Slides - University of Adelaide

Picture Perfect: Image Formats and Resolution
Peter Murdoch
March 2014
The University of Adelaide
Picture Perfect: Image Formats and Resolution
When preparing teaching materials you may want to include
images to display information graphically, to highlight an
idea or concept, or to provide decoration.
There are three key stages to consider when using images:
image capture, image editing and image output.
Understanding the features of image files and formats can
help you make appropriate choices to ensure that your use
of images is as successful as possible.
Slide 1
The University of Adelaide
What is an Image File?
• Image files can be one of two types: a vector or a raster file.
• A vector file uses geometric shapes and lines and
mathematical equations to describe an image.
• A raster file uses pixels laid out in a grid to describe an
image. A pixel is the smallest possible unit of image data.
• Vector images have to be converted to raster files to be
displayed or printed.
Slide 2
The University of Adelaide
Image capture / creation
• Original image files can be obtained in a number of ways.
• Existing files can be acquired from stock photo websites,
some of which supply images for free and some which
charge fees and apply restrictions on using the image.
• An image can be captured using a digital camera or a
scanner. A photocopier can be used as a scanner but it
usually only captures data in black and white.
• You can also take a screen capture from your computer
using the Snipping Tool (Windows) or Grab (Mac).
• An image can be created using either vector graphics or
raster graphics software.
Slide 3
The University of Adelaide
Capture: It's all about size!
• camera = megapixels (MP)
• scanner = dots per inch (dpi) – at 100%
• creation = dimensions (width x height)
• the more pixels captured the larger the file size
• the more pixels captured the larger the possible print size
• less pixels cannot be made more without losing quality,
but more pixels can be made less
• how you will use the image can influence
your choice of file size
Slide 4
The University of Adelaide
Capture: It's all about size!
Minimum megapixels for quality prints, printed at 300 dpi
Max Print Size
Minimum MP
4 x 6"
2 megapixels
1600 x 1200
5 x 7"
3 megapixels
2048 x 1536
8 x 10"
5 megapixels
2560 x 1920
11x 14"
6 megapixels
2816 x 2112
16 x 20"
8 megapixels
3264 x 2468
16 x 24"
12 megapixels
4200 x 2800
Slide 5
The University of Adelaide
Capture: Quality counts!
• Colour space – the bit-depth of an image file determines
the potential number of different colours it can store.
• A higher bit-depth
means a larger file size.
1 bit = 2 colours
8 bit = 256 colours
16 bit = Thousands of colours
24 bit = Millions of colours
36 bit = 4 channel colour - CMYK
Slide 6
8 bits = 1 byte
The University of Adelaide
Capture: Quality counts!
Colour 'gamut'
The dynamic range
of colours capable of
being captured for
an image file.
Also the colour
range capable of
being displayed on a
screen or produced
by a printer.
Slide 7
The University of Adelaide
Capture: Quality counts!
• Focus (the human element) – an image that is 'out of focus'
when it is taken cannot be made in-focus later, so careful
control of the camera is important.
• Compression – is a technique for storing more data in less
space. There are two types of compression used for image
files, lossless and lossy compression.
• Lossless compression saves space by packing up and
grouping common colours. All the colour pixels are restored
when the file is opened and no data is lost.
• Lossy compression saves space by discarding pixels that are
not necessary for viewing the file. When opened the pixels
are restored using colours from the available set.
Data is lost every time a lossy file is edited and saved.
Slide 8
The University of Adelaide
Capture: Formats for safe keeping
• Saving your original images in a suitable file format to
maximise their future use is very important.
• Store or archive your image files in a format that will be
readily available in the future – a recognised 'standard'
format is best.
• These standard formats are also likely to be well supported
in any new versions of image editing software.
• For vector images – EPS, PNG and SVG.
• For raster images – BMP, TIF and JPG.
Slide 9
The University of Adelaide
Editing: Why?
• There are many reasons why you might need to edit your
image files:
– to Resize them
– to Crop them
– to Adjust their tone or colour balance, or
– to Convert them into a format suitable for display
on-screen or for printing.
• BUT...
Never use your original (captured/created) image files
Once you have saved over your original
you cannot get it back.
Slide 10
The University of Adelaide
Editing: Tools for the job!
Computer based editors
• These have basic editing capabilities.
• Picture Manager (Windows); Preview or iPhoto (Mac)
Commercial editors
• For raster images: Adobe Photoshop, Corel PaintShop Pro
• For vector images: Adobe Illustrator, Corel Draw, AutoCAD
Open Source editors
• For raster images: GIMP
• For vector images: Inkscape
Slide 11
The University of Adelaide
Output: What's your target?
• How you prepare your image file in the editing stage,
and what format you choose to save the image file in,
will depend on where you plan to use the image – on
what 'device' you are targeting.
• This 'device' will most likely be a computer screen or a
• Image files for computer screens are generally low
resolution, RGB based files that are usually small in size.
• Image files for printing are usually large high resolution files
that support high quality output in black and white,
grayscale or full colour.
Slide 12
The University of Adelaide
Output: to Screen
• Images for display on screen use a resolution of 72 ppi.
• Monitor size in pixels is also known as screen resolution.
File formats for screen output
• gif – compact, suits flat colour, only 256 colours, animation
• png – more colours, variable transparency, no animation
• jpg / jpeg – excellent for photographs, high compression
• svg – for vector images on the Web, not well supported yet
Slide 13
The University of Adelaide
PPT = 720 x 540
Output: to Screen
• Scale your images to the size required to keep file size down.
Slide 14
300 ppi
2130 x 993 px
18.03 x 8.41 cm
1.45 MB
72 ppi
511 x 238 px
4.33 x 2.02 cm (300 ppi)
176 KB
1/8 th
The University of Adelaide
Output: to Print
• Image resolution for printer output files is much higher;
150/200 dpi for photographs and 300/600 dpi for line art.
• The 'line screen' setting for a printer affects print quality;
the higher the line screen, the finer an image can be printed.
File formats for print output
• tif / tiff – best for raster images, very good for b&w, colour
and transparency, files can be very large.
• eps – best for vector images, uses a low-res tiff in software
for placement but prints at high resolution.
• pdf – a page description file, supports embedded images
Slide 15
The University of Adelaide
Output: to Print
• Vector images scale up better than raster images.
Slide 16
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Output: B&W halftones
• In black and white printing shades of gray are produced by
using dots of varying size and density.
• More dots closer together for darker shades, smaller dots
further apart for lighter shades.
Slide 17
The University of Adelaide
Output: full colour CMYK
• CMYK images are separated into 4 'screens' – one for each
colour that is printed. The dots of the four colours merge
visually to represent a wider spectrum of colours.
Slide 18
The University of Adelaide
Output: Gamut mismatch
• Different 'devices' are capable of outputting different results
so it is important to 'target' your file to the right colourspace.
• The range of colours available to display on a monitor is much
greater than the range that can be reproduced by a printer.
Slide 19
The University of Adelaide
There are three key stages to consider when using images.
Slide 20
The University of Adelaide
Understanding image files and their different formats will
help you to work with them successfully.
When creating image files
– the more data you can capture the better.
When editing image files
- always work with a copy, never the original.
When using image files
- use a format suited to the output required,
using no more data than is necessary.
Slide 21
The University of Adelaide
Thank You
© The University of Adelaide 2014
Slide 22
The University of Adelaide