Volume Visualization

Question 1

What is volume visualisation?

Answer 1

Visualization of volume data.

Question 2

What are volumes?

Answer 2

Samples in a 3D space.

Question 3

What are pictures/images?

Answer 3

Samples in a 2D space.

Question 4

What is the goal of volume visualisation?

Answer 4

• To gain insight into 3D data
• Depends on the context + focus

Question 5

What is a voxel?

Answer 5

Volume element (point sample in 3D space)

Question 6

When we say data is a set of cells, what is a cell?

Answer 6

A 3D primitive (can be cubic or not)

Question 7

How are values within cells calculated?

Answer 7

Interpolation

Question 8

What is interpolation used for?

Answer 8

• Creating smoother images
• Resampling data (typically at a higher resolution)
• Reconstructing missing information

Question 9

What is nearest neighbor interpolation?

Answer 9

The simplest and fastest interpolation technique. Uses filter width of 1 or 0

Question 10

When is recommended not to use nearest neighbor interpolation?

Answer 10

When the resampling rate is greater than the original sampling rate (e.g. when zooming in or out on an image)

Question 11

What are the different types of continuities for interpolation?

Answer 11

C0: only the reconstructed signal is continuous

C1: The derivative of the reconstructed signal is continuous.

C2, C3, …: Second, third, … derivatives are continuous

Question 12

What is linear interpolation?

Answer 12

Takes the mid value between two points

Question 13

What is Gaussian filtering?

Answer 13

The use of a Gaussian function or filter

Question 14

What is image order volume rendering technique??

Answer 14

• For every pixel: DO …
• Cost is related to the size of image
• Also known as ray casting

Question 15

What is object-order volume rendering techniques?

Answer 15

• For every object (voxel) Do …
• Cost is related to number of objects
• projection of volume data onto the image plane

Question 16

What are transfer functions?

Answer 16

Assigning each scalar value a different color and opacity value. Implemented as a look up table

Question 17

What is a level set for isosurfaces?

Answer 17

The set of all points where the function equals some value

Question 18

What is an isocountour?

Answer 18

Surface made from the level set of a 2D scalar function

Question 19

What is an isosurface?

Answer 19

A surface made from the level set of a 3D scalar function

Question 20

What are some challenges with volume rendering?

Answer 20

• Large data sets and information
• Computational speed

Question 21

What are the two color compositing orders in optical models?

Answer 21

• Back-to-front order
• Front-To-back order

Question 22

What is maximum intensity projection?

Answer 22

For all the volume samples along a ray we find the maximum valued sample

Question 23

What is front-to-back volume rendering composition?

Answer 23

Loop over all the samples on the ray from front to back and accumulate the color with the opacity. Can terminate early when transparency is 0 (full color has been hit)

Question 24

What is back-to-front volume rendering composition?

Answer 24

Starting from the end of the ray loop over all the samples on the ray

Question 25

What is gradient shading?

Answer 25

Computing the gradient at each pixel according to its neighbors and using this value to shade the pixel

Question 26

What is the ray equation for a point along the ray?

Answer 26

P(t) = P_o + tV
o = origin of the ray
t = scalar value
V = direction/vector

Question 27

What is an octree?

Answer 27

An octree is a recursive 3D subdivision of space. Every non-leaf node has 8 children.

Question 28

How would you use an octree with volume data?

Answer 28

Recursively divide a volume into groups of eight until each unit either contains a data point or is empty, thus creating a hierarchical structure that efficiently captures the varying density of the volume data.

Question 29

Why can we not calculate the gradient for points that are on the edge?

Answer 29

For our central difference equation we wouldn’t have the neighbour values to calculate.W

Question 30

What are the benefits of using an octree?

Answer 30

• Speeds up isosurface and MIP algorithms (skips empty spaces)
• Precomputed
• Each node stores the max or min of the equivalent region in space

Question 31

What are gradients used for?

Answer 31

• shading in volume rendering

Question 32

What is the correlation between gradient and isosurfaces?

Answer 32

Gradient is always perpendicular to the isosurface

Question 33

What is direct volume rendering?

Answer 33

Using the original dataset to generate visualizations. Use of techniques such as interpolation and ray intersection.

Question 34

What is indirect volume rendering?

Answer 34

Using a different representation of the datasets, for example isosurfaces being visualized using level sets.

Question 35

What are some common ray composition techniques?

Answer 35

• Accumulation
• Maximum intensity
• Optical models
• Non-photo realistic models

Question 36

What is accumulation ray composition technique?

Answer 36

The combinations/sum of points along a ray. Good for x-ray renderings

Question 37

What is maximum intensity projection?

Answer 37

For each ray selections the value with the highest intensity value

Question 38

What are optical models (ray composition)?

Answer 38

Optical models visualize how light reacts to surfaces.

Question 39

What are partical models (ray composition)?

Answer 39

Representing the voxels as particles, with density. Partical behavior is described with phase functions.

Question 40

What are non photo-realistic renderings?

Answer 40

Renderings with exaggerated features to display more information. Most often based on contour enhancements.

Question 41

What are some examples of indirect volume rendering methods?

Answer 41

• Slicing
• Marching cubes
• Contour tracking
• Cuberillies

Question 42

What are some challenges we face when volume rendering?

Answer 42

• Rendering a large sum of 3D data to a 2D space
• Large data sets
• Computation speed and power