CS2405-COMPUTER GRAPHICS VIVA

CS2405-COMPUTER GRAPHICS LAB

VIVA QUESTIONS

Define Computer graphics.
Computer graphics remains one of the most existing and rapidly growing computer fields. Computer graphics may be defined as a pictorial representation or graphical representation of objects in a computer.

Define Random scan/Raster scan displays?
Random scan is a method in which the display is made by the electronic beam which is directed only to the points or part of the screen where the picture is to be drawn.
The Raster scan system is a scanning technique in which the electrons sweep from top to bottom and from left to right. The intensity is turned on or off to light and unlight the pixel.

List out the merits and demerits of Penetration techniques?
The merits and demerits of the Penetration techniques are as follows
• It is an inexpensive technique
• It has only four colors
• The quality of the picture is not good when it is compared to other techniques
• It can display color scans in monitors
• Poor limitation etc.

What are the features of Inkjet printers?
• They can print 2 to 4 pages/minutes.
• Resolution is about 360d.p.i. Therefore better print quality is achieved.
• The operating cost is very low. The only part that requires replacement is ink cartridge.
• 4 colors cyan, yellow, majenta, black are available.

What are the advantages of laser printer?
• High speed, precision and economy.
• Cheap to maintain.
• Quality printers.
• Lasts for longer time.
• Toner power is very cheap.

What are the advantages of electrostatic plotters?
• They are faster than pen plotters and very high quality printers.
• Recent electrostatic plotters include a scan-conversion capability.
• Color electrostatic plotters are available. They make multiple passes over the paper to plot color pictures.

Define pixel.
Pixel is shortened forms of picture element. Each screen point is referred to as
pixel or pel.

What is frame buffer?
Picture definition is stored in a memory area called frame buffer or refresh buffer.

DDA algorithm:
Digital differential analyzer is a scan conversion line algorithm based on calculating either dx or dy at unit intervals in one coordinate and determine corresponding integer values nearest to the line path for the other coordinate.

Merits and de-merits of DDA:

Merit:
1. It is a faster method for calculating pixel positions than direct use.
2. It eliminates multiplication by making use of raster characteristics.

De-merits:
1. It is time consuming
2. It is difficult to perform rounding and arithmetic operations.

 Need for Bresenhams algorithm
Since this algorithm uses only incremental integer calculations we need to decide which of the two possible pixel positions is closer to the line path at each sample step.

Decision parameter and its needs
Decision parameter is an integer parameter whose value is proportional to the difference between the separations of two pixel positions from the actual line path. It is used to decide which of the two possible pixel positions is closer to the line path at each sample step.

Properties of circle
1. Circle type
2. Width
3. Color
4. dot-dash patterns
5. Pen or brush options

Attribute primitive
The parameter that affects the way a primitive is to be displayed is referred to as attribute parameters. Eg. Color, size that determine fundamental characteristics of a primitive.

 Attributes of a line
1. Line type
2. Line width
3. Pen and brush options
4. Line color

What is PHIGS?
PHIGS is Programmers hierarchical interactive graphics standard. It is the second software standard to be developed and approved by standard organization. It is an extension of graphical kernel system (GKS). It increases capabilities for object modeling, color specifications, surface rendering and picture manipulations.

What are the different types of line type attributes?
solid lines
dotted lines
dashed lines

What is pixel mask?
It is a string containing the digits 1 and 0, to indicate which positions to plot along the line path.
What is the function used to set line width attribute?
setLineWidthScaleFactor (LW)
Where LW is line width parameter and is assigned a positive no. to indicate the relative width of the line.
 Function used to set color index
setPolylinecolourIndex(5)
Here 5 represents the color index.

 How will you specify the line width for lines with |m| < 1?
For lines with slope magnitude less than 1, we can modify a line drawing routine to display thick lines by plotting a vertical span of pixels at each x position along the line. The no. of pixels in each span is set equal to the integer magnitude of parameter lw.

How will you specify the line width for lines with |m| >1?
For lines with slope magnitude greater than 1, we can modify a thick line with horizontal spans alternatively picking up pixels to the right and left of the line path.

What is a Look-up table?
It is a scheme for storing color values in a color look-up table where frame buffer values are now used as indices into the color table. This scheme would allow the user to select any 256 colors for simultaneous display from a palette of nearly 17 million colors.

