Graphics

Question 1

What is a frame buffer?

Answer 1

A memory space that stores a grid

Question 2

What is double buffering?

Answer 2

Where the front buffer’s image is being displayed on screen, and the back buffer contains the image that is currently being rendered. Then you swap the two buffers so the image that was being rendered earlier is now being displayed. WebGL does this for you automatically.

Question 3

Vertex shader vs fragment shader

Answer 3

Vertex shader: The programmable shader that handles the processing of individual vertices

Fragment shader = pixel shader: takes care of how the pixels between the vertices look

Question 4

Vertex Buffer Objects

Answer 4

Contain the data WebGL needs to describe the geometry that will be rendered

Question 5

Index Buffer Objects

Answer 5

Contain integers that are used as references pointing to data in VBOs, in order to enable the reuse of the same vertex

Question 6

Local coordinates and world coordinates

Answer 6

Each object in a 3D space is constructed on a dedicated coordinate system, and the purpose of this is to reduce complication for 3D scene construction. Then we apply the model transform which gives us the world coordinates (the single coordinate system for all objects in the scene). The view transform gives us the inverse, turning the world coordinates into view coordinates.

Question 7

Perspective projection vs orthographic projection

Answer 7

Perspective: near and far planes of frustum are different
Orthographic: near and far planes are the same

Question 8

View frustum

Answer 8

Name of the visible region, defined by six planes: near/far, top/bottom, right/left.

Question 9

Viewport transform

Answer 9

The viewpoint transform maps the projected view to the available space in the computer screen (canvas).

Question 10

Scaling operation

Answer 10

(a, 0 | 0, b)

Question 11

Anticlockwise rotation

Answer 11

(cos, -sin | sin, cos)

Question 12

Shear along x

Answer 12

(1, k | 0, 1)

Question 13

Reflect in x-axis

Answer 13

(1, 0 | 0, -1)

Question 14

Homogeneous coordinates

Answer 14

Coordinates in “projective space”. The W component scales all the others. If we have the homogeneous 2D coordinate (x, y, w), the Cartesian form is (x/w, y/w). Under homogeneous coordinates, 2D transformation matrices become 3D. Multiplying homogeneous coordinates by a scalar represents the same point as before.

Question 15

Perspective transformation

Answer 15

Transforms each object so that the distant object appears smaller. The view volume in the shape of a frustum will become a regular parallelepiped.

Question 16

Perspective transformation equations

Answer 16

x’ = dx/z
y’ = dy/z
z’ = z

d is the distance of the image plane from the centre of projection

Question 17

Shading

Answer 17

The process of altering the colour of an object/surface/polygon, based on the type of light source emitting light.

Question 18

Flat shading

Answer 18

Assign a single colour to each face of an object

Question 19

Smooth shading

Answer 19

Apply lighting against the normal vector at each vertex, to calculate vertex colours (vertex shader). The colours across a face are generated by interpolating the colours obtained at the corner vertices of the face (rasterisation).

Question 20

Phong shading

Answer 20

Normal vector at each point over an object surface is obtained by interpolating normal vectors of the corner vertices of the surface (rasterisation). The colour of each surface point is then calculated by applying lighting against the interpolated normal vector at the point (fragment shader)

Question 21

Types of light source

Answer 21

Directional light: like the sun, generates light from very far away, parallel rays
Point light: like a light bulb that emits light artificially in all directions from a point
Ambient light: represents indirect light, light emitted from all light sources and reflected by walls

Question 22

Calculating overall surface colour

Answer 22

surface colour by diffuse reflection + surface colour by ambient reflection

Question 23

Ambient reflection

Answer 23

Reflection of light from indirect light sources, illuminates an object equally from all directions with the same intensity, its brightness is the same at any position

Question 24

Diffuse reflection

Answer 24

Reflection of light from a directional or point light. Light is reflected equally in all directions from where it hits (due to rough surface). “t” is the angle between light direction and surface orientation (direction perpendicular to surface).

surface colour by diffuse = light colour * base colour of surface * cos(angle between light and normal)

Question 25

Using a point light object

Answer 25

The direction of the light from a point light source differs at each position in the 3D scene, so when calculating shading, you need to calculate the light direction at the specific position on the surface where the light hits. Pass the position of the light source and then calculate the light direction at each vertex position.

Question 26

Different coordinates used in mapping

Answer 26

Parametric coordinates (uv): a logical coordinate system for processing the surface and internal space of a 3D object
Texture coordinates (ts): used to identify points in the image to be mapped
Local or world coordinates: used to position 3D objects
Window coordinates: where the output image is really produced

Question 27

Forward texture mapping

Answer 27

Compute 3D positions of the texture points, and then project them onto the image plane. Mapping from texture coordinates to points on a surface requires three functions: x = x(s, t), y = y(s, t), z = z(s, t). The main problem is that adjacent texture points may project onto non-adjacent image points, creating an uncoloured area.

Question 28

Backwards texture mapping

Answer 28

Given a point on an object, we identify which point in the texture it corresponds to. We need a map of the form s = s(x, y, z), t = t(x, y, z). Good: makes sure every object point has a corresponding texel. Bad: mapping functions are hard to find.

Question 29

Two-part mapping

Answer 29

Map the texture to a simple intermediate surface, then the textured intermediate surface is mapped to the object.

Second phase: normals mapped from intermediate to actual and vice versa, vectors from center of intermediate

Question 30

MIP-mapping

Answer 30

MIP-mapping uses an image pyramid to precompute coarse versions of a texture. It allows us to solve the problem of aliasing caused by over-sampling. Only requires 1/3 more storage, reduces memory consumption of running applications, supports anti-aliasing.

Question 31

Normal mapping

Answer 31

Where normal vectors are encoded as an image, allowing us to generate a visually 3D effect by applying lighting to perturbed normal vectors on the object surface.
Advantage: we are using textures to alter the surface normal, which does not change the actual shape of the surface, meaning it does not impose performance overhead, it’s just shaded as if it was a different shape.

Question 32

Bump mapping

Answer 32

We treat the texture as a single-valued height function, and compute the normal from the partial derivatives of the texture. The heights encode the amount by which to perturb N in the (u, v) direction of the space describing the object surface. Greyscale encodes height, harder to implement but easier to specify (opposite of normal map)

Question 33

Displacement mapping

Answer 33

Use texture map to actually move surface points, meaning geometry must be displaced before visibility is determined. Done as a preprocess or with complicated vertex/fragment shader implementation.

Question 34

Environment maps

Answer 34

We can simulate reflections by using the direction of the reflected ray to index a spherical texture map at infinity

Question 35

Texture maps for illumination

Answer 35

Also known as light maps
The lighting is set up beforehand, and all the expensive lighting calculations are done during pre-process time, avoiding runtime overhead. Visual quality of the lighting is directly dependent on the size of the light map textures. A diffuse texture map is applied first, and then a light map is multiplied with it. The lightmaps for different parts of an object are packed into a large lightmap texture.

Question 36

Fog maps

Answer 36

Fog maps allow dynamic modification of light maps. We put fog objects into the scene, compute where they intersect with geometry and then paint the fog density into a dynamic light map, using the same mapping as a static light map.