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Chapter: Mechanical : Computer Aided Design : Visual Realism

Important Questions and Answers: Visual Realism

Mechanical - Computer Aided Design - 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?




Easy to implement Hardware supported


Polygons can be processed in arbitrary order-Fast: ~ #polygons, #covered pixels




-                     Costs memory


9.             What are the advantages and disadvantages of Ray-casting Algorithm in hidden surface removal?




+ Relatively easy to implement


+               For some objects very suitable (for instance spheres and other quadratic surfaces)


+ Transparency can be dealt with easily




-    Objects must be known in advance


-    Slow: ~ #objects*pixels, little coherenc


10.        List the two types of smooth shading.


Gouraud shading


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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