Computer Graphics, BCA, March 2020

 1. Picture definition is stored in a memory area called the refresh buffer or frame

buffer. This memory area holds the set of intensity values for all the screen

points. Stored intensity values are then retrieved from the refresh buffer and

"painted" on the screen one row (scan line) at a time . Each screen point is

referred to as a pixel .

2. Typefaces (or fonts) can be divided into two broad groups: Serif type and sans

serif. Serif type has small lines or accents at the ends of the main character

strokes, while sans-serif type does not have accents.

3. Another kind of constraint is a grid of rectangular lines displayed in some part of

the screen area. When a grid is used, any input coordinate position is rounded to

the nearest intersection of two grid lines.

4. Clipping determines each element into the visible and invisible portion. Visible

portion is selected. An invisible portion is discarded.

Types of Clipping:

1. Point Clipping

2. Line Clipping

3. Area Clipping (Polygon)

4. Curve Clipping

5. Text Clipping

5. A simple method for translation in the xy plane is to transfer a rectangular block of

pixel values from one location to another. Sequences of raster operations can be

executed to produce real-time animation of either two-dimensional or three-

dimensional objects, as long as we restrict the animation to motions in the

projection plane.

6. Emissive displays are devices that convert electrical energy into light; where the

image is produced directly on the screen. Non-emissive displays use optical

effects to convert sunlight into graphical patterns and shapes; where the light is

produced behind the screen and the image is formed by filtering this light

7. Pencil-shaped devices are used to select screen positions by detaching the light

coming from points on the CRT screen. They are sensitive to the short burst of

light emitted from the phosphor coating at the instant the electron beam strikes a

particular point.

8. A keyframe is a detailed drawing of the scene at a certain time in the animation

sequence. Within each key frame, each object is positioned according to the time

for that frame.

9. The most straightforward method for defining a motion sequence is direct

specification of the motion parameters. Here, we explicitly give the rotation

angles and translation vectors. Then the geometric transformation matrices are

applied to transform coordinate positions.

10. Bitmap fonts require more space, because each variation (size and format) must

be stored in a font cache. It is possible to generate different sizes and other

variations, such as bold and italic, from one set, but this usually does not produce

good results.

11. A world-coordinate area selected for display is called a window. An area on a

display device to which a window is mapped is called a viewport. The window

defines what is to be viewed; the viewport defines where it is to be displayed.

12. Solid-modeling packages often provide a number of construction techniques.

Sweep representations are useful for constructing three-dimensional objects that

possess translational, rotational, or other symmetries.

(10 x 2 = 20

Marks)

Section B

Answer any 6 questions. Each question carries 5 marks.

13. For a given radius r and screen center position ( x , y,), we can first set up our

algorithm to calculate pixel positions around a circle path centered at the

coordinate origin (0,O),

Then each calculated position (x, y) is moved to its proper screen position by

adding x,

to x and y, toy. Along the circle section from x = 0 to x = y in the first quadrant,

the slope of the curve varies from 0 to -1. Therefore, we can take unit steps in

the positive x direction over this octant and use a decision parameter to

determine which of the two possible y positions is closer to the circle path at

each step. Positions in the other seven octants are then obtained by symmetry.

Given a circle radius r = 10, we demonstrate the midpoint circle algorithm by

determining positions along the circle octant in the first quadrant hum x = 0 to x =

y. The initial value of the decision parameter is For the circle centered on the

coordinate origin, the initial point is (x,, yo) - (0, lo), and initial increment terms for

calculating the division parameters are

Successive decision parameter values and positions along the circle path are

calculated

using the midpoint method as

14. Transformation of object shapes from one form to another is called morphing,

which is a shortened form of metamorphosis. Morphing methods can he applied

to any motion or transition involving a change in shape. Given two key frames for

an object transformation, we first adjust the object specification in one of the

frames so that the number of polygon edges (or the number of vertices) is the

same for the two frames. A straight-line segment in key frame k 15 transformed

into two line segments in key frame k t 1. Using linear interpolation to generate

the in-betweens.

15. Another technique for solid modeling is to combine the volumes occupied by

overlapping three-dimensional objects using set operations. This modelling

method, called constructive solid geometry (CSG) creates a new volume by

applying the union, intersection, or difference operation to two specified volumes.

A CSG application-starts with an k t i a l set of three-dimensional objects. Ray-

casting methods are commonly used to implement constructive solid geometry

operations when objects are described with boundary representations.

16. To generate a rotation transformation for an object, we must designate an axis of

rotation(about which the object is to be rotated) and the amount of angular

rotation.

Unlike two-dimensional applications, where all transformations are carried out in

the xy plane, a three-dimensional rotation can be specified around any line in

space. The easiest rotation axes to handle are those that are parallel to the

coordinate axes.

