Computer graphics iv unit

Computer Graphics
 Visible Surface Detection Methods

Learning Objectives
 Classification of Visible Surface Detection Algorithms
 Back-Face Detection
 Depth-Buffer Method
 A Buffer Method
 Scan-Line Method
 Depth-Sorting Method
 BSP – TREE Method
 Area Sub Division Method
 Octree Method
 Ray Casting Method
 Curved Surfaces
 Wire-Frame Visibility Methods
 Visibility Detection Functions

Visible Surface Detection
 A major consideration in the generatation of
realistic graphics is determining what is visible
within a scene from a chosen viewing position
 Algorithms to detect visible objects are referred
to as visible-surface detection methods

Classification of Visible Surface
Detection Algorithms
 Object-space methods
 Compares objects and parts of object to each other
within the scene definition to determine which
surfaces should be marked as visible
 Image-space methods
 Visibility is decided point by point at each pixel
position

Back-Face Detection
 A fast and simple object-space method for
locating back faces
 A point (x,y,z) is “inside” a polygon surface with
plane parameters A, B, C, D if :
Ax + By + Cz + D < 0
 When an inside point is along the line of sight to
the surface, the polygon must be a back face and
so cannot be seen

Back-Face Detection
 The test is simplified by considering the normal vector
N to the polygon and the viewing vector V
 This is back face if
V · N > 0

Back-Face Detection …
 V.N = Vz . C > 0
 In right handed viewing system, the polygon is a
back face
if C <= 0

Depth(Z) Buffer Method
 A commonly used image-space approach
 Each surface is processed separately, one point
at a time
 Also called the z-buffer method
 It is generally hardware implemented

Depth Buffer Method
 3 surfaces overlap at (x, y). S1 has the smallest
depth value

Depth Buffer Method
 Two buffers are needed
 Depth buffer (distance information)
 Frame buffer (intensity/color information)

Depth Buffer Method
 Depth-Buffer Algorithm
for all (x,y)
depthBuff(x,y) = 1.0, frameBuff(x,y)=backgndcolor
for each polygon P
for each position (x,y) on polygon P
calculate depth z
if z < depthBuff(x,y) then
depthBuff(x,y) =z
frameBuff(x,y)=surfColor(x,y)

Depth Calculation
 Calculate the z-value on the plane
 Incremental calculation
C
DByAx
zDCzByAx

 0
C
B
z
C
DyBAx
z
C
A
z
C
DByxA
z
yxyx
yxyx








),()1,(
),(),1(
)1(
)1(
),(positionofdepththe:),( yxz yx

Accumulation Buffer (A-Buffer)
 An extension of the depth-buffer for dealing
with anti-aliasing, area-averaging, transparency,
and translucency
 The depth-buffer method identifies only one
visible surface at each pixel position
 Cannot accumulate color values for more than one
transparent and translucent surfaces
 Even more memory intensive
 Widely used for high quality rendering

Accumulation Buffer (A-Buffer)
 Each position in the A-buffer has two
fields
 Depth field: Stores a depth value
 Surface data field
 RGB intensity components
 Opacity parameter (percent of transparency)
 Depth
 Percent of area coverage
 Surface identifier

Scan-Line Method
 Image space method
 Extension of scan-line algorithm for polygon
filling
 As each scan line is processed, all polygon
surface projections intersecting that line are
examined to determine which are visible

Scan-Line Method (cont…)
Pixel positions across each scan-line are
processed from left to right
At the left intersection with a surface the surface
flag is turned on (S1flag, S2 flag – on/off)
At the right intersection point the flag is turned
off
We only need to perform depth calculations
when more than one surface has its flag turned on
at a certain scan-line position

Two important tables are maintained:
 The edge table
 The surface facet table
The edge table contains:
 Coordinate end points of reach line in the scene
 The inverse slope of each line
 Pointers into the surface facet table to connect
edges to surfaces

The surface facet tables contains:
 The plane coefficients
 Surface material properties
 Other surface data
 Maybe pointers into the edge table

Scan-Line Method Limitations
The scan-line method runs into trouble when
surfaces cut through each other or otherwise
cyclically overlap
Such surfaces need to be divided
ImagestakenfromHearn&Baker,“ComputerGraphicswithOpenGL”(2004)

