visual realism Unit iii

UNIT III
VISUAL REALISM
ARUNPRAKASH S
ASSISTANT PROFESSOR
K.RAMAKRISHNAN COLLEGE OF
ENGINEERING

VISUALIZATION
 Visualization can be defined as a technique for
creating images , diagrams or animations to
communicate ideas.
 Projection and shading are common methods for
visualizing geometric models.
 CAD uses isometric and perspective projection in
addition to orthographic projection for generating
rich visual images with complete design
information.

 To project 3D to 2D objects we need to remove
the ambiguities of the different views, which can
be got by the elimination of hidden lines ,
surfaces , solid removal approaches.
 Shading,
 Lighting
 Transparency
 Coloring approaches provide more visual realism

Model clean up process
 Generate orthographic views
 Eliminate hidden lines
 Changing necessary hidden lines as dashed line or
adding dimension and text to the different views.

Application of realism
 Robot Simulations : Visualization of movement of their links and joints and end
effector movement etc.
 CNC programs verification of tool movement along the path prescribed and
estimation of cup height and surface finish etc.
 Discrete Even Simulation : Most of DES packages provide the user to create
shop floor environment on the screen to visualize layout of facilities, movement
of material handling systems, performance of machines and tools.
 Scientific Computing : Visualization of results of FEM analysis like iso-stress
and iso-strain regions, deformed shapes and stress contours. Temperature and
heat flux in heat-transfer analysis. Display and animation of mode shape in
vibration analysis.
 Flight Simulation : Cockpit training for pilots is first being provided with flight
simulators, which virtually simulates the surrounding that an actual flight will
pass through.

HIDDEN LINE REMOVAL
 “For a given three dimensional scene, a given
viewing point and a given direction eliminate
from an appropriate two dimensional projection
of the edges and faces which the observer cannot
see”
 Object space method
 Image space method

Two main types of algorithms:
– Object space: Determine which part of the object are visible. Also called
as World Coordinates. Object is described in physical coordinate system.
– It compares the object and parts to each other within the scene definition
to determine which surface is visible.
– Image space: Determine per pixel which point of an object is visible.
Also called as Screen Coordinates. Visibility is decided point by point at
each pixel position on view plane.
– Zooming does not degrade the quality.
Object space Image space

Two main Hidden Surface Removal Algorithm
Techniques:
Object space: Hidden Surface Removal for all
objects
Image space:
Objects: 3 D Clipping
Transformed to Screen Coordinates
Hidden Surface Removal

HIDDEN LINE ELIMINATION PROCESS
DISPLAY OF RESULTS
ELIMINATION OF HIDDEN LINES
APPLICATION OF VISIBILITY TECHNIQUES
CHECKS FOR OVERLAPPING
DEPTH COMPARISION IS USED TO DETERMINE PART OR ALL OF POLYGON IS
HIDDEN
SORTING OF 2D IMAGE DATA
2D OBJECT DATA
TRANSFORMATIONS-CONTAINS VISIBLE AND INVISBLE EDGES.
3D OBJECT DATA
GEOMETRY AND TOPOLOGY

VISIBILITY TECHNIQUES
 MINIMAX TEST
 CONTAINMENT TEST
 SURFACE TEST
 COMPUTING SILHOUETTES
 EDGE INTERSECTION
 SEGMENT COMPARISONS
 HOMOGENITY TEST

MINIMAX TEST
 Minimax test (also called the overlap or bounding box test) checks if two
polygons overlap. The test provides a quick method to determine if two
polygons do not overlap.
 It surrounds each polygon with a box by finding its extents (minimum and
maximum x and y coordinates) and then checks for the intersection for any two
boxes in both the X and Y directions.
 If two boxed do not intersect, their corresponding polygons do not overlap (see
Figure 1). In such a case, no further testing of the edges of the polygons is
required.
 If the minimax test fails (two boxes intersect), the two polygons may or may not
overlap, as shown in Figure 1. Each edge of one polygon is compared against
all the edges of the other polygon to detect intersections. The minimax test can
be applied first to any two edges to speed up this process

