1 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Classical Viewing.
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Transcript of 1 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Classical Viewing.
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1Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Classical Viewing
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2Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Objectives
• Introduce the classical views•Compare and contrast image formation by computer with how images have been formed by architects, artists, and engineers
•Learn the benefits and drawbacks of each type of view
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3Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Classical Viewing
• Viewing requires three basic elements One or more objects
A viewer with a projection surface
Projectors that go from the object(s) to the projection surface
• Classical views are based on the relationship among these elements
The viewer picks up the object and orients it how she would like to see it
• Each object is assumed to constructed from flat principal faces
Buildings, polyhedra, manufactured objects
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4Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Planar Geometric Projections
•Standard projections project onto a plane•Projectors are lines that either
converge at a center of projection
are parallel
•Such projections preserve lines but not necessarily angles
•Nonplanar projections are needed for applications such as map construction
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5Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Classical Projections
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6Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Perspective vs Parallel
•Computer graphics treats all projections the same and implements them with a single pipeline
•Classical viewing developed different techniques for drawing each type of projection
•Fundamental distinction is between parallel and perspective viewing even though mathematically parallel viewing is the limit of perspective viewing
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7Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Taxonomy of Planar Geometric Projections
parallel perspective
axonometric multivieworthographic
oblique
isometric dimetric trimetric
2 point1 point 3 point
planar geometric projections
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8Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Perspective Projection
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9Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Parallel Projection
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10Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Orthographic Projection
Projectors are orthogonal to projection surface
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11Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Multiview Orthographic Projection
• Projection plane parallel to principal face
• Usually form front, top, side views
isometric (not multivieworthographic view) front
sidetop
in CAD and architecture, we often display three multiviews plus isometric
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12Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Advantages and Disadvantages
•Preserves both distances and angles Shapes preserved
Can be used for measurements• Building plans• Manuals
•Cannot see what object really looks like because many surfaces hidden from view
Often we add the isometric
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13Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Axonometric Projections
Allow projection plane to move relative to object
classify by how many angles ofa corner of a projected cube are the same
none: trimetrictwo: dimetricthree: isometric
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14Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Types of Axonometric Projections
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15Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Advantages and Disadvantages
• Lines are scaled (foreshortened) but can find scaling factors
• Lines preserved but angles are not Projection of a circle in a plane not parallel to the
projection plane is an ellipse
• Can see three principal faces of a box-like object
• Some optical illusions possible Parallel lines appear to diverge
• Does not look real because far objects are scaled the same as near objects
• Used in CAD applications
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16Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Oblique Projection
Arbitrary relationship between projectors and projection plane
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17Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Advantages and Disadvantages
• Can pick the angles to emphasize a particular face
Architecture: plan oblique, elevation oblique• Angles in faces parallel to projection plane are
preserved while we can still see “around” side
• In physical world, cannot create with simple camera; possible with bellows camera or special lens (architectural)
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18Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Perspective Projection
Projectors converge at center of projection
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19Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Vanishing Points
• Parallel lines (not parallel to the projection plan) on the object converge at a single point in the projection (the vanishing point)
• Drawing simple perspectives by hand uses these vanishing point(s)
vanishing point
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20Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Three-Point Perspective
• No principal face parallel to projection plane
• Three vanishing points for cube
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21Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Two-Point Perspective
• On principal direction parallel to projection plane
• Two vanishing points for cube
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22Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
One-Point Perspective
• One principal face parallel to projection plane
• One vanishing point for cube
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23Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Advantages and Disadvantages
• Objects further from viewer are projected smaller than the same sized objects closer to the viewer (diminution)
Looks realistic
• Equal distances along a line are not projected into equal distances (nonuniform foreshortening)
• Angles preserved only in planes parallel to the projection plane
• More difficult to construct by hand than parallel projections (but not more difficult by computer)