lec 8 - surface rendering and shading

Question 1

What shading techniques are there?

Answer 1

flat shading and full shading

Question 2

What is flat shading?

Answer 2

When you compute the shading at some
representative point of each (visible) polygon face (may be a vertex, or the centroid of the face, assuming it lies within the face). Then use this computed value to fill the whole polygon.

Question 3

What is full shading?

Answer 3

do the complete shading calculation at each pixel as the polygon is displayed

Question 4

What are some problems of flat shading?

Answer 4

1. the same value is used to render the entire polygon,
2. it misses out on shading variations.
   It is therefore realistic only for distant viewer and distant light source.

Question 5

When are either flat shading or full shading inappropriate?

Answer 5

If our polyhedron is really intended as an approximation to a curved object.

Question 6

Why are either flat shading or full shading inappropriate for curved surfaces?

Answer 6

The discontinuities in surface normal where the face meet will lead to artefactual discontinuities in the shading.

Question 7

What are alternatives to either flat shading or full shading?

Answer 7

Gouraud shading and Phong shading. In both, compute an “average” surface normal at each visible vertex, by averaging the normals of the faces that meet
there.

Question 8

What is Gouraud shading?

Answer 8

Normalised polygon surface normals averaged to obtain vertex normals in Gouraud shading

Question 9

How do you calculate surface normals?

Answer 9

Assuming all polygons store vertices in same walking order, the sign for surface normals should be correct and obtained simply calculating the cross products of the vectors (p2 - p1) and (p3 - p2), where p1, p2 and p3 are the points that make up the polygon.

Question 10

How does Gouraud shading work?

Answer 10

In Gouraud shading, compute the shading at each vertex.
When filling a polygon in a scan-line fashion, interpolate the shading linearly along each edge, and then interpolate linearly along each scan-line segment

Question 11

What is Gouraud shading also known as?

Answer 11

intensity interpolated shading.

Question 12

Is Gouraud shading dependant on orientation?

Answer 12

yes, The results of interpolated-shading is not independent of the projected polygons position

Question 13

When do you get perspective distortion with Gouraud shading?

Answer 13

you have a polygon with vertex 1 more distant than vertex 2.
In this case you get perspective distortion, since perspective foreshortening means that the difference from one scan line to another increases in the direction of the further coordinate.

Question 14

What are the side effects of perspective distortion with Gouraud shading?

Answer 14

you can get polygonal silhouettes and

edges can look weird.

Question 15

What is Phong Shading?

Answer 15

Phong shading is similar to Gouraud, except interpolate the surface normals, and do full shading calculation at every pixel
using these interpolated normals

Question 16

Give an example of when Phong would be better than Gouraud.

Answer 16

If you have a surface where there is a highlight on the vertex, Gouraud will pick it up, however if the highlight is in the centre of the object, Gouraurd will completely ignore it as it will be interpolated over. Phong picks it up in both cases.

Question 17

Can you use Gouraud shading with specular reflection?

Answer 17

yes, but get less pronounced highlights (according to specular reflection coefficient).

Question 18

What is an issue with averaged vertex normals?

Answer 18

Averaged vertex normals may not represent surface geometry adequately (e.g. on slide 18) so that there will be little or no variation in the shade, if the light source is distant.

Question 19

Why is there an issue with averaged vertex normals?

Answer 19

because we interpolate between the normals, therefore the actual surface normal information in between is lost!
Particularly true for objects represented by with lots of small polygons.

Question 20

Gourard vs. Phong - differences?

Answer 20

Gouraud shading:

• interpolation over intensity
• only requires calculating the illumination model at each vertex.

Phong shading:

• interpolation over surfaces normals
• requires calculation of the illumination model at each pixel