COMPUTER GRAPHICS (XCS 354) UNIT – IV

25. What do you mean by Perspective projection? 

Perspective projection is one in which the lines of projection are not parallel. Instead, they all converge at a single point called the center of projection. 

26. What is Projection reference point? 

In Perspective projection, the lines of projection are not parallel. Instead, they all converge at a single point called Projection reference point. 

27. What is the use of Projection reference point? 

In Perspective projection, the object positions are transformed to the view plane along these converged projection line and the projected view of an object is determined by calculating the intersection of the converged projection lines with the view plane. 

28. What are the different types of parallel projections? 

The parallel projections are basically categorized into two types, depending on the relation between the direction of projection and the normal to the view plane. They are orthographic parallel projection and oblique projection. 

29. What is orthographic parallel projection? 

When the direction of the projection is normal (perpendicular) to the view plane then the projection is known as orthographic parallel projection 

30. What is orthographic oblique projection? 

When the direction of the projection is not normal (not perpendicular) to the view plane then the projection is known as oblique projection. 

COMPUTER GRAPHICS (XCS 354) UNIT – IV

13. Differentiate between interpolation spline and approximation spline? 

When the spline curve passes through all the control points then it is called interpolate. When the curve is not passing through all the control points then that curve is called approximation spline. 

14. What do you mean by parabolic splines? 

For parabolic splines a parabola is fitted through the first three points p1,p2,p3 of the data array of kot points. Then a second parabolic arc is found to fit the sequence of points p2, p3, p4. This continues in this way until a parabolic arc is found to fit through points pn-2, pn-1 and pn. The final plotted curve is a meshing together of all these parabolic arcs. 

15. What is cubic spline? 

Cubic splines are a straight forward extension of the concepts underlying parabolic spline. The total curve in this case is a sequence of arcs of cubic rather than parabolic curves 32 Each cubic satisfies :ax+ bx + cx + d 

16. What is a Blobby object? 

Some objects do not maintain a fixed shape, but change their surface characteristics in certain motions or when in proximity to other objects. That is known as blobby objects. Example – molecular structures, water droplets. 

17. Define Octrees? 

Hierarchical tree structures called octrees, are used to represent solid objects in some graphics systems. Medical imaging and other applications that require displays of object cross sections commonly use octree representation. 

18. Define Projection? 

The process of displaying 3D into a 2D display unit is known as projection. The projection transforms 3D objects into a 2D projection plane. 

COMPUTER GRAPHICS (XCS 354) UNIT – IV

1. What are the various representation schemes used in three dimensional objects? 

• Boundary representation (B-res) – describe the 3 dimensional object as a set of surfaces that separate the object interior from the environment. 
• Space-portioning representation – describe interior properties, by partitioning the spatial region containing an object into a set of small, no overlapping, contiguous solids. 

2. What is Polygon mesh? 

Polygon mesh is a method to represent the polygon, when the object surfaces are tiled, it is more convenient to specify the surface facets with a mesh function. The various meshes are Triangle strip – (n-2) connected triangles Quadrilateral mesh – generates (n-1)(m-1) Quadrilateral 

3. What is Bezier Basis Function? 

Bezier Basis functions are a set of polynomials, which can be used instead of the primitive polynomial basis, and have some useful properties for interactive curve design. 

4. What is surface patch? 

A single surface element can be defined as the surface traced out as two parameters (u, v) take all possible values between 0 and 1 in a two-parameter representation. Such a single surface element is known as a surface patch. 

5. Write short notes on rendering bi-cubic surface patches of constant u and v method? 

The simple way is to draw the iso-parmetric lines of the surface. Discrete 

approximations to curves on the surface are produced by holding one parameter constant 

and allowing the other to vary at discrete intervals over its whole range. This produce 

curves of constant u and constant v. 

6. What are the advantages of rendering polygons by scan line method? 

i. The max and min values of the scan were easily found. 

ii. The intersection of scan lines with edges is easily calculated by a simple incremental method. 

iii. The depth of the polygon at each pixel is easily calculated by an incremental method.