I Power

Multimedia Technology

Graphics

Higher Computing

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Digital Cameras: CCD Array 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 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) 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) 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) 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.

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24-bit Bitmap 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) 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.) Let us take a single line in a graphic with B

representing a black pixel and W representing white: WWWWWWWWWWWWBWWWWWWWWWWWW

BBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW

If we apply a simple Run-Length Code to the above graphic line, we get the following: 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

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GIF 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 AnimationAn animation is a series of graphics that are displayed in rapid sequence in a web browser to appear as a moving picture.

In animation software each still image is called a frame. GIF animation is a common way of creating animated graphics.

Frames

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GIF Animation (Cont.) 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 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.

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Joint Photographic Experts Groups (JPEG) 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.

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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 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: 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.) PNG also has:

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 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

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Calculating File Sizes You need to be able to perform calculations using

the relationship: File Size =

Resolution

(Dots per inch)

Size

(no of Sq inches)

X Colour Depth (no of bits)

X

1600 x 1200

X 8 x 10 X 8E.g.

=1228800000bits=153600000 bytes

=146.4844 Megabytes

Divide by 8 to get answer in BytesDivide by 1024 twice to get answer in Mb.

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Dithering 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 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 Anti-Aliased 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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Aliased

Anti-aliasing Example

Anti-Aliasied

The higher resolution will get rid of the problem of aliasing since this occurs only at low resolutions.

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Vector Graphics 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.

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Vector Graphics (Cont.) 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.

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Vector Graphics (Cont.) 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.

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Vector Graphics (Cont.) 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 Most vector graphics packages have their

own way of storing objects and their attributes, but a few standard formats exist. 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) 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.)

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.

This involves performing millions of calculations that have to be processed each time a complex 3-D graphic is drawn.

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Digital signal processor (DSP) 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).