 The containment test checks whether a given point lies inside a given polygon or
polyhedron. There are three methods to compute containment or surroundness.
 For a convex polygon, one can substitute the X and Y coordinates of the point into
the line equation of each edge. If all substitutions result in the same sign, the
point is on the same side of each edge and is therefore surrounded.
 For non-convex polygons, two other methods can be used. In the first method, we
draw a line from the point under testing to infinity as shown in Figure 2a. The semi-
infinite line is intersected with the polygon edges. If the intersection count is
even, the point is outside the polygon ( in Figure 2a). If it is odd, the point is
inside.
 If one of the polygon edges lies on the semi-infinite line, a singular case arises which
needs special treatment to guarantee the consistency of the results.
 The second method for non-convex polygons (Figure 2b) computes the sum of the
angles subtended by each of the oriented edges as seen from the test point. If the
sum is zero, the point is outside the polygon. If the sum is -360 or +360 the point is
inside. The minus sign reflects whether the vertices of the polygon are ordered in a
clockwise direction instead of counter clockwise.

Computing silhouettes
 A set of edges that separates visible faces from invisible faces of an
object with respect to a given viewing direction is called silhouette
edges (or silhouettes).
 The signs of the components of normal vectors of the object faces
can be utilized to determine the silhouette.
 An edge that is part of the silhouette is characterized as the
intersection of one visible face and one invisible face.
 An edge that is the intersection of two visible faces is visible, but
does not contribute to the silhouette.
 The intersection of two invisible faces produces an invisible edge.

Edge intersection
 The hidden algorithm initially calculates the
edges intersections in 2D.
 To find out partially visible lines.
 The two edges intersect at a point where y2-y1=0.
 Then segment comparisons are used to further
determine visibility

Segment comparison
 The image is computed scan line by line that is in
segments and displayed in the same order.
 The scan line is divided into spans(dashed lines).
 The visibility is determined within each span by
comparing the depths of the edge segments that
lie in the span.
 Segments with maximum depth are visible
throughout the span.

Homogeneity test
 Points are compared for visibility.
 Homogeneously visible
 Neighborhood of point P can be projected objectively
onto neighborhood of the projection of a point.
 Homogeneously invisible
 Neighborhood of point P cannot be projected
objectively onto neighborhood of the projection of a
point.
 In-homogeneously visible
 Pr(N(P))=N(Pr(P))
 Pr(N(P))≠N(Pr(P)) inhomogeneously invisible.

Surface Back-face elimination
We cannot see the back-face of solid objects:
Hence, these can be ignored
VN
faceback:0NV

Back-face elimination
We cannot see the back-face of solid objects:
Hence, these can be ignored
facefront:0NV
V
N

Back-face elimination
 Object-space method
 Works fine for convex polyhedra: ±50% removed
 Concave or overlapping polyhedra: require
additional processing
 Interior of objects can not be viewed
Partially visible front faces

 Hidden line removal algorithm
 Depth Algorithm or Z algorithm or Priority
algorithm
 Area oriented algorithms
 Overlay algorithm-Curved surface
 Roberts algorithm
 Hidden surface removal algorithm
 Depth buffer algorithm or z-buffer algorithm
 Area coherence algorithm or Warnock’s
algorithm
 Scan-line algorithm or Watkin’s algorithm
 Hidden solid removal algorithm
 Ray tracing algorithm

Depth or priority algorithm
 This algorithm is also known as the depth or z algorithm. The algorithm is
based on sorting all the faces (polygons) in the scene according to the
largest z coordinate value of each.
 This step is sometimes known as assignment of priorities. If a face
intersects more than one face, other visibility tests besides the z depth are
needed to resolve any ambiguities.
 Its basis is on the view according to the biggest Z co-ordinate value.
 If face intersects more than one face, other visibility test beside z-depth is
required to solve any issue.

Depth or priority algorithm
 Painter’s algorithm
 As we utilize the procedure the painter’s way of
creating the background first and then the overlaying
layer and then the outermost layer with reducing depth.
 When we view from z and x axis there is no
overlapping view.

Painter’s Algorithm
 Assumption: Later projected polygons overwrite earlier
projected polygons
Graphics Pipeline
1 12 23 3
Oops! The red polygon
Should be obscured by
the blue polygon

 Main Idea
 A painter creates a picture
by drawing background
scene elemens before
foreground ones
 Requirements
 Draw polygons in back-to-
front order
 Need to sort the polygons
by depth order to get a
correct image
from Shirley

 Sort by the depth of each polygon
Graphics Pipeline
1 12 23 3
depth

 Compute zmin ranges for each polygon
 Project polygons with furthest zmin first
(z) depth
zmin
zmin
zmin
zmin

 Problem: Can you get a total sorting?
zmin
zmin
zmin
zmin
Correct?

