The University of Adelaide Picture Perfect: Image Formats and Resolution Peter Murdoch March 2014 PREPARING GOOD LOOKING DOCUMENTS

The University of Adelaide

Picture Perfect: Image Formats and Resolution

Peter Murdoch

March 2014

PREPARING GOOD LOOKING DOCUMENTS

The University of AdelaideSlide 2

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.


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.

source: www.VectorOpenStock.com


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.


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


Capture: It's all about size!

Minimum megapixels for quality prints, printed at 300 dpi

Max Print Size Minimum MP Resolution

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

source: http://www.digicamhelp.com/camera‐features/camera‐parts/megapixels/


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 colours8 bit = 256 colours16 bit = Thousands of colours24 bit = Millions of colours36 bit = 4 channel colour - CMYK 8 bits = 1 byte



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.

source: http://www.viovio.com/wiki/Understanding+Color



• 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.


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.


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

ALWAYS EDIT ON A COPY.

Once you have saved over your original you cannot get it back.

Limitations


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


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 printer.

• 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.


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


Output: to Screen

• Scale your images to the size required to keep file size down.

• 72 ppi

• 511 x 238 px

• 4.33 x 2.02 cm (300 ppi)

• 176 KB

• 1/8 th

• 300 ppi

• 2130 x 993 px

• 18.03 x 8.41 cm

• 1.45 MB

• 8x

PPT = 720 x 540


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


Output: to Print

• Vector images scale up better than raster images.

JPEG EPS


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.

source: http://help.adobe.com/en_US/indesign/cs/using/WSa285fff53dea4f8617383751001ea8cb3f-7069a.html


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.

source: http://www.unisa.edu.au/printing/New/LVL3/printing-colour.asp


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.

http://www.cambridgeincolour.com/tutorials/color-space-conversion.htm


Summary

capture

There are three key stages to consider when using images.

edit output


Summary

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.


Thank You

© The University of Adelaide 2014

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The University of Adelaide Picture Perfect: Image Formats and Resolution Peter Murdoch March 2014 PREPARING GOOD LOOKING DOCUMENTS