GIF format used on web pages – two versions available;
1. Non-interlaced – the picture downloads on e line of pixels at a time and
gradually builds the image up. The image can take a while to appear especially
with dial-up connections.
2. Interlaced – a fuzzy image of the whole picture is downloaded. More data is
then received until the picture is completed and becomes clear. Good for larger
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images.
Vector Graphics Data
Good format for;
GIF format
Images with few
colours.
Poor format for;
Images with flat
areas of colour.
Photographs
High quality pictures
Animated images.
High quality videos
Images which need
transparent areas.
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Compression cont.
Compresses by
looking for
adjacent colours
but allows the
user to vary the
reduction of
colours from
256 to 16
million.
PNGs can be
non-interlaced
or interlaced
Lossless – no data
is lost in
compression.
PNG (Portable Network
Graphics) for storing
compressed graphics
PNG cannot
be animated
Allows transparency
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Compression cont.
Main standard for
compressing high quality
images and photos
Uses a mathematical formula to look for
and remove colours that won’t be noticed
by the human eye.
Lossy compression
JPEG (Joint Photographic
Expert Group) for
compressing files
The level of the
compression can
be varied to
reduce file size
further but
quality may also
be reduced.
Does not support
interlacing or
transparency.
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Technical Terms
Dithering – uses a combination of colours to create an illusion of
having additional colours. This improves the appearance of an
image and simulates more colours than there are available.
Anti-aliasing – the graphic has its edges slightly blurred and
mixed with background colours to eliminate the jagged edges
that can occur.
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Calculating File Sizes
File Size = Resolution x Colour Depth
Height x Width or Pixels
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Number of Bits
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Output Hardware
Inside the computer a graphics card has the task of generating the images.
The card has the following components;
GPU – to perform complex calculations each time a complex graphic is
drawn freeing the computer’s main processor.
Memory – called VRAM (video RAM) to store the images to be displayed
instead of the computer’s RAM. More VRAM means higher colour depth
and resolution.
Digital to Analogue Converter (DAC) – converts the digital computer signal
into an analogue electrical signal which can be displayed on the monitor.
Monitors
CRT – Cathode Ray Tube
LCD – Liquid Crystal Display
TFT – Thin Film Transistor
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End of Topic 215
Vector Graphics Data
Vector drawn graphics (object oriented graphics) works
by creating objects (lines/shapes) and defining them
mathematically. When that object is saved, all that is
actually saved is a formula which describes that shape’s
attributes.
Common attributes of vector graphics objects;
shape
rotation
position
line
size
fill
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Can be scaled to large
sizes, keeping original
quality and file size.
(Resolution Independent)
Relatively small
file sizes
Advantages of using
Vector Drawn
Graphics
Individual
objects can
be edited.
Easily converted
to bitmapped
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Difficult to recreate
realistic images.
Drawbacks of using
Vector Drawn
Graphics
Only individual objects
can be edited not pixel
by pixel.
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Dependent on
hardware &
software for
appearance &
quality.
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The quality of a vector graphic is not affected
when resized because;
The vector graphics stores the attributes
of each object and the object is drawn
again at it new size. This means that it will
always be displayed at the best possible
resolution for the output device.
The most common standard file format for objectorientated graphics is called SVG (Scalable Vector
Graphics). This is the format used for vector images
that appear on the internet.
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3D Vector Images
A 3D vector image is store as a description with all the
attributes associated with 2D images, such as type of
shape, fill colour and dimensions.
In addition other attributes need to be stored, and
these include;
the angle of rotation
surface texture (bit mapped patterns to cover a
surface)
shadows
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VRML (Virtual Reality Modelling or Mark-up Language)
VRML is a standard for describing interactive threedimensional vector images. VRML is capable of
representing static and animated dynamic 3D and
multimedia objects.
Each object in the file is known as a node.
The properties or attributes of the objects are known as
the fields.
WRL (World Representation Language) is a file type for
3D vectors.
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