COMPUTER GRAPHICS (XCS 354) UNIT V

1. Explain Back face detection method and Depth buffer method 
Backface Detection method 
• Fast and simple object space method
• For identifying the back faces of a polyhedron
• Based ion inside outside test
• Plane parameters A, B, C, D Ax +By+ Cz +D < 0
• Inside point is along the line of sight to the surface V.N = Vz C
• Polygon is back face if C < 0
• Polygon cannot see on any face if C = 0
• Z component value is C <= 0

Depth Buffer Method 

• To compare the surface depths at each pixel position on the plane projection
• Also referred as z- buffer method
• Two buffer areas
• Depth buffer to store depth values
• Refresh buffer to store intensity values
• Depth value for a surface position (x, y) Z( -Ax – By– D)/C
• Depth z’ of next position (x + 1, y) Z’( -A(x + 1)-By – D)/C


2. Explain area subdivision and A- Buffer method 
A Buffer Method 
• Extension idea of the depth buffer method
• It represents an antialiased, area averaged, accumulation-buffer method
• Only one visible surface
• Two fields
• Depth field

• Stores a positive or negative real number 

• Intensity field

• Stores a surface intensity information or a pointer value 

• Data for each surface in the linked list includes
• RGB intensity components
• Opacity parameters
• Depth
• Percent of area coverage
• Surface identifier
• Pointer to next surface
Area Sub Division Method 

• Image space method
• The relative characteristic of surface are
• Surrounding surface
• Inside surface
• Outside surface
• Conditions for subdivision
• All surfaces are outside surfaces with respect to the area

• Only one inside
• A surrounding surface with in the area boundaries


3. Explain Depth sorting method 
• Both image and object space operations]
• Perform the basic function
• Surface are sorted in order of decreasing depth
• Surface are scan converted in order, starting with the surface of greatest depth
• Often referred as painters algorithm
• Test listed in the order of increasing difficulty
• Surface do not overlap
• Projections of the two surfaces on to the view plane do not overlap

4. Explain key frame systems 
Key frame systems
• Motion path can be given with a kinematics description
• Physically based on force acting object
• Frame in to individual component or object called cells Morphing
• Transformation of object shapes from one form to another is called morphing
• Two key frames for an object transformation
• Preprocessing using vertex count Simulating accelerations
• Time interval between keyframe is divided in to n + 1
t = (t2 – t1) / n = 1
tBj = t1 + j t, j = 1,2,3 ,……
• Time for jth in- between is
tBj = t1 + t[ ( 1-cos[j / (n + 1)] ) / 2], j= 1, 2, 3, …. 31