Understanding

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Understanding working with computer graphics

• EDU1472

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Objectives 1

• Compare digital and traditional images
• Compare bit map and object images
• Discuss storage requirements of images
• Describe various graphic file formats
• Compare print with other images
• Define colour space and gamut
• Explain basics of colour printing
• Examine relationship between colour computer
  storage

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Objectives 2

• Compare painting drawing tools
• Describe common characteristics of graphics
  software
• Describe common graphics tools
• Discuss some applications of computer graphics

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Representing images

• 'Traditional' images
• Represented on a medium
• Paper, canvas, etc.
• Lines, strokes can be smooth continuous
• 'Digital' images
• Represented on medium
• Screen, paper, etc.
• Reproduced from memory
• Vector graphics
• Beam scans as required
• Raster scan graphics

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Raster scan images

• Electron beam scans constantly
• Switched on off to produce points of light
• All images represented as dots
• Picture elements or pixels
• Colour uses 3 beams RGB

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Image production
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Image types

• Bit map (Paint)
• Information stored about each dot in the image
• Problems in scaling
• Components difficult to separate
• Fast redraw - simple data transfer
• Print resolution depends upon stored resolution

• Object (Draw)
• Instructions stored for drawing the image
• Scaling is easier
• Components can be moved, resized, edited
• Redraw requires calculations - may be slow
• Print resolution depends on output device

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Memory usage

• Screen resolution 70 - 100 dpi
• 640 by 480 pixels
• Colour uses more memory
• Colour uses a palette or look-up table
• Detailed colour images are large

Colours bits per pixel bytes per screen 2
(black white) 1 38400 4 2 76800 16 4 153600 2
56 8 307200 thousands 16 614400 millions 24 9216
00
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Graphics file formats

• Balancing image detail storage space
• TIFF (Tag Image File Format)
• Colour grey scale bit maps, default scan format
• GIF (Graphics Interchange Format)
• Compressed colour bit maps
• EPS (Encapsulated PostScript)
• Stored as PostScript with optional bit map
  preview
• JPEG (Joint Photographic Experts Group)
• Compressed full colour bit maps
• BMP
• Windows bit map

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Text as graphic images

• Fonts are graphic elements
• Bit map
• Fast reproduction on screens
• Poor scaling qualities
• Outlines
• Slower on-screen reproduction
• May have associated bit maps
• Better scaling
• Postscript
• Programming language
• Page description, graphics, fonts
• TrueType
• Scalable fonts

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Displaying colour

• Electromagnetic spectrum
• Visible light
• Blue-violet to red
• White all wavelengths
• Black absence of light
• Human eye
• Rods - light dark
• Cones - 3 bands of spectrum RGB
• Perception
• Pure yellow red/green mix

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Colour space

• RGB Additive
• Red, Green, Blue
• Colours formed by addition of light
• Screens, scanners, projectors
• CYM Subtractive
• Cyan, Yellow, Magenta
• Colours formed by reflected light
• Printers
• CYMK Printing
• Pure black is added

http//dx.sheridan.com/advisor/cmyk_color.html
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Gamut

• Range of colours able to be produced and/or
  detected by a device
• Generally less than the full range of a colour
  space

http//dx.sheridan.com/advisor/cmyk_color.html
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Colour printing

• Flat (match) colour
• 1 to 4 colours (2 most common)
• Separate plates/runs for each colour
• Spot colour, highlights
• Various systems - Pantone
• Four-colour process (CYMK)
• Four plates/runs
• Screens separations
• Registration of plates

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Palettes

• Storage of colour data requires extra memory
• Monochrome (1 bit), 4 colour (2 bits),
• 16 colour (4 bits), 256 colour (8 bits),
• Thousands 65536 (16 bits)
• Full colour, millions 16777216 (24 bits)
• 256 colours (shades) generally sufficient
• 8 bits (1 byte) per pixel
• Selected from range of possible colours
• Palette may be edited
• 640 x 480 screen at 8 bits requires 307200 bytes
• 800 x 600 screen at 8 bits requires 480000 bytes
• 1024 x 768 screen at 16 bits requires 1572864
  bytes