17. There are two basic projection methods. In a parallel projection, coordinate

positions are transformed to the vied plane along parallel lines. For a perspective

projection, object positions are transformed to the view plane along lines that

converge to a point called the projection reference point (or center of projection).

The projected view of an object is determined calculating the intersection of the

projection lines with the view plane.

18. Some graphics packages (for example, PHlCS) provide several polygon

functions for modeling object F . A single plane surface can be specified with a

function such as fillArea. But when object surfaces are to be tiled, it is more

convenient to specify the surface facets with a mesh function. One type of

polygon mesh is the triangle strip.

19. Story board layout

The Story board is an outline of the action. It defines the motion sequence as a

set of basic events that are to take place. Depending on the type of animation to

be produced, the storyboard could consist of a set of rough sketches or it could

be a list of the basic ideas for the motion.

Keyframe specifications

A keyframe is a detailed drawing of the scene at a certain time in the animation

sequence. Within each key frame, each object is positioned according to the

time for that frame. Some key frames are chosen at extreme positions in the

action; others are spaced so that the time interval between key frames is not two

great. More key frames are specified for intricate motions than for simple, slowly

varing motions.

Object definitions

An object definition is given for each partidpant in the action. Objects can be

defined in terms of basic shapes, such as polygons or splines. In addition, the

associated movements for each object are speded along with the shape.

20.A translation is applied to an object by repositioning it along a straight-line path

from one coordinate location to another. We translate a two-dimensional point by

adding translation distances, f, and t,, to the original coordinate position (x, y) to

move the point to a new position ( x ' , y') .

x' = x + t,, y' = y + t,

21. Data Glove

A data glove that can be used to grasp a "virtual" object. The glove is

constructed with a series of sensors that detect hand and finger motions.

Electromagnetic coupling between transmitting antennas and receiving antennas

is used to provide information about the position and orientation of the hand. The

transmitting and receiving antennas can each be structured as a set of three

mutually perpendicular coils, forming a three-dimensional Cartesian coordinate

system. Input from the glove can be used to position or manipulate objects in a

virtual scene.

Digitizer

A position on an object is a digitizer. These devices can be used to input

coordinate values in either a two-dimensional or a three-dimensional space.

Typically, a digitizer is used to scan over a drawing or object and to input a set of

discrete coordinate positions, which can be joined with straight-Iine segments to

approximate the curve or surface shapes.

Touch Panel

Touch panels allow displayed objects or screen positions to be selected with the

touch of a finger. A typical application of touch panels is for the selection of

processing options that are repmented with graphical icons.Other systems can

be adapted for touch input by fitting a transparent device with a touchsensing

mechanism over the video monitor screen. Touch input can be recorded using

optical, electrical, or acoustical methods. Optical touch panels employ a line of

infrared light-emitting diodes (LEDs) along one vertical edge and along one

horizontal edge of the frame. The opposite vertical and horizontal edges contain

light detectors. These detectors are used to record which beams are interrupted

when the panel is touched.

Light pen

Pencil-shaped devices are used to select screen positions by detaching the

light coming from points on the CRT screen. They are sensitive to the short burst

of light emitted from the phosphor coating at the instant the electron beam

strikes a particular point. Other Light sources, such as the background light in

the room, are usually not detected by a light pen. An activated light pen, pointed

at a spot on the screen as the electron beam lights up that spot, generates an

electrical pulse that causes the coordinate position of the electron beam to be

recorded. As with cursor-positioning devices, recorded Light-pen coordinates

can be used to position an object or to select a processing option.

(6 x 5 = 30 Marks)

Section C

Answer any 2 questions. It carries 15 marks.

22.

Explain its working

23. A polygon boundary processed with a line clipper may be displayed as a series

of unconnected line segments , depending on the orientation of the polygon to the

cIipping window. For polygon clipping, we require an algorithm that wiIl generate one

or more closed areas that are then scan converted for the appropriate area fill. The

output of a polygon clipper should be a sequence of vertices that defines the clipped

polygon boundaries.

24.A keyframe is a detailed drawing of the scene at a certain time in the animation

sequence. Within each key frame, each object is positioned according to the time for

that frame. Some key frames are chosen at extreme positions in the action; others

are spaced so that the time interval between key frames is not two great. More key

frames are specified for intricate motions than for simple, slowly varing motions.

Explain with an example in morphing.

25. An accurate and efficient raster line-generating algorithm, developed by

Bresenham,

scan converts lines using only incrementa1 integer calculations that can be adapted

to display circles and other curves.The vertical axes show-scan-line positions, and

the horizontal axes identify pixel columns.Sampling at unit x intervals in these

examples, we need to decide which of two possible pixel positions is closer to the

line path at each sample step.Write Bresenham’s line drawing algorithm with

example.