pc graphics from megabytes to modest bytes n.
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PC Graphics - From Megabytes To Modest Bytes. By Murray Niman G6JYB. 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?

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Presentation Transcript
the problem
The Problem


  • 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)


  • I must have the latest 6-Megapixel camera - and I get the same problem
common formats
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

other formats
Other Formats

Powerpoint - combines any Pictures with text and graphics

Shockwave (SWF) & Flash - Vectored animation


  • 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

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 2^8 = 256 colours = a bit low for Colour photos
  • Three Bytes = 24 bits, 2^24 = 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

pixels pages and sizes
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

cameras scanners screens printers
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 1:1 size from Scanner to Screen to Printer
  • If you know its final use, choose the size in advance
size area
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 doesn’t save much but can ruin photos

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
how many colours
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

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)



  • 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