IVR Chapter 5 Rendering

Questions about the lecture 'Introduction to Virtual Reality' of the RWTH Aachen Chapter 5 Rendering

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This flashcard set covers advanced rendering techniques at the university level, focusing on algorithms, shading models, and texture mapping. It delves into topics like the Phong model, scan conversion, and hidden surface removal, exploring how to enhance realism in virtual scenes. Ideal for computer science students or professionals, this set provides a comprehensive overview of rendering processes and their applications in creating visually accurate and immersive digital environments.

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03.02.2017 / 10.02.2017

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Define the Bresenham algorithm for detecting lines in scan conversion? [3]

1. Compute x=x1, y=y1, dx=x2-x1, dy=y2-y1 and d=2*dy -dx

2. Define incrE = 2*dy and incrNE = 2*(dy-dx)

3. Loop as long as x<x2

3.1 Update x++

3.2 If d>0 then y++ and d+=incrNE

3.3 Else d+=incrE

3.4 Output (x,y)

Define the pixel loop algorithm for detecting lines in scan conversion? [2]

1. For every polygon

1.1 Peform DDA with inserting round(x) into liner list A[y]

2. For every possible y value

2.1 For every pair (x_i,x_i+1) of A[y]

2.2 If z-value(x,y) > z-buffer(x,y) then update z-buffer

2.3 Compute diffuse intensity I_d and set framebuffer (x,y,I_d)

What are the characteristics of the polygon edge list regarding scan conversion? [3]

1. Even number of list elements for closed polygons

2. Does also work for concave polygons with holes

3. Perform insertion sort

Name know display solutions from 1993 till nowadays? [6]

From 1. CGA (1993), 2. NTSC (1995), 3. VGA (1997), 4. XGA (1999) to 5. WUXGA and 6. 4K (today)

What are the characteristics of anti-aliasing regarding display resolution? [2]

1. Pixel treated as a plane with multiple sub-pixels // Virtually higher resolution

2. Color of pixel results of averaging intensities of sub-pixels

What are the characteristics of multi-sampling regarding display resolution? [3]

1. 1 to 16 samples randomly distributed in a 64x64 subpixel area

2. Samples are filtered within a 5x5 pixel large circle shaped area

3. Filter consists of weighting, normalizing and accumulating and is programmable

Increases realism of virtual scenes as the purpose of texture gradients with...? [3]

1. photos

2. simulating mirror reflections without ray tracing

3. semi-transparent surfaces as glass

How does the texture gradient algorithm works? [3]

1. Texture map [Texel T(u,v)] → Surface parametrization → 2. Object space [x_w,y_w,z_w] → Projection → 3. Screen [x_s,y_s]

What is an alternative to surface parametrization for the texture gradient algorithm? [3]

Perform 1. S- and 2. O-Mapping

3. Avoid distortion of polygon models

What are the possible strategies for O-Mapping? [4]

1. Surface normal vector and interim surface

2. Interim normal vector and object surface

3. Projection of object midpoint to interim surface

4. Mirror reflection of sight vector at interim surface // Walking texture while moving viewer

What are the extreme aliasing effects regarding texture gradient? [2]

1. Viewer is close to object → one texel for many pixels // tiling

2. Viewer is fare of object → many texels for one pixel

How can we react on the ‘viewer close to object’-case regarding texture gradient? [2]

1. Texture hierarchies with 3 colors in ¾ and so on in ¼ of image

2. Mip mapping with tri-linear interpolation

How can we react on the ‘viewer far of object’-case regarding texture gradient? [2]

1. Shrink wrap method

2. Inverse mapping

How does inverse mapping works regarding texture gradient? [2]

1. 4 corner pixels transformed into texture coordinates

2. Integrate over surface in texture map covered by pixel

What has to be considered for HDM regarding stereo and motion parallax? [2]

1. Real and virtual left right screens

2. Larger virtual screens at longer distance for porper focus

What has to be considered for ego-centric perspective regarding stereo and motion parallax? [2]

1. Projection coupled to user’s viewpoint

2. Requires real-time head tracking without latency // motion sickness

What are the equations for shearing to symmetric view volume regarding stereo and motion parallax?

