Graphics workshop Library and Information Services University of St Andrews

Structure 1. Raster vs vector graphics 2. Scanning 3. Purpose of use 4. Decreasing file size 5. File formats

Vector graphics object-oriented graphics or drawings consist of a series of mathematically defined points that are joined together by a line Object: independent unit characterised by e.g. colour and shape. independent of other objects

Raster graphics Also called bitmaps The image consists of many pixels. Each pixel has its own colour. Pixels are arranged that a pattern is formed. Original image Same image 4x its original size Original image is enlarged 6x.

Scanning Scanning only captures an incomplete representation of the original image. Have a clear idea of the use of the image at the time of scanning it. Print On-screen presentation Long-term preservation (?) Resolution

“Frequency” at which samples are taken from the original image during the process of scanning. expressed as dpi (dots per inch) or ppi (pixels per inch) Capture image at appropriate resolution. Avoid interpolated resolution.

Purpose of use Print On-screen Long-term preservation

Print Don’t exceed resolution of printer. On non-professional equipment : Not much difference between different file formats Resolution can be relatively low. On professional equipment: File formats: TIFF (Tag(ged) Image File Format) or EPS (Encapsulated Postscript). Contact printer prior to scanning your images about details.

On-screen presentation Issues involved: Image quality Downloading time/ file size Browser compatibility File formats: GIF (Graphics Interchange Format) JPEG (Joint Photographic Experts Group) PNG (Portable Network Graphics) Resolution: 72 ppi

Long-term preservation? The standard archival format is uncompressed TIFF. Documentation and metadata are important. A conversion from JPEG or GIF to TIFF will not restore the information that has been filtered out during the process of saving the initial file.

Decreasing file size Cropping: Selecting part of the image Resizing : Decreasing dimensions of the image File compression Colour models

Cropping Select the part of the image you want to display

Brightness and contrast Original image Brightness increased by 28 Contrast increased by 25

Resizing Don’t drag image Use software’s resize option

Compression Reducing the file size of the image whilst maintaining image quality Different forms of compression Lossy – some information is discarded (JPEG) Lossless – all image information is preserved (GIF, PNG) Compression level can be variable

Colours Images can contain a variable number of colours 8-bit colour (256 colours) 24-bit colour (16.7 million colours) Using more colours increases file size

GIF Compression Lossless Reduction in the number of colours, which may have a negative influence on image quality Colour models Black & white file size: 8Kb Palette 256 predefined colours file size: 65Kb

JPEG: Compression Lossy compression, i.e. data not necessary for on-screen display filtered out Loss of image quality through compression Compression: 0% (file size: 78 Kb) Compression: 45% (file size: 13 Kb) Compression: 96% (file size: 2 Kb)

JPEG: Colour models Greyscale: 256 shades of grey 0 (black) 255 (white) Greyscale: Compression: 33% File size: 15 Kb RGB: Compression: 33% File size: 17 Kb RGB: 256 shades each for red, green and blue values between 0 and 255 red, green and blue = 255: white red, green and blue = 0: black

PNG Compression: Lossless Supported colour models: Greyscale Palette RGB

Comparison: GIF, JPEG, PNG GIF (31.1Kb) JPEG (33% comp, 11.2KB) PNG (32.5Kb)