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CSE452 Computer Graphics. Lecture 6: Projection. Model. Lect 2-3: Images. Lect 17-21: Curves, surfaces, fractals, and dynamic objects. Lect 4-5: Basic shapes. Skeleton of CSE452. Render. Lect 6-9: Camera projection. Lect 10-15: Illumination. Interact. Lect 16:

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cse452 computer graphics

CSE452 Computer Graphics

Lecture 6: Projection

slide2

Model

Lect 2-3:

Images

Lect 17-21:

Curves, surfaces, fractals, and dynamic objects

Lect 4-5:

Basic shapes

Skeleton of CSE452

Render

Lect 6-9:

Camera projection

Lect 10-15:

Illumination

Interact

Lect 16:

User interfaces

classification of projections
Classification of Projections

Perspective Projection

Parallel Projection

parallel projections
Parallel Projections
  • Preserves object size
    • Edges parallel to projection plane maintain their lengths after projection
  • Preserves parallelism
    • Lines that are parallel stay parallel after projection
types of parallel projection
Types of Parallel Projection
  • Orthographic projection
    • Projectors orthogonal to the view plane
  • Oblique projection
    • Projectors not orthogonal to the view plane
types of parallel projection1
Types of Parallel Projection

Orthographic

Oblique

multi view orthographic projection
Multi-view Orthographic Projection
  • Projection plane is one of coordinate planes

“3D Max” software interface

axonometric orthographic projections
Axonometric Orthographic Projections
  • Projection plane is not one of coordinate planes

Dimetric

Isometric

oblique projections
Oblique Projections
  • Projectors not orthogonal to projection plane
    • The projection plane is typically parallel to a face of the object
  • Classified by the angle between projector and plane
      • Pi/4: Cavalier type
        • Preserves the lengths of edges orthogonal to projection plane
      • ArcTan(2): Cabinet type
        • Halves the lengths of edges orthogonal to projection plane

Cavalier: 45 degree

Cabinet: arctan(2) =63.4 degree

parallel projection in drawing
Parallel Projection in Drawing

Earliest known technical drawing: Plan view (orthographic projection) from

Mesopotamia, 2150 BC

perspective projections
Perspective Projections
  • How our eyes see the world
    • Objects further away look smaller (foreshortening)
    • Parallel lines may not remain parallel
vanishing points
Vanishing Points
  • Perspective projection of a group of parallel lines intersects at a single vanishing point
    • Unless the group is parallel to the projection plane
  • Why?
slide16

3D line

Viewpoint

slide17

3D line

  • Find P on the viewing plane so that PE is parallel to AB
  • P,A’,B’ lie on the same line, because of these facts:
    • A,B,P,E defines a plane.
    • P,A’,B’ all lie on the plane of ABPE
    • P,A’,B’ also lie on the viewing plane.
    • P,A’,B’ all lie on the intersecting line between viewing plane and the plane of ABPE.

Viewpoint

slide18

Similarly, P,C’,D’ are co-linear for any CD parallel to PE.

Conclusion: P is the vanishing point (unless AB is parallel to the viewing plane, in which case P does not exist)

Viewpoint

types of perspective projections
Types of Perspective Projections
  • Based on number of vanishing points for lines parallel to the three coordinate axes
    • Determined by # of axes parallel to the viewing plane

A unit cube:

One-point Perspective

(view plane parallel to 2 axes)

Two-point Perspective

(view plane parallel to 1 axis)

Three-point Perspective

(view plane not parallel to any axis)