Embed Size (px)
Citation preview
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Graphics File Formats
• File formats have developed with applications.
• At least 50 currently in use.
• Examples include: GIF, JPEG, TIFF, BMP, DIB, PCD, PNG …….
• Each has been developed for specific reasons.
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Graphics File Formats
• File formats have developed with applications.
• At least 50 currently in use.
• Examples include: GIF, JPEG, TIFF, BMP, DIB, PCD, PNG …….
• Each has been developed for specific reasons. Many are obsolete, irrelevant, and there is much crossover in functionality.
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Makeup
• Graphics files typically contain info on– Type / subtype (e.g. GIF 87a , GIF 89a,
Greyscale)– Size - width and height (e.g. 120x80)– Colour depth in bits (2, 4, 8, 16, 24, 32)– Palette information (if less than 16 bits in
depth)– Internalised image data (e.g. PCD has 5 sub
images)
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Colour Depth
Pixels make up an image
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#FF00A3#255,0,163#11111111,00000000,01100011
Colour - DepthEach pixel has a colour depth. A certain number of ‘bits’ are used to define a pixels colour e.g. held as an RGB value.
For 256 (or less) colours a palette is used as an index describing which 256 actual colours to use in an image.
240,200,171
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Colour DepthThe colours themselves consist of up to 24 bits of information to descibe them.
8 bits for Red, Green and Blue - 256x256x256colours = 24 Million possible colours.
In web page design we use these 24 bits to define our colours for page detail. e.g.
<BODY BGCOLOR=“#F0D200”>
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Size / Depth
• Obviously, less colour - less size in the final image. e.g (in uncompressed raw format). – 300x200x1 (2 colours) = 8k– 300x200x4 (16 colours) = 30k– 300x200x8 (256 colours) = 60k– 300x200x24 (full colour) = 180k
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Size / Depth
8k 60k1 bit 8 bit
180k24 bit
30k4 bit
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Colourspace• Less colour /size also means lower quality.
• When working / creating / manipulating images for production we should use 24 bit colour.
• To conserve space (or transmission time), we can reduce colour depth - but only at the end of the process.
• Continual reduce/ increase cycles lowers quality in a noticeable manner.
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Compression• We can compress images to further reduce
size.– LZW / RLE - lossless– JPEG - lossy– Wavelet - lossy– Fractal - lossy
greater compression
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Compression - LZW / RLE• This technique is used in common lossless
compression formats - GIF / TIFF / BMP ...Runs of colour can be defined in a simple Run / Colour / Number format. e.g. for yellowR0206, for gray R0304, for red R04FE, R0101 for white, R0614 for blue - taken from the palette below.
321
654
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Compression - LZW / RLE
TIF uncompressed = 289kTIF lzw compressed = 248k
TIF uncompressed = 90kTIF lzw compressed = 5k
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Compression - JPEG• JPEG (Joint Pictures Expert Group) uses
advanced mathematical techniques to store image information.
• It is ‘lossy’ - compression causes the loss of image data.
• Compression can be tailored to need.
• Useful on post production only.
TIF lzw compressed = 248kJPEG (15%) = 29k
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Compression - JPEG
212k original 12k 50%31k 8%
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Compression - Fractal• Patented (not public domain) technique,
again, using very complex maths.
• Images are represented as mathematical expressions.
• Used by commercial business to store large image databases. E.g. Microsoft Encarta uses FIF files to allow over 25000 images on one CD.
