Week 5 - OpenGL and Texture & Environment Mapping

Question 1

What is the OpenGL rendering pipeline?

Answer 1

Vertex specification
Vertex shader
Tessellation
Geometry shader
Vertex pre-processing
Primitive assembly
Rasterization
Fragment shader
Per-sample operations

Question 2

What do OpenGL’s “shaders” do?

Answer 2

They are small programs compiled at run-time by the OpenGL program that can be programmed in a C-like language called GL Shading Language (GLSL). There are two types; vertex shaders and fragment shaders

Question 3

What do vertex shaders do?

Answer 3

They perform basic processing of individual vertices in the scene (e.g. viewpoint transformation)

Question 4

What do fragment shaders do?

Answer 4

They process fragments generated by the rasterization

Question 5

Name 3 GLSL data types.

Answer 5

Basic types (similar to C++; bool, int, uint, float, double)

Vector types

Matrix types

Question 6

What are the GLSL vector operations?

Answer 6

cross, dot, length, normalize, distance

Question 7

In terms of GLSL, what are uniforms?

Answer 7

Variables that are shared by all instances of the shader, denoted by the key word uniform

Question 8

In terms of GLSL, what are inputs?

Answer 8

Variables specific to each instance, denoted by keyword in

Question 9

In terms of GLSL, what are outputs?

Answer 9

The result of the shader computation, sent down the rendering pipeline, denoted by keyword out

Question 10

What is the purpose of vertex shaders?

Answer 10

To perform operations applied to all vertices in the model, as all vertices need to be transformed (ie rotated, translated) from world coordinates to the camera view space, including normals

Question 11

What are the default input and output for vertex shaders?

Answer 11

input: index of the current vertex
output: vertex position in clip space (after projection)

Question 12

What is the purpose of fragment shaders?

Answer 12

To calculate shading for all fragments (ie pixels), e.g. using Gouraud, Phong or Blinn shading

Question 13

What is texture mapping?

Answer 13

Mapping a two-dimensional texture (image) onto the surface of a three-dimensional object

Question 14

How are textures represented in OpenGL?

Answer 14

Textures are images, and each pixel is also called a texture element (texel)

Each vertex of the mesh is associated to a point in the texture ([0, 1] x [0, 1])

Colour is samples from the texture at each fragment at interpolated locations

Question 15

What are the four texture wrapping options in OpenGL?

Answer 15

GL_REPEAT (the texture pattern is repeated periodically)

GL_MIRRORED_REPEAT (same as above but the texture is flipped in alternance)

GL_CLAMP_TO_EDGE (the coordinate is clamped to [0, 1])

GL_CLAMP_TO_BORDER (anything outside of [0, 1] is set to a default value

Question 16

Where is texture coordinate information stored?

Answer 16

At vertex locations, and when rendering, each fragment’s texture coordinates are interpolated from the triangle it belongs to, e.g. using Barycentric interpolation

Question 17

How can you sample colour from a texture in OpenGL (as pixels don’t always align with textures)?

Answer 17

GL_NEAREST (returns the texel closest to (s, t))
or
GL_LINEAR (returns the weighted average of the four neighbours)

Question 18

What is forward mapping?

Answer 18

The texture space is mapped to object space via surface parameterization, and then it is mapped to the screen space via projection

Question 19

What is inverse mapping?

Answer 19

A pixel is mapped to a pre-image of the pixel

Question 20

Name a benefit of forward mapping.

Answer 20

Simple to comprehend

Question 21

Name 2 benefits of inverse mapping.

Answer 21

Suited to scan-line algorithms

Efficient (only required textures are computed)

Question 22

How does OpenGL use textures?

Answer 22

1. Create the texture in Python - create the texture object, bind it, and set the texture data
2. In the shader, add texture coordinates per fragment, create a texture object, and an output colour to display
3. Sending the texture in Python - activate texture zero, get the location of the texture uniform by name, set this uniform to point of texture unit zero, and bind the texture we want to this unit

Question 23

What is environment mapping?

Answer 23

Representing the distant “environment” as a texture

Intensity and colour of the reflected ray found by texture mapping the environment map onto the rendered polygon

Question 24

How does sphere mapping work?

Answer 24

Map the environment onto the surface of a distant sphere surrounding the scene

Simple calculations to find incident illumination

Inadequate resolution of texels near boundaries of map leads to distortions

Question 25

How does cube mapping work?

Answer 25

The environment is mapped onto 6 faces of a cube

Simple to render map and calculate incident light

Each image easily rendered by placing “camera” at the centre of the cube facing each face in turn

6 textures need to be accessed (easy on GPUs)

Cube maps need to be recalculated for objects moving in the environment or changes in viewpoint

Question 26

How is a cube map rendered?

Answer 26

Need to render the view from all six faces of the cube with framebuffers

Question 27

How do you sample from a cube map texture?

Answer 27

Calculate which texel from the cube map would reflect on which fragment by reflecting the view vector with the surface normal (dome by the GLSL reflect() function)

Question 28

The result of what is a sequence of fragments?

Answer 28

Rasterization

Question 29

We calculate texture coordinates (s, t) for each pixel using what of the texture coordinates at the triangle’s vertices?

Answer 29

Barycentric interpolation

Question 30

When Scissor test is enabled, it fails if the fragment’s pixel lies outside of a specified rectangle of the screen. Where in the graphics pipeline is this test located?

Answer 30

Per sample operations