VISUAL REALISM
1. List some
hidden-space algorithms.
a. Depth
–Buffer Algorithm
b. Scan-line
coherence Algorithm
c. Area-coherence
algorithm (Warnock’s algorithm)
d. Priority
algorithm (Newell, Newell and Sancha algorithm)
2. Enumerate
usage of Shading in Computer Graphics.
Shading is used in drawing for
depicting levels of darkness on paper by applying media more densely or with a
darker shade for darker areas, and less densely or with a lighter shade for
lighter areas. There are various techniques of shading including cross hatching
where perpendicular lines of varying closeness are drawn in a grid pattern to
shade an area. The closer the lines are together, the darker the area appears.
Likewise, the farther apart the lines are, the lighter the area appears.
3. What are
the two main ingredients in shading of model?
•
Properties of the model surface
•
Properties of illumination falling
on it
4. Differentiate
Point and Spotlight lighting.
Point lighting
Light originates from a single point, and spreads outward in all
directions.
Spotlight
lighting
Models a Spotlight. Light originates from a single point, and spreads outward in
a cone.
5. Distinguish
Flat and Smooth shading.
Flat
Shading
Uses the same color for every pixel in a face -
usually the color of the first vertex.
Edges appear more pronounced than they would on a
real object because of a phenomenon in the eye known as lateral inhibition
Same color for any point of the face
Individual faces are visualized
Not suitable for smooth objects
Less computationally expensive
Smooth
shading
Smooth
shading uses linear
interpolation of colors between vertices
The edges disappear with this technique
Each point of the face has its own color
Visualize underlying surface
Suitable for any objects
More computationally expensive
6. Enumerate Painter's algorithm.
It sorts polygons by their bary center and draws
them back to front. This produces few artifacts when applied to scenes with
polygons of similar size forming smooth meshes and back face culling turned on.
The cost here is the sorting step and the fact that visual artifacts can occur.
7. How Warnock algorithm works?
It divides the screen into smaller areas and sorts
triangles within these. If there is ambiguity (i.e., polygons overlap in depth
extent within these areas), then further subdivision occurs. At the limit,
subdivision may occur down to the pixel level.
8. What are the advantages and disadvantages of
Depth-Buffer Algorithm?
Advantages
Easy to implement Hardware supported
Polygons can be processed in arbitrary order-Fast: ~
#polygons, #covered pixels
Disadvantages
-
Costs memory
9.
What are the advantages and disadvantages of
Ray-casting Algorithm in hidden surface removal?
Advantages
+ Relatively
easy to implement
+
For some objects very suitable (for instance
spheres and other quadratic surfaces)
+ Transparency
can be dealt with easily
Disadvantages
- Objects
must be known in advance
- Slow: ~
#objects*pixels, little coherenc
10.
List the two types of smooth shading.
O Gouraud shading
O Phong shading
11.
Write short note on Goura ud shading.
1. Determine
the normal at each polygon vertex
2. Apply an
illumination model to each vertex to calculate the vertex intens ity
3. Interpolate
the vertex intensities using bilinear interpolation over the surface polygon
12. Write Advantages of Gour aud shading.
Polygons, more c omplex than triangles, can also
have different colors specified for each vertex. In these instances, the
underlying logic for shading c an become more intricate.
13. What are the Problems encountered in
Gouraud shading?
Even the smoothness introduced by Gouraud shading
may not preve nt the appearance of the shading differences between adjacent
polygons.
Gouraud shading is more CPU intensive and can become
a problem when rendering real time environments with many polygons.
T-Junctions with adj oining polygons can sometimes
result in visu al anomalies. In general, T-Junctions should be avoided.
14. List some hightlights of Ph ong shading over
Gouraud shading model.
Phong shading is similar to Gouraud shading,
except that the Normals are interpolated. Thus, the spec ular highlights are
computed much more pre isely than in the Gouraud shading model:
a. Compute a
normal N for each vertex of the polygon.
b. From
bilinear interp olation compute a normal, Ni for each pixel. (This must be
renormalized each tim e)
c. From Ni
compute an intensity Ii for each pixel of the polygon.
d. Paint
pixel to shade corresponding to light.
15.
Catalog Hidden surface removal algorithms
i. Z-bufferin
g
ii. Coverage
buffers (C-Buffer) and Surface buffer (S-Buffer)
iii. Sorted
Active Edge List
iv. Painter's
algorithm
v. Binary sp
ace partitioning (BSP)
vi. Ray
tracin g
vii. The
Warnock algorithm
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