Week 7 - Bump & Shadow Mapping and Visibility, Occultations & Clipping

Question 1

What is bump mapping?

Answer 1

Simulating height variations by adjusting directions of surface normals

Question 2

What his a heightmap (elevation map)?

Answer 2

A texture provided to the shader that defines for each pixel on the model’s surface how far from the polygonal mesh’s surface it should lie

Question 3

What are the steps for using a height map (elevation map)?

Answer 3

Look up the heightmap for the current surface position (implemented as a texture lookup)

Calculate surface normal perturbation at this location from the height map

Combine the calculated normal with the original one

Calculate illumination for the new normal using a standard shading

Question 4

What are the direct shadow methods?

Answer 4

Shadow volumes and shadow maps

Question 5

What are the indirect shadow methods?

Answer 5

Raytracing, radiosity and photon mapping

Question 6

What is the idea of shadow volumes?

Answer 6

Determine volumes which are created by shadows

Shadow volume represented by a polyhedral generated by object silhouette and receiver object

A point is in shadow if it is in the volume

Question 7

What is the shadow volume algorithm?

Answer 7

Trace rays from eye point into scene for each pixel

Initialise a shadow counter to zero

Calculate intersections with shadow volumes(s)

For each intersection (sorted by distance from eye), increase shadow counter if entering the volume, and decrease shadow counter if leaving the volume

If the counter is zero, the point is not in any shadow volume

Question 8

What does OpenGL’s depth buffer contain?

Answer 8

The depth value for each fragment, and used for occlusion control (ie z-buffering)

Question 9

What does OpenGL’s stencil buffer contain?

Answer 9

Which fragment should be drawn, and therefore allowing fine grained clipping to arbitrary shapes

Question 10

How are shadow volumes created using the stencil buffer proposed by Tim Heidmann in 1981?

Answer 10

Render the whole scene without the light source (ie everything in the shadows)

Then, for all light sources:
- Calculate a mask using the stencil buffer with caps on the illuminated areas
- Render the scene with lighting applying the stencil buffer
- Use blending to add this to the scene

Question 11

What is the idea of shadow mapping?

Answer 11

Only the objects which are visible from the light source can receive direct lighting

Question 12

What is the basic method for shadow mapping?

Answer 12

Step 1 (generation of depth maps):
- Render the scene from the light source as usual, but only keep track of the depth buffer
- Be aware of aliasing

Step 2:
- Look up the shadow maps

Step 3:
- Lookup in shadow map texture

Question 13

What are the three types of culling?

Answer 13

View-Frustum culling

Back-face culling

Occlusion culling

Question 14

What is also known as View-Frustum culling?

Answer 14

Rendering and clipping

Question 15

What can you clip to?

Answer 15

Camera view frustum
Device boundaries
Viewport boundaries
User specified clipping path

Question 16

What is the most common shape for a clipping region?

Answer 16

A convex polygon

Question 17

What is object-level culling also known as?

Answer 17

Painter’s algorithm

Question 18

What is image-level culling also known as?

Answer 18

Z-buffer algorithm

Question 19

What are the two types of occlusion culling?

Answer 19

Object-level culling and image-level culling

Question 20

What is the idea of the Painter’s algorithm?

Answer 20

Drawing objects from the most distant one to the closest one

Question 21

When does the Painter’s algorithm have an issue and how is this addressed?

Answer 21

When the objects cannot be ordered from back to front - this is addressed by the Z-buffer algorithm

Question 22

What is a Z-buffer?

Answer 22

Aka depth buffer, it is a buffer with an entry for every pixel on the screen, containing (xs, ys) element as the transformed surface that is closest to the screen at (xs, ys)

Question 23

What can’t a Z-buffer handle?

Answer 23

Transparency (including partial transparency)