Raster Graphics

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Raster Graphics

• Main file formats, colour models and resolution
• Overview
• Editing in Adobe Photoshop 7

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What are 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.
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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 (?)
• Capture image at appropriate resolution.
• Avoid interpolated resolution.

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Issues to consider

• File format
• TIFF, EPS, JPEG, GIF, PNG
• Colour model
• black white, palette, RGB, CMYK

The choice of the file format may restrict the
choice of possible colour models.

• Resolution
• Often expressed as dpi (dots per inch), more
  accurately ppi (pixels per inch).
• Frequency at which samples are taken from the
  original image during the process of scanning.

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Print

• There is no point for the image resolution to
  exceed the resolution of the printer.
• On non-professional equipment (e.g. printer
  connected to office or home computer)
• The print quality between images saved in
  different file formats may not vary much.
• Resolution can be relatively low.
• On professional equipment
• Acceptable file formats are usually TIFF
  (Tag(ged) Image File Format) or EPS (Encapsulated
  Postscript).
• You will need to inquire about the resolution
  particularly if you are reproducing coloured
  images. Required standards may differ from one
  printer to the next.
• Contact printer prior to scanning your images
  about details.

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On-screen presentation

• e.g. on web pages
• 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

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GIF

• Compression
• It is possible that the compression influences
  the quality of the colours in an image
  negatively.
• Supported colour models
• Black white
• Palette (256 colours)

Black white (file size 8Kb)
Palette (file size 65Kb)
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JPEG Compression

• Lossy compression, i.e. data not necessary for
  on-screen display is filtered out in the process
  of saving
• Loss of image quality through compression

Compression 0 (file size 78 Kb)
Compression 45 (file size 13 Kb)
Compression 96 (file size 2 Kb)
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JPEG Colour models

• Greyscale 256 shades of grey ranging from 0
  (black) to 255 (white)
• RGB consists of red, green and blue, whereby
  each of these colours can take a value from
  between 0 and 255. The higher the values are for
  a colour the lighter it is, e.g. if red, green
  and blue are all 255 white is displayed. If the
  value for all three colours is 0, the displayed
  colour is black.

Greyscale Compression 33 File size 15 Kb
RGB Compression 33 File size 17 Kb
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PNG

• Compression
• Lossless
• Supported colour models
• Greyscale
• Palette
• RGB
• Not all browsers support PNG

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Comparison GIF, JPEG, PNG
GIF(file size 65Kb)
JPEG (compr 33, file size 17 Kb)
PNG (file size 158 Kb)
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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.

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Adobe Photoshop 7
4. Colour model
3. File format
5. Resolution image dimensions
1. File browser (File management)
2. History palette (Snapshots)
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File browser

• View details before opening the file

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Selecting a file
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History Palette

• Snapshot
• A snapshot is a temporary copy of any state of
  the image.
• It can be stored for an entire work session.
• It can be used to compare effects of changes made
  or to recover work after undesired changes.
• Snapshots are not saved with the image, and
  closing the image deletes them.
• Steps

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File format
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Changing the colour model

• Photoshop calls Palette Indexed Colour
• If changing colour model be sure it is one
  supported by the file format, e.g. you can save
  .jpg files in CMYK (leads to problems for some
  computers)

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Image dimensions resolution
Dont drag your image to the right dimensions on
the web page. Use Image Size.
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File sizes

• TIFF 600ppi 36.2MB 4128 x 2992
• JPEG 72ppi 863Kb 4128 x 2992
• JPEG 72ppi 54KB 412 x 299