What are the area-fill attributes?
1. Fill styles
2. Pattern fill
3. Soft fill

 How areas are displayed with 3 basic fill styles?
1. Hollow with a color border- using only the boundary outline
2. Filled with solid color- displayed in a single color including the borders
3. Filled with specified pattern or design

How a color is chosen for solid interior or hollow area outline?
setInteriorColorIndex (fc)

where, fc is the fill color parameter to set the desired color code.

How do you select fill pattern?
SetInteriorStyleIndex(pi)
Where, pi is the pattern index no.

Soft or tint fill?
Modified boundary fill and flood filled procedures that are applied to repaint areas so that the fill color is combined with the background colors are referred to as soft or tint fill.

Character attributes
1. Text-font, size, color and orientation
2. Marker

Command used to set the font

SetTextFont(tf)

Where, tf is the text font parameter.

To set size of character

setCharacterHeight(ch)

where, ch is the character height

How will you set the orientation of the character?
setCharacterUpVector(upvect)
Where, upvect is assigned two values that specify the x and y vector components.

Marker attributes
A marker symbol is a single character that can be displayed in different colors and sizes. Marker attributes are implemented by procedures that load the chosen character into the raster at the defined positions with the specified color and size.

What is unbundled attribute?
The specification that exactly describes how a primitive is to be displayed with the attribute settings is called as individual or unbundled attributes.

 Bundled attributes
A particular set of attribute values for a primitive on each output device is then chosen by specifying the appropriate table index. Such attributes are called as bundled attributes.

Bundle table
The table for each primitive that defines groups of attribute values to be used when displaying that primitive on a particular output device is called bundled table.

Nyquist sampling frequency
To avoid loosing information from periodic objects we need to set a sampling frequency to at least twice that of the highest frequency occurring in the object

 Fs = 2 fmax
Fs- Nyquist sampling interval


What is Transformation?
Transformation is the process of introducing changes in the shape size and orientation of the object using scaling rotation reflection shearing & translation etc.

Define clipping.
Clipping is the method of cutting a graphics display to neatly fit a predefined graphics region or the view port.

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.

Define B-Spline curve.
A B-Spline curve is a set of piecewise (usually cubic) polynomial segments that pass close to a set of control points. However the curve does not pass through these control points, it only passes close to them.

What is the use of control points?
Spline curve can be specified by giving a set of coordinate positions called control
points, which indicates the general shape of the curve, can specify spline curve.

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.

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.

Define projection.
The process of converting the description of objects from world coordinates to viewing coordinates is known as projection

Define computer graphics animation.
Computer graphics animation is the use of computer graphics equipment where the graphics output presentation dynamically changes in real time. This is often also called real time animation.

What is key frame?
One of the shape photographs that a film or video is made of the shape of an object is known initially and for a small no of other frames called keyframe.


What is Transformation?
Transformation is the process of introducing changes in the shape size and
orientation of the object using scaling rotation reflection shearing & translation etc.

What is translation?

Translation is the process of changing the position of an object in a straight-line
path from one coordinate location to another.

What is rotation?
A 2-D rotation is done by repositioning the coordinates along a circular path, in
the x-y plane by making an angle with the axes.

What is scaling?
The scaling transformations changes the shape of an object and can be carried out
by multiplying each vertex (x,y) by scaling factor Sx,Sy where Sx is the scaling factor of x and Sy is the scaling factor of y.

What is shearing?
The shearing transformation actually slants the object along the X direction or the
Y direction as required.ie; this transformation slants the shape of an object along a
required plane.

What is reflection?
The reflection is actually the transformation that produces a mirror image of an
object. For this use some angles and lines of reflection.

Distinguish between window port & view port.
A portion of a picture that is to be displayed by a window is known as window
port. The display area of the part selected or the form in which the selected part is
viewed is known as view port.

Define clipping.
Clipping is the method of cutting a graphics display to neatly fit a predefined
graphics region or the view port.

What is covering (exterior clipping)?
This is just opposite to clipping. This removes the lines coming inside the
windows and displays the remaining. Covering is mainly used to make labels on the
complex pictures.

What is the need of homogeneous coordinates?
To perform more than one transformation at a time, use homogeneous coordinates
or matrixes. They reduce unwanted calculations intermediate steps saves time and
memory and produce a sequence of transformations.