Area oriented algorithm
It is based on subdivision of given data set in a stepwise
fashion until all visible areas are determined and displayed.

 Identify silhouette polygons – silhouette edges are
recognized and connection of silhouette edges to form closed
polygons by sorting all edges for equal end points
 Assign quantitative hiding(QH) values to silhouette
polygon.
 This is achieved by intersecting the polygons. The intersection points
define points where QH may change. Find QH value using depth test. 0
is visible and 1 is invisible.
 Determine the visible silhouette segments.
 If closed silhouette polygon is completely invisible it need not be
considered any further. If it is visible the segments with least QH values
are considered.

 Intersect the visible silhouette segments with
partially visible faces.
 To find out the partially visible and fully visible faces.
 Display the interior of the visible or partially
visible polygons.
 By using stack and simply enumerates all faces lying
inside a silhouette polygon.
 The stack is started with a visible face and a loop
begins popping of face F2

Overlay algorithm
 The curved surfaces are approximated as planar
surfaces.
 The u-v grid is used to create grid surface which
consists of regions having straight edges.
 The curves in each region are approximated as
line segment.
 The 1st step is to use surface equation and grid
linear edges are created.

Hidden line removal for curved surface
 To compute the exact visibility we introduce a
notion of visibility curves obtained by projection of
silhouette and boundary curves and decomposing the
surface into nonoverlapping regions.
 The nonoverlapping and visible portions of the
surface are represented as trimmed surfaces and we
present a representation based on polygon
trapezoidation algorithms.
 The curved surface is converted in polygon mesh and
calculated for visiblity.

Hidden surface removal algorithm
 The elimination of parts of a solid objects that are
covered by others is called hidden surface
removal.
 Depth buffer or Z-buffer Algorithm
 Area coherence or Warnock’s algorithm
 Scan-line algorithm or Watkin’s algorithm

Depth-Buffer Methods
43
Three surfaces overlapping pixel position (x,y) on the view plane.
The visible surface, S1, has the smallest depth value.
vx
vy
vz
3S
2S
1S
 ,x y
view plane

Depth buffer or z-buffer algorithm

45
Z-Buffer Algorithm
As we render each polygon, compare the depth of each
pixel to depth in z buffer
If less, place shade of pixel in color buffer and update z
buffer

Z-buffer: A Secondary Buffer
Color buffer Depth buffer

 Two buffer areas are required
• Depth buffer
 Store depth values for each (x, y) position
 All positions are initialized to minimum depth
 Usually 0 – most distant depth from the viewplane
• Refresh buffer
 Stores the intensity values for each position
 All positions are initialized to the background
intensity

Z-BUFFER ALGORITHM:
• Its an extension of Frame Buffer
• Display is always stored on Frame Buffer
• Frame Buffer stores information of each and every
pixel on the screen
• Bits (0, 1) decide that the pixel will be ON or OFF
• Z- Buffer apart from Frame buffer stores the depth
of pixel
• After analyzing the data of the overlapping
polygons, pixel closer to the eye will be updated
• Resolution of X,Y => Array[X,Y]

Given set of polygon in image space
Z-Buffer Algorithm:
1. Set the frame buffer & Z-Buffer to a background
value
(Z-BUFFER=Zmin) where, Zmin is value
To display polygon decide=>color, intensity and depth
2. Scan convert each polygon
i.e, for each pixel, find depth at that point
If Z(X,Y)>Z-BUFFER(X,Y)
Update Z-BUFFER(X,Y)=Z(X,Y)
& FRAME BUFFER
This process will repeat for each pixel

• By this way we can remove hidden lines and display
those polygons which are closer to eye
• X*Y space required to maintain Z-Buffer=> X*Y
times will be scanned
• Expensive in terms of time and space as space is very
large
x
z
display
S
S’
x
y S
S’

Area coherence or warnock’s algorithm
Area Subdivision
 Exploits area coherence: Small areas of an
image are likely to be covered by only one
polygon
 Three easy cases for determining what’s in front
in a given region:
1. a polygon is completely in front of everything else in
that region
2. no surfaces project to the region
3. only one surface is completely inside the region,
overlaps the region, or surrounds the region

Identifying Tests
 Four possible relationships
 Surrounding surface
 Completely enclose the area
 Overlapping surface
 Partly inside and partly outside the area
 Inside surface
 Outside surface
 No further subdivisions are needed if one of the following conditions is
true
 All surface are outside surfaces with respect to the area
 Only one inside, overlapping, or surrounding surface is in the area
 A surrounding surface obscures all other surfaces within the area boundaries 
from depth sorting, plane equation
Surrounding
Surface
Overlapping
Surface
Inside
Surface
Outside
Surface

