Topic 2 Bit mapped

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Topic 2 Bit mapped Vector graphics
Still images the general name for any graphic
that doesnt 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 cameras 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 s
pecial 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
computers 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 gre
y 111 black
Rule To Learn The number of colours
2 bit depth Eg a bit depth of 5 would give
25 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 f
ormat 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 st
retches 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 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 images.
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Compression cont.
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Compression cont.
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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
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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 computers
main processor.
Memory called VRAM (video RAM) to store the
images to be displayed instead of the computers
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
End of Topic 2
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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 shapes attributes.
Common attributes of vector graphics objects
shape rotation
position line size fill
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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
object-orientated 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
three-dimensional vector images. VRML is capable
of representing static and animated dynamic 3D
and multimedia objects. Each object in the file i
s known as a node. The properties or attributes o
f the objects are known as the fields.
WRL (World Representation Language) is a file
type for 3D vectors.