Distinguish between uniform scaling and differential scaling.
When the scaling factors sx and sy are assigned to the same value, a uniform
scaling is produced that maintains relative object proportions. Unequal values for sx and sy result in a differential scaling that is often used in design application.

What is fixed point scaling?
The location of a scaled object can be controlled by a position called the fixed
point that is to remain unchanged after the scaling transformation.

List out the various Text clipping?
_ All-or-none string clipping - if all of the string is inside a clip window, keep it
otherwise discards.
_ All-or-none character clipping – discard only those characters that are not
completely inside the window. Any character that either overlaps or is outside a
window boundary is clipped.
_ Individual characters – if an individual character overlaps a clip window
boundary, clip off the parts of the character that are outside the window.

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.

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

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.

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.


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.

What are the advantages of rendering by patch splitting?
i. It is fast- especially on workstations with hardware polygon-rendering
pipeline.
ii. It’s speed can be varied by altering the depth of sub-division.

Define B-Spline curve.
A B-Spline curve is a set of piecewise (usually cubic) polynomial segments that
pass close to a set of control points. However the curve does not pass through these
control points, it only passes close to them.

What is a spline?
To produce a smooth curve through a designed set of points, a flexible strip called
spline is used. Such a spline curve can be mathematically described with a piecewise
cubic polynomial function whose first and second derivatives are continuous across
various curve section.

What is the use of control points?
Spline curve can be specified by giving a set of coordinate positions called control
points, which indicates the general shape of the curve, can specify spline curve.

What are the different ways of specifying spline curve?
• Using a set of boundary conditions that are imposed on the spline.
• Using the state matrix that characteristics the spline
• Using a set of blending functions that calculate the positions along the curve path by specifying combination of geometric constraints on the curve

What are the important properties of Bezier Curve?
• It needs only four control points
• It always passes through the first and last control points
• The curve lies entirely within the convex half formed by four control
points.

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.

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.

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.

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.

What are the steps involved in 3D transformation?

• Modeling Transformation
• Viewing Transformation
• Projection Transformation
• Workstation Transformation

What do you mean by view plane?
A view plane is nothing but the film plane in camera which is positioned
and oriented for a particular shot of the scene.

Define projection.
The process of converting the description of objects from world
coordinates to viewing coordinates is known as projection

What you mean by parallel projection?
Parallel projection is one in which z coordinates is discarded and parallel
lines from each vertex on the object are extended until they intersect the view plane.

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.

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.



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.

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.

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

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.

What is an axonometric orthographic projection?

The orthographic projection can display more than one face of an object.
Such an orthographic projection is called axonometric orthographic projection.

What is cavalier projection?

The cavalier projection is one type of oblique projection, in which the
direction of projection makes a 45-degree angle with the view plane.


What is vanishing point?
The perspective projections of any set of parallel lines that are not parallel
to the projection plane converge to appoint known as vanishing point.

What do you mean by principle vanishing point.

The vanishing point of any set of lines that are parallel to one of the three
principle axes of an object is referred to as a principle vanishing point or axis
vanishing point.


What is view reference point?
The view reference point is the center of the viewing coordinate system. It
is often chosen to be close to or on the surface of the some object in the scene.

Define computer graphics animation.
Computer graphics animation is the use of computer graphics equipment
where the graphics output presentation dynamically changes in real time. This is often also called real time animation.

Define frame.
One of the shape photographs that a film or video is made of is known as
frame.

What is key frame?
One of the shape photographs that a film or video is made of the shape of
an object is known initially and for a small no of other frames called keyframe

What is pseudo animation?

Pseudo animation is creating a sequence of stills, photographing or video
graphing each still as one frame, and then later playing back the frames at a faster
speed.


What is a Particle system?

A particle system is a method for modeling natural objects, or other
irregularly shaped objects, that exhibit “fluid-like” properties. Particle systems are
suitable for realistic rendering of fuzzy objects, smoke, sea and grass.

Give some examples for computer graphics standards.
i. CORE – The Core graphics standard
j. GKS -- The Graphics Kernel system
k. PHIGS – The Programmers Hierarchical Interactive Graphics System.
l. GSX – The Graphics system extension
m. NAPLPS – The North American presentation level protocol syntax.