Warnock’s Area Subdivision
(Image Precision)
 Start with whole image
 If one of the easy cases is satisfied (previous slide), draw
what’s in front
 Otherwise, subdivide the region and recurse
 If region is single pixel, choose surface with smallest depth
 Advantages:
 No over-rendering
 Anti-aliases well - just recurse deeper to get sub-pixel
information
 Disadvantage:
 Tests are quite complex and slow

Characteristics
 Takes advantage of area coherence
 Locating view areas that represent part of a single surface
 Successively dividing the total viewing area into smaller rectangles
 Until each small area is the projection of part of a single visible
surface or no surface
 Require tests
 Identify the area as part of a single surface
 Tell us that the area is too complex to analyze easily
 Similar to constructing a quadtree

55
Process
 Staring with the total view
 Apply the identifying tests
 If the tests indicate that the view is sufficiently
complex
 Subdivide
 Apply the tests to each of the smaller areas
 Until belonging to a single surface
 Until the size of a single pixel
 Example
 With a resolution 1024  1024
 10 times before reduced to a point

Warnock’s Algorithm
 Regions labeled with case
used to classify them:
1) One polygon in front
2) Empty
3) One polygon inside,
surrounding or
intersecting
 Small regions not labeled
2 2 2
2222
2
2
3
3
3
3 33
3
3
3
3
3
333
3
3
1
1 1 1
1

SCAN LINE Z-BUFFER ALGORITHM:
• An image space method for identifying visible
surfaces
• Computes and compares depth values along the
various scan-lines for a scene

Scan-Line Method Basic Example
 Scan Line 1:
 (A,B) to (B,C) only inside S1, so color from S1
 (E,H) to (F,G) only inside S2, so color from S2
 Scan Line 2:
 (A,D) to (E,H) only inside S1, so color from S1
 (E,H) to (B,C) inside S1 and S2 , so compute & test depth
In this example we color from S1
 (B,C) to (F,G) only inside S2, so color from S2
B
A
D
C
G
F
E
H
S1 S2
Scan Line 1
Scan Line 2
Scan Line 3

• Scanning takes place row by row
• To facilitate the search for surfaces crossing a given
scan-line an active list of edges is formed for each
scan-line as it is processed
• The active list stores only those edges that cross the
scan-line in order of increasing x
• Pixel positions across each scan-line are processed
from left to right
• We only need to perform depth calculations
• In Scan Line, Z-Buffer(X) whereas earlier it was X*Y

Use Z-Buffer for only 1 scan line / 1 row of pixel
• During scan conversion in the Active Edge
List(AEL)=> Calculate Z(X,Y)
i.e, -Pixel info between 1 & 2 active edges will only
be stored
-Next pixel will be stored as z=z1+∆z
-∆z will be constant but can change anywhere in
case of slope
• If Z(X,Y)>Z BUFFER(X) => Update
• If 2 polygons are present-along with Active Edge List,
Active Polygon List will also be included
• Active Polygon List=> List of polygons intersecting a
scan line

Hidden solid removal
 The hidden solid removal of B-rep model are
algorithms such as z-buffer.
 Convert CSG to B-rep.
 Render it with standard hidden surface removal
techniques.
 RAY TRACING
 The complex 3D solid/solid intersection problem
is converted into a 1D ray/solid intersection
calculation

Ray tracing
 If we shoot a ray from the viewpoint through the
pixel, the first object which hits is the one that is
visible at the pixel.
 It can be used for both flat and curved surface.
 Shoot the ray from the eyepiece one per pixel
 Find the closest object blocking the path of the
ray.
 Since it has infinite rays, the light rays are traced
backwards, a ray from the viewpoint is traced
through a pixel until it reaches a surface.

Ray casting
 If resolution is x,y then there are xy pixels so xy
light rays are traced.
 Each ray is tested for intersections with each
object in the picture including the non clipping
plane.
 The intersection closest to the viewpoint is
determined since rays intersect many objects.

 The main advantage is that it can create extremely
realistic rendering of pictures by incorporating
laws of optics for reflection and transmitting light
rays
 The major disadvantage is the performance since
it starts the process a new and treat each eye ray
separately.

visual realism Unit iii

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visual realism Unit iii