Lecture 7 (Surface Rendering and Shading)

Question 1

What are the shading techniques? Elaborate on each.

Answer 1

Flat shading -> One colour per polygon (no interpolation)

Smooth shading (Gouraud and Phong) -> Gradient in polygons

Question 2

What are the Smooth Shading techniques? Elaborate on each.

Answer 2

Gouraud and Phong.

• Gouraud = Vertex shading
• Phong = Pixel by pixel shading, slower but better for highlights

Question 3

WTF is ‘Texture mapping’?

Answer 3

Maps a planar image (typically 2D) onto a 3D object.

- This adds visual detail

Question 4

WTF is ‘Bump mapping’?

Answer 4

Generate rough surfaces, without increasing the number of polygons.

*** The surface does not change, but shading makes it look like it did

• Can be used to add realism to textures

Question 5

WTF is ‘Displacement mapping’?

Answer 5

Actually, changes the shape of object, unlike Bump Mapping which only changes the shading of the object

• Displaces each point on the surface

Question 6

What is the difference between Gouraud and Phong?

Answer 6

Gouraud will miss out on highlights in the middle of the polygon / vertex - as it only applies the lighting model at the vertices.

Gouraud applies an illumination model to each vertex to calculate the vertex intensity first; then interpolates the vertex intensities over the surface polygon.

Phong does the opposite.

Question 7

What is the Shading Model?

Answer 7

Determines how to render the FACES OF POLYGONS in the scene, given the illumination

Question 8

What is the Illumination Model?

Answer 8

Determines HOW LIGHT SOURCES INTERACT with object surfaces

- i.e. intensity of the light that is reflected at any given point on a surface

Question 9

What are the difference Light Source Models?

Answer 9

Point source : All light rays originate at a point and radically diverge

Parallel source : Light rays are all parallel. It may be modelled as a point source at infinite distance (the sun)

Distributed source : All light rays originate at a finite area in space. It models a nearby source (lightbulb)

Question 10

What is Flat Shading and why use it?

Answer 10

Entire polygon is the same colour

• It is much faster and simpler to compute compared to smooth shading
• Realistic in certain cases:
  
  • Polygon is small enough
  • Eye is very far away (Mach Band Effect is not noticeable)

Question 11

What is Smooth Shading?

Answer 11

Colour changes within the polygon - lighting is computed at MULTIPLE points on each polygon (no Mach Band Effect)

Question 12

What is Gouraud Shading?

Answer 12

VERTEX SHADING

• Lighting calculated for each polygon vertex
• Colours are interpolated for the interior pixels
• Linear change from one vertex colour to another

Question 13

What are the drawbacks of Gouraud Shading?

Answer 13

Doesn’t work well with polygon surfaces with high curvature

Assumes linear change across the polygon
- This is a problem because one part may have specular lighting

Question 14

Describe the Gouraud Shading Algorithm.

Answer 14

1) Determine the normal at each polygon vertex = this is the average of the normals of adjacent faces
2) Apply an illumination model to each vertex to calculate the vertex intensity
3) Linearly interpolate the vertex intensities over the surface polygon

Question 15

What is Phone Shading?

Answer 15

PIXEL BY PIXEL SHADING

Similar to Gouraud, except interpolates the surface normals. and do full shading calculation at EVERY PIXEL using these interpolated normals

*** Much slower than Gouraud shading, but does a much better job of handling highlights

Question 16

Describe the Phong Shading Algorithm.

Answer 16

1) Determine the normal at each polygon vertex (average the normals of adjacent faces)
2) Linearly interpolate the vertex normals over the surface polygon
3) Apply the illumination model along each scan line to calculate intensity of each surface point

Question 17

What is Cell Shading?

Answer 17

A non-realistic shading technique aimed at making objects look like comic books.

Also known as Toon Shading.

Question 18

What are the two things to be aware of when using Texture Mapping?

Answer 18

1) A texture map may be SMALLER than the surface it is applied to
- If the texture gets replicated, the image may not look natural
- If you don’t replicate the texture, then the texture will not cover the whole surface
* ** Texture Synthesis can solve this

2) When applying the same textures across multiple objects, it may be necessary to CORRECTLY ALIGN them

Question 19

What is texture synthesis?

Answer 19

Used to deal with textures being too small for the surface they’re mapped onto

Generates synthetic textures of arbitrary size based on the characteristics of a sample input image