ISYS 3074 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.
ISYS 3074 Graphics File Formats File formats have developed with applications. At least 50 currently in use. Examples include: GIF, JPEG, TIFF, BMP, DIB,
Dec 20, 2015
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ISYS 3074
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.
ISYS 3074
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.
ISYS 3074
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)
ISYS 3074
Colour Depth
Pixels make up an image
ISYS 3074
#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
ISYS 3074
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”>
ISYS 3074
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
ISYS 3074
Size / Depth
8k 60k1 bit 8 bit
180k24 bit
30k4 bit
ISYS 3074
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.
ISYS 3074
Compression• We can compress images to further reduce
size.– LZW / RLE - lossless– JPEG - lossy– Wavelet - lossy– Fractal - lossy
greater compression
ISYS 3074
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
ISYS 3074
Compression - LZW / RLE
TIF uncompressed = 289kTIF lzw compressed = 248k
TIF uncompressed = 90kTIF lzw compressed = 5k
ISYS 3074
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
ISYS 3074
Compression - JPEG
212k original 12k 50%31k 8%
ISYS 3074
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.
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