www.hope.ac.uk faculty of sciences and social sciences hope website development graphics stewart...

www.hope.ac.uk Faculty of Sciences and Social

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Website DevelopmentGraphics

Stewart Blakeway

FML 213

[email protected]://hopelive.hope.ac.uk/computing/

0151 291 3113


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Session Objectives

• Describe the different file types used in Web pages

• Demonstrate the correct use of file types in Web pages

• Demonstrate graphics optimisation• Describe the importance of the image file

size


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RGB Colour Model

• Known as the additive model• Cathode Ray tubes use RGB Model• All colours are made up of proportions of red,

green and blue.• Pure red, green and blue = white


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Colours on the Web

• RGB Colours– Numerical colour specification– Hexadecimal values from #00 to #FF– RGB code = #000000 – Each pair of digits represent combinations of of

red, green and blue• #0000FF = represents pure blue• #CCCCFF = represents a lighter shade of blue


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Display Definitions

• Display is organised into a grid– Pixels

• Resolution is defined in terms of pixels• Monitors display pixels per inch according to

their resolutions


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Web graphics

• Web graphics– Different to preparing a image for print

• Consider how the other person will receive the image– Download time

• File type• Compression

– The monitor they will use• Colours• Resolution


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Computer images

Two types of Computer images:• Vector–Mathematical description– Lineart – blueprint

• Raster or Bitmap– Pixels - individual dots– Screenart – photograph


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Vector images

• Drawing• Graphic described in terms of lines and curves– Vectors

• Vector information– Position– Colour– Line thickness

• Resolution independent


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Raster or Bitmap images

• Coloured pixels• Arranged on a grid– Like a mosaic

• A bitmap can be reduced in size• Can be enlarged...but– Degradation of quality


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Image file types

Many available• BMP• GIF• JPG• PNG• PSD• PSP • TIFF………………plus many more


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File types used by Web Browsers

• Web Browsers - image file types– GIF– JPG– PNG

Supported by browsers because of compression capabilities


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Images on the Web

• Images should be – Small– or– Worth seeing

• They may have a much larger file size than the Web page itself!

• Longer download time


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Download Times

• Vary according to the connection• Use images selectively• Not too many per page

• Typical guidance – total of 20 KB per page– no more than 6 images


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Obtaining an Image

• Use someone else’s– Scanners– Web– Disk

• Make your own– Digital Camera– Draw it


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JPEG or JPG format

• Raster images• Joint Photographic Experts Group• Most Common file type for photographs on

the Web– .jpg or .jpeg

• Progressive encoding– Image is brought into focus as it is loaded– Slight increase in file size


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JPEG - Colours

• 24 bits per pixel - 16 million colours– Smooth colour quality– Subtle tone and colour changes

• Separates the brightness information from colour hues


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JPEG Compression

• Lossy process – Information is lost in the compression

• Compression Ratios– Typically 10:1 down to 100:1– Greater the compression the smaller the file size– There is no ‘right’ compression ratio – Experiment, starting with the highest compression

ratio and smallest file size


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JPG Lossy Compression Method

• Algorithm divides the image into squares• Mathematics called Discrete Cosine

Transformation– Divides the squares into curves– Depending on the compression level, discards

some of the smaller (less significant) curves

• If over compressed ‘blockiness’ occurs


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JPEG Optimisation

Reduce the size of the image• Reduce the resolution to 70-75 pixels per

inch• Increase the compression as much as the

image can allow


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GIF Format

• Raster (bit-mapped) Images• Graphics Interchange Format• Most Common file type on the Web • Lossless compression

filename.gif


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Interlacing

• Means of storing a image file• As the image loads more and more detail

appears• Full resolution once downloaded completely• Users with slow connections need not wait

for full resolution to download


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GIF Colours

• 8 bits per pixel– 256 colours– Colour indexing

• Mac and PCs handle 40 colours differently• Browser Safe Palettes – Palette of 216 colours


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Palettes

• Adaptive• WebSnap Adaptive• Web216• Exact• System• Grayscale• Black and White• Uniform


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Transparency

• One colour of the palette is designated as transparent

• Image file sizes are slightly larger• Avoid a halo effect – make sure that the background of the images is

close to the background colour used in the web page (before making the background transparent)


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Example

Transparent GIFs blend very nicely into a web

page,

rather than appearing with a surrounding box.


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GIF Compression• Average compression ratio of

4:1 (depends on number of repeated colours)

• Uses pattern-recognition– Looks for patterns of same

coloured pixels – Represents the pattern with a

number (colour indexing)

1.6 KB

1.8 KB


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GIF files

• File size after compression

– Complex images • Many of different colours• Around 80% of the original file size

– Simple images• Few colours• Around 10% of the original file size


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Animated Gif Images

• Animated gif file– Single file stored in .gif format– Contains series of images / frames – User defined delay sequence– Animation can be

• Looped (Forever)• Finite (once, twice etc.)


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Anti-aliasing

• Adds intermediate colours to smooth edges (by mixing the colours of the image’s edges and the background’s colour)

• Increases the file size

• Curves need it more than rectangular shapes


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GIF optimisation

• Make the image as small as possible• Use as few as possible colours• Avoid anti-aliasing– But not at the cost of spoiling your image

• Reduce the number of colours in the palette


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PNG

• Portable Network Graphicsfilename.png

• Used to replace GIFs– Less common and not supported by older

browsers

• 48 bits per pixel• Good compression without loss of quality• Transparency and interlacing supported


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PNG optimisation

• Make the image a small as possible• Use as few colours as possible• Avoid anti-aliasing– But not at the cost of spoiling your image

• Reduce the number of colours in the palette


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Thumbnail images

• Smaller image linking to a larger image• Need two versions of the image– Small– Larger

• Fireworks has a Batch Process (File Batch Process..) to create the smaller images (thumbnails)


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Thumbnail code

<a href=“large_image.jpg”>

<img src=“small_image.jpg” alt=“click here to see a larger picture” height=“” width=“”>

</a>


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Examples of an Optimised image

• The next set of slides demonstrate download times for a jpg image.


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Original101 Seconds loading time621 KB1600 x 1200


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20% Compression81 Seconds loading time460 KB1600 x 1200


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• Reducing the quality of the picture will reduce the file size

• The picture you have just seen is– 1600 x 1200


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• Reduce the size of the image to the minimum you can get away with

• Apply the maximum compression you can get away with

– More compression, less quality– Easy to reduce an image, not so easy to enlarge


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Fireworks Demonstration

• How to set transparency• How to reduce the size of the image• How to change the palette• How to reduce the quality• How to create a thumbnail


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Questions?

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