Depth-Sorting Method
 Both image-space and object-space operations
 Also called painter’s algorithm
 Surfaces sorted in order of increasing depth
 Surfaces scan-converted in order, starting with the surface of greatest
depth
B behind A as seen by viewer Fill B then A

Depth Sorting
 We make the following tests for each polygon that has
a depth overlap with S
 If any one of these tests is true, no reordering is
necessary for S and the polygon being tested
 Polygon S is completely behind the overlapping surface
relative to the viewing position
 The overlapping polygon is completely in front of S relative
to the viewing position
 The boundary-edge projections of the two polygons onto the
view plane do not overlap

BSP Trees
 Binary space partitioning is an efficient method
for determining object visibility
 Paint surfaces into the frame buffer from back
to front
 Particularly useful when the view reference point
changes, but the objects are at fixed positions

BSP Tree Construction
1. Choose a polygon T and compute the equation of the plane it
defines
2. Test all the vertices of all the other polygons to determine if
they are in front of, behind, or in the same plane as T.
3. If the plane intersects a polygon, divide the polygon at the
plane
4. Polygons are placed into a binary search three with T as the
root
5. Call the procedure recursively on the left and right subtree

Area Subdivision
 Four possible relationships between polygon surfaces
and a rectangular section of the viewing plane
 Terminating criteria
 Case 1: An area has no inside, overlapping, or surrounding
surfaces (all surfaces are ourside the area)
 Case 2: An area has only one inside, overlapping or
surrounding surfaces
 Case 3: An area has one surrounding surface that obscures all

Octrees
 Visible-surface identification is accomplished by
searching octree nodes in a front-to-back order

Ray Casting
 We consider the line of sight
from the a pixel position through
the scene
 Useful for volume data
 Ray casting is a special case of
ray tracing that we will study
later

Curved Surfaces
Surfaces are described using special curves;
representations for curves generalize to representations
for surfaces
 Explicit form:
 Implicit form:
 Parametric form:
),( yxfz 
0),,( zyxf
))(),(),((),,( tztytxzyx p

Surface contour plots
 Y = f(x,z)
 Identify visible curve section
maintain ymin to y max values previously
calculated for the pixel x
 Increment x by 1 and find y, if y in the range, y
not visible. Otherwise visible
 Similarlly find YZ plane and XZ plane to find
visible area

 Display all object edges – difficult to determine front and back
 Depth cueing – displayed intensity of a line is a function of its
distance from the viewer
Wire-Frame Visibility Methods

Wire-Frame Visibility Methods
 Detect hidden lines and display them differently

Wire-Frame Surface-Visibility
 Compare edge positions with surface positions
 Similar to line clipping, but we need to compare depths as well

Wire-Frame Depth Cueing
 Vary the brightness of objects in a scene as a function of
distance from the viewing position
minmax
max
)(
dd
dd
dfdepth



 Multiply each pixel color by:
Where d is the distance of a point
from the viewing position

Visibility Detection Functions
 Back-face removal:
glEnable(GL_CULL_FACE);
glCullFace(mode);
GL_BACK, GL_FRONT, GL_FRONT_AND_BACK
glDisable(GL_CULL_FACE);
default
 GL_BACK is default. Sometimes we want to see the back faces,
for example if we are in a room we can use GL_FRONT

Depth-buffer functions:
 Request depth buffer when initializing:
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB|GLUT_DEPTH);
 Clear depth buffer each time a new frame is displayed:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 OpenGL depth-buffer visibility-detection routines can be
activated with
glEnable(GL_DEPTH_TEST);
 and deactivated with
glDisable(GL_DEPTH_TEST);

 Wire-frame surface visibility:
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

 Depth cueing:
glEnable(GL_FOG);
glFogi(GL_FOG_MODE, GL_LINEAR);
 This applies the linear depth function to object colors using
dmin=0.0 and dmax=1.0. We can set different values for dmin and
dmax with the following function calls
glFogf(GL_FOG_START, minDepth);
glFogf(GL_FOG_END, maxDepth);
 Hidden line hidden surface removal(HLHSR) FUNCTION
setHLHSRidentifier(visibilityfunctionIndex)

Computer graphics iv unit

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Computer graphics iv unit