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ECM3423 Computer Graphics > Week 4 - Models and Illumination > Flashcards

Week 4 - Models and Illumination Flashcards

1
Q

How are continuous objects modelled?

A

Using polygonal approximations - a number of connected polygons (faces): a mesh

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2
Q

When rendering opaque objects, why is only the outer boundary/surface important?

A

Since only the outer boundary/surface interacts with light

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3
Q

What is a mesh?

A

A number of connected polygons/faces

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4
Q

What are the two main types of mesh?

A

Triangle mesh and quadrangle mesh

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5
Q

What does a polygon mesh consist of?

A

A set of vertices (3D points described by (x, y, z) coordinates)

A set of edges (line segments connecting two vertices)

A set of faces (polygons)

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6
Q

When are two edges chained?

A

If they share one vertex

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7
Q

What are surface normals?

A

Vectors perpendicular to the surface

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8
Q

What 3 conditions must a polygon mesh satisfy?

A

The intersection of two faces is either empty or a vertex or an edge

Manifold meshes - every edge must be shared by exactly two faces (except for the border edges)

The faces must have consistent normals (defining the same outward direction)

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9
Q

What is a Mobius strip?

A

A shape with inconsistent normals

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10
Q

How are smooth surfaces approximated?

A

By a discrete set of points, connected by edges

Typically meshes have more polygons in high curvature areas

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11
Q

What methods can you use to define a triangle mesh using OpenGL?

A

glBegin/glEnd and glVertex, using one of GL_TRIANGLES (plots each triangle separately), GL_TRIANGLE_STRIP (plots in a line, avoids repeating vertices), or GL_TRIANGLE_FAN (plots in a circle, avoids repeating vertices)

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12
Q

What does explicit representation of a triangle mesh mean?

A

Each triangle is encoded by its three vertices

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13
Q

What does redundant representation of a triangle mesh mean?

A

Each vertex is shared by three triangles

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14
Q

How many float variables are required for redundant triangle mesh representation?

A

3 x 3 x n = 9n floats for n faces

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15
Q

What are VBOs?

A

Vertex Buffer Objects - arrays for object models that can remain in GPU memory

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16
Q

What is shared vertex/indexed face set representation?

A

Array of coordinates of all vertices (3m floats for m vertices) and an array of triangles with indices into the vertex list (3n integers for n faces)

It gives less redundancy than explicit representation

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17
Q

Name 3 properties of polygon models per-vertex.

A

Coordinate (x, y, z)

Colour (r, g, b)

Normal (nx, ny, nz)

18
Q

Name 4 light and material properties.

A

Light source

Colour

Reflectivity

Transparency

19
Q

How can meshes be made to look solid?

A

By filling the triangles with colour

20
Q

How can you get rid of the polygonal edges when filling a mesh with colour?

A

Interpolate the colour across every face

21
Q

Name 4 types of light sources.

A

Point lights (very small candle flame)

Directed lights (sun, assumed at infinite distance)

Spot lights (torch etc)

Distributed lights

22
Q

What attenuation does directed light have?

A

No attenuation

23
Q

What is light attenuation?

A

The reduction in its intensity as it travels through a medium due to absorption or scattering of photons

24
Q

What attenuation does a point light source have and why?

A

Quadratic attenuation -> 1/d^2 because it is a sphere (A = 4pid^2)

25
Q

When a ray of light hits a surface, what 4 things can happen?

A

Absorption, reflection, refraction and fluorescence

26
Q

What does global illumination consider?

A

All interactions of light within the scene, such as lighting directly from light sources, and indirect lighting

27
Q

What does Lambert’s cosine law describe?

A

Light reflected with equal intensity in all directions

28
Q

What is perfect specular reflection?

A

Reflection from a perfect mirror. Incident and reflected light make equal and opposite angles with surface normal

29
Q

What is imperfect specular reflection?

A

Some light is scattered away from principal reflected direction

30
Q

Give 2 examples of a specular reflection model.

A

The Phong Model and the Blinn-Phong Model

31
Q

Name the light source options for the Phong reflection model

A

Point light at a given position

Spot light at a given position with a limited set of light directions

32
Q

How do you do flat shading of polygon surfaces?

A

A single intensity is calculated for each polygon

Shade is determined by polygon normal and colour at “centre”

A triangular face’s normal is given by the cross product

33
Q

How does Gouraud shading work?

A

Calculate vertex normals by (possibly weighted) averaging of adjacent polygons

(Bi)linerarly interpolate intensity between vertices

34
Q

How does Phong shading work?

A

Calculate vertex normals by averaging (possibly weighted) adjacent polygons

(Bi)linearly interpolate normals between vertices

35
Q

How does scanline interpolation work?

A

Uses Z-buffer algorithm to determine screen locations of vertices

Intensities or normals interpolated from vertices along vertices

Interpolate along scanline from edges

36
Q

Name 3 methods and give a brief explanation of how to shade polygons in screen space.

A

Flat shading - shade determined by polygon normal and colour at “centre”

Gouraud shading - calculate shades at vertices and interpolate to interior pixels

Phong shading - interpolate normal direction to interior pixels and then calculate shade

37
Q

Name a positive and a negative of using flat shading.

A

Fast and avoids interpolation

Doesn’t look great

38
Q

Name a positive and a negative of using Gouraud shading.

A

Basic shading, good for diffuse reflections

Poor rendering of highlights and specular reflections

39
Q

Name a positive and a negative of Phong shading.

A

Highest quality without volumetric rendering

4-5 times slower than Gouraud

40
Q

How can you use Phong shading and Gouraud shading together?

A

Use Phong for surfaces with specular reflection

Use Gouraud for diffuse surfaces

ECM3423 Computer Graphics (11 decks)

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