DIRECT VOLUME RENDERING

Question 1

What is volume rendering

Answer 1

Concerned with rendering the inside of things
Both Amorphous (clearly defined boundaries) or not
its not about surfaces

Question 2

Common things modelled that dont have boundaries or surfaces

Answer 2

weather fronts
heat
sea

Question 3

What is a core issue of data points in volume rendering

Answer 3

Knowing how they are classified: core piece or external

Question 4

What are the data point units in a CAT scanner

Answer 4

radiodensity values measured in Hounsfield units
-1000 Air
-600 lung tissue
0 water
40 muscle
350 bones
1800 skull bone
(don’t need to remember these)

Question 5

What is the transfer function

Answer 5

Transforming data values to visualisable data

Question 6

What is a voxel

Answer 6

a single sample, or data point, on a regularly spaced, three-dimensional grid
We can make voxels smaller and sample at a higher frequency to get a more definitive image

Question 7

Raycasting for volume rendering

Answer 7

A more sophisticated method

Fire a ray from the viewpoint, through the pixel in our viewplane and into the volume
when the ray enters the volume, we march along in tiny steps and measure the colour and opacity associated with the voxel we happen to be in
We store this information

Repeat this process, casting many rays

Question 8

What is additive reprojection

Answer 8

The technique being used in raycasting for volume rendering
flat image of a 3d object, pixels change colour depending on what the ray has passed through

Question 9

Create a surface using surface normals

Answer 9

We create a fake surface normal by detecting changes in the voxel in three different directions (x,y,z)

The directions do not actually mean anything, the normal is symbolic - but still helps us create the surface

We can plug this faux surface normal vector into local illumination equations to create surface shading

Question 10

The fundamentals of direct volume rendering

Answer 10

Essentially we are just picking colour and opacity values - we are not calling anything as directly transparent or naming colours too crudely
All our data should pass through this direct volume rendering process

Really important when say looking for a large clump of cancerous cells - but we are still relying on our brain to do a lot of the visualisation

Question 11

Cons of direct volume rendering

Answer 11

If we want to move the viewpoint, we have to re do the whole ray casting step - computationally long

Good quality costs at significant computational costs
- DVR is not supported end to end by computer hardware