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Transcript of ch12 view1
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University of PalestineComputer Graphics
ITGD3107
Prepare:Mohammed J. el-masre
Nidal M. El-Borbar
Supervision:Dr. Sana'a
Presentation name :Three Dimensional Viewing
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Contents
3D ViewingCamera AnalogyViewing Pipeline
Camera Modeling w/ OpenGLProjectionsTypes of ProjectionsPerspective ProjectionParallel ProjectionProjection Matrix
Projection of Lines3D Viewing with OpenGLMatrix ModesThree Dimensional Viewing
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3D ViewingThe steps for computer generation of a view of a three
dimensional scene are somewhat analogous to the
processes involved in taking a photograph.
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Camera Analogy
Position
Orientation
Window (aperture)
of the camera
1. Viewing position
2. Camera orientation
3. Size of clippingwindow
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Viewing PipelineThe general processing steps for modeling and
converting a world coordinate description of a
scene to device coordinates:
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Viewing Pipeline1. Construct the shape of individual objects in a scene within modeling
coordinate, and place the objects into appropriate positions within the scene
(world coordinate).
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Viewing PipelineWorld coordinate positions are converted to viewing
coordinates.
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Viewing PipelineConvert the viewing coordinate description of the
scene to coordinate positions on the projection
plane.
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Viewing Pipeline
Positions on the projection plane, will then mapped
to the Normalized coordinate and output device.
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Camera Modeling w/ OpenGLcamera coordinatesystem
viewport coordinate
system
device/screen
coordinate system
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Coordinate Systems
World Coordinate System-This is object space or the space in
which the application model is defined.
Screen Coordinate System-The space in which the image is
displayed.World Window (or clipping)-This is the rectangle in the world
defining the region that is to be displayed.
Viewport-The rectangular portion of the interface window that
defines where the image will actually appear (usually theentire interface window but in some cases modified to be a
portion of the interface window).
Viewing Transformation-The process of mapping a world
window in World Coordinates to the Viewport.12
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Window to Viewport mapping
Window-to-Viewport mapping is the process of
mapping or transforming a two-dimensional,world-coordinate scene to device coordinates. In
particular, objects inside the world or clipping
window are mapped to the viewport. The viewport
is displayed in the interface window on the screen.In other words, the clipping window is used to
select the part of the scene that is to be displayed.
The viewport then positions the scene on the
output device. 13
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Projection plane
1. Projection plane called the view plane in
computer graphics
2. It is defined by a point called view
reference point (VRP) and a normal (a
vector perpendicular to the plane) called
view plane normal (VPN).
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Viewing Coordinates
View Reference point: It is the center of viewing
coordinate system. It is chosen to be close to or onsurface of object in a scene.
View Plane normal vector: This normal vector is in
a direction perpendicular to view plane.
View Distance: It is the distance from view plane to
view reference point. 15
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Positioned by a View distance
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Projections
Once the view volume isdetermined, aprojectiontransformation willconvert the object inthe 3D view volume toa 2D image on the
projection plane.
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Types of Projections
Perspective ProjectionParallel Projection
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Perspective Projection (1/2)
Give a realistic view of 3D objects, but doesnot preserve shape of object or scale
Perspective images of parallel lines notparallel with projection plane converge to apoint called vanishing point or projectionreference point.
Size of object is diminished with distance Used in architectural, engineering, industrial
design, and advertising drawings
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Perspective Projection (2/2)
3-point perspective
1-point perspective
2-point perspective
Most popular
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Parallel Projection
Accurate and same-scale measurements
Not realistic, and may require multipleprojections to get a complete feeling of theobject
Used in drawings of machine parts and inworking architectural drawings
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Orthographic ProjectionProjectors are orthogonal to projection surface
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Multiview Orthographic Projection
1. Projection plane parallel to principal face2. Usually form front, top, side views
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Axonometric Projections
Allow projection plane to move relative
to object
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Types of
Axonometric Projections
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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. Cavalier: 45 degree ArcTan(2): Cabinet type
Halves the lengths of edges orthogonal to projection plane
Cabinet: arctan(2) =63.4 degree
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Types of Perspective Projections
Based on number of vanishing points for
lines parallel to the three coordinateaxes
Determined by number of axes parallel
to the viewing plane
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Types
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