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Drawing basics

• Produces 'object graphics'
• Complete image consists of discrete objects
• Lines, boxes, circles, etc
• Select and edit elements
• Data stored as instructions for redraw
• Resolution determined by output device
• Precision work
• Accurate size
• Alignment
• Layers

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Drawing software

• Geometric shapes
• Compact files
• Instructions vs every pixel
• Time required for periodic redraw
• Black and white or colour

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Basic drawing tools

• Freehand drawing
• Straight line
• Rectangle
• Round corner rectangle
• Circle or oval
• Text

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Additional tools

• Arc
• Regular polygon
• Irregular polygon
• Bezigon

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Attribute control

• Fill controls
• Colour, pattern, texture, gradient palettes
• Editable via Accents panel
• Line controls
• Colour, pattern, thickness, arrows
• Pick up attributes (line fill)

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Useful moves

• Shift key constrains drawing
• Circles, squares, 0/45/90 lines
• Selection
• Arrow tool or drag around several objects
• Deselection
• Click on blank area to deselect all
• shift-click on object to deselect one
• Duplication
• Alignment

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Arranging objects
Stacking
Alignment
Transformation
Grouping
ungrouped
grouped
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Reshaping

• Rectangle drawn as irregular polygon
• Select 'Reshape'
• Click to add a point
• Drag point out
• Select 'Smooth' while point is selected
• Drag handles to adjust curve

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Applications

• Diagrams
• Flow charts
• Organization charts
• Plans
• Buildings
• Rooms with objects
• Forms
• Spaces for data entry
• Check boxes

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Painting

• Produces bit map images
• Image consists of pixels (dots) on screen
• Elements are merged into image
• Cannot be easily separated
• Data stored pixel by pixel
• Suited to artistic work

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Painting basics

• Visual effect depends on placement of pixels
• Mixing may occur in the eye
• Similarity to 'painting'?
• Dithering
• Impression of intermediate shades by mixing of
  colours
• Texture and shading can be simulated by placement
  of pixels
• Black and white or colour

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Pixel by pixel
With skill it is possible to create pleasing
visual effects with simple black and white
Detail of the second fence post
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Artistic effects 1
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Artistic effects 2
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Basic painting tools

• Pencil
• fine lines with irregular shape
• Brush
• strokes of varying width and pattern
• characteristics controlled by dialog box
• Spray can (air brush)
• shading and softening edges
• spray size and flow rate controlled by dialog box
• Bucket
• fill enclosed area using current colour or pattern

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Shape text tools

• Rectangle
• Round rectangle
• Oval
• Regular polygon
• Irregular polygon
• Arc
• Freehand line curve
• Bezier curve
• Text

empty or filled current line thickness
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Editing tools

• Selection rectangle (marquee)
• rectangular areas
• Lasso
• irregular areas
• Magic wand
• irregular areas of the same colour
• Eye dropper
• pick up colour for matching
• Eraser
• remove image parts

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Attribute controls

• Image resolution and depth
• Memory requirements increase
• Fill controls
• Colour, pattern, texture, gradient palettes
• Editable via accents panel
• Line controls
• Colour, pattern, thickness, arrowheads

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Modifier keys

• Shift
• Constrains movement or growth
• Usually to vertical, horizontal 45
• Option
• Option-drag to copy a selection
• Option-eyedropper for line colours
• Command
• Command-drag selection rectangle to tighten
  selection

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Special effects

• Select all or part of the image
• Choose an effect from the Transform menu
• Drag handles as necessary
• Deselect
• Options include
• Shear, distort, perspective
• Rotation, scaling, flipping
• Filling, inverting, picking up, blending
• Tinting

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Applications

• Artistic graphics
• Simulated painting
• Simulated sketches
• Blended colours
• Retouching scanned images
• Pixel by pixel editing