PC Graphics From Megabytes To Modest Bytes

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PC Graphics- From Megabytes To Modest Bytes
ByMurray Niman G6JYB
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The Problem

• SIZE MATTERS !
• How is it I can download a web page such as BBC
  News which may have 40 Images in a few seconds?
• But my mates send me Megabyte photos which take
  ages to load and cost a fortune on the phone bill
  (or inkjet cartridges)
• OR
• I must have the latest 6-Megapixel camera - and I
  get the same problem

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Common Formats

• BMP - Bit Map (1, 16, 256,32K or 16M Colours, no
  compression)
• GIF - Graphics Interchange Format (Compuserve)
  (1, 16, 256)
• JPG/JPEG - Joint Photographic Experts Group (16m
  only)
• TIF - Tagged Image Format (Usually 16m)
• Also lots of others-
• PNG - Portable Network Graphics, PCX, WMF, PBM,
  EPS etc etc
• Most of the ones we are concerned with are raw
  pixel data.
• Do not confuse with WMF, PDF, Flash, smart
  shapes, Wordart etc

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Other Formats

• Powerpoint - combines any Pictures with text and
  graphics
• Shockwave (SWF) Flash - Vectored animation
• Movies-
• AVI Microsoft Video (but can be compressed into
  WMV, DivX)
• MPG - Motion Picture Experts Group
• MOV - Apple Quicktime Movie
• GIF - Animated - Very inefficient but ok for web
  pages

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Colours

• A Pixel A Picture Cell - one dot that is a
  colour
• Everything is binary
• 1 bit 0 or 1 Black or White
• 16 colours 4bits or half a byte
• One Byte 28 256 colours a bit low for
  Colour photos
• Three Bytes 24 bits, 224 16million colours
  (256 Red, Blue and Green)
• Note - a black/white photo is grey (RGB are all
  the same)
• Colour uses RGB so needs upto three bytes instead
  of one

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Pixels Pages and Sizes

• DPI - Dots per Inch (or pixels per inch)
• An A4 page is 12x8
• dpi Pixels Total Pixels
• 75 900x600 540,000
• 100 1200x800 960,000
• 150 1800x1200 2,160,000
• 200 2400x1600 3,840,000
• 300 3600x2400 8,640,000
• 600 7200x4800 34,560,000
• 1200 14400x9600 138,240,000
• 2400 28800x19200 552,960,000
• - and remember is often 3bytes per pixel for
  full colour

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Cameras, Scanners, Screens Printers

• They are all different !
• Screens are 75-90dpi
• Printers are 300-2400
• Cameras / Scanners can zoom (Optical Zoom saves
  megapixels)
• You cannot get a 11 size from Scanner to Screen
  to Printer
• If you know its final use, choose the size in
  advance

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Size Area

• This can give Big Gains . . .
• A picture that is half the size is a quarter of
  the area/no of pixels
• Crop unnecessary items around the borders
• Resizing the remainder down by 30 or 50
• Line Graphics
• If an image is a graph, circuit diagram, map etc
  it will only need between 2 and 256 colours -
  Using GIF is very economic and better quality
  than Jpeg
• High JPEG Compression doesnt save much but can
  ruin photos

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Noise

• Need good quality originals
• Clean high contrast images use far less space
  than noisy/busy ones (Lots of pixels can be tied
  up on grass blades, leaves etc)
• Raising contrast and decreasing/merging colours
  can save a lot
• Flooding a slightly noisy background with black
  or white etc can save a lot
• If it is almost grey, then make it a greyscale
  andreduce to 16 colours

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How Many Colours?

• Graphs/diagrams may only need 16 colours
• A photo may use 16million! (but rarely needs them
  all)
• A picture that is half the size is a quarter of
  the area/no of pixels
• Many pictures on the web are reduced to 256 or
  less
• So not only does size matter, colour count does
  too

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Example
JPG 56418 BMP 751470 TIF 750666 GIF(256)
103552 GIF(16) 33770 GIF(16) as JPG 73647!
3200 Colours in Use in Original
16 Colours in Use in GIF(16)
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Conclusions

• Formats
• Use the most appropriate for both quality and
  efficiency
• JPG for most Photos, GIF for Graphs, Circuit
  Diagrams etc
• Use a decent PhotoEditor like Paintshop Pro etc -
  not the freebies with scanners/cameras - they
  give you control over colours, pixels etc
• Sizes/Pixels, No of colours - Do you really need
  it all?
• Resize, crop. Keep a nice original, but use/send
  only reduced versions