P = SH_xy(-Ex/Ez,-Ey/Ez) = 4x4 diag with P_31 -Ex/Ez and P_32 -Ey/Ez

How does S-mapping works?

T(u,v) → T’(x_i,y_i,z_i)

Explain the vectors L, N, R and V of the Phong model?

[phong.rendering, 4]

1. L is the incoming light vector

2. N is the vector orthogonal to the screen

3. R is the reflected vector caused by L

4. V is the vector to viewer from reflection point

What can happen calculating L*N or R*V?

[phong.rendering, 2]

1. Dot product might be negative, assuming all light gets reflected away

2. Clamp result to [0,1] first

What are the steps and transitions of the rendering pipeline? [4]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. Transformation →[Culling] 2. Projection →[Clipping] 3. Scan Conversion →[Hidden surface removal] 4. Shading

Name three types of reflection on surfaces regarding rendering? [3]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. Direct

2. Multi-surface

3. Subsurface

Name three types of direct reflection on surfaces regarding rendering? [3]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. Mirror // Perfect specular

2. Directed diffuse

3. Diffuse

Which reflection is considered in real time CG and VR?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

Local reflection

Which reflection is considered in photo realistic CG?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

Global reflection

What has to be considered for global reflection? [3]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. Interaction of different object surfaces

2. Ray tracing

3. Radiosity

Name a model for reflection?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

Phong model

What is the principle of the phong model regarding reflection? [2]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. Linear combination of diffuse, specular and ambient component

2. I = I_a*k_a + I_i*(k_d*(L*N) + k_s*(R*V)^n)

What are the parameters of the phong model regarding reflection? [3]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. I_i the source intensity

2. theta angle between L and N

3. omega angle between R and V

What are the definitions for the diffuse intensity of the phong model regarding reflection? [2]

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

1. I_d = I_i*k_d*cos(theta) = I_i*k_d*(L*N)

2. I_d = k_d* Sum_n I_in*L_n*N

What are the definitions for the specular intensity of the phong model regarding reflection? [2]

1. I_s = I_i*k_s*cos^n(omega) = I_i*k_s*(R*V)^n

2. With n index for surface roughness inf for perfect mirror

What are the definitions for the ambient intensity of the phong model regarding reflection?

I_g = I_a*k_a

Name three types of shading regarding reflection? [3]

1. Flat 2. Gouraud and 3. Phong

What is the definition of flat shading regarding reflection?

No interpolation thus inner points get same intensity

What is the definition of gouraud shading regarding reflection? [5]

1. Bilinear interpolation for inner points

2. I_x = 1/y-y’ *(I*(y_s-y’) + I’*(y-y_s)) with point between point p and p' and x of {a,b}

3. I_s = 1/x_b-x_a *(I_a*(x_b-x_s) + I_b*(x_s-x_a)) with I_a left and I_b right

4. No highlights in the middle of polygons // E.g. virtual table

5. Corrugations are smoothed out _AAA_ -> _____

What is the definition of phong shading regarding reflection?

Interpolation of vertex normal vectors

What is the motivation for hidden surface removal (HSR) compared to culling? [2]

1. Culling detects viewable sides of single objects

2. HSR detects viewable sides of all objects

What are the characteristics of Z-buffering? [3]

1. Special memory on graphics HW with one 24bits entry per pixel

2. Updated with highest z value of 3D object point covered by the pixel

3. Increased accuracy with front and back clipping plane value

What are the characteristics of scan conversion? [3]

1. Also called rasterization

2. Determine pixels for which intensity values are required

3. Polygon fill via horizontal line segments (So-called scanline order) for solid objects

What is the idea of scan conversion for lines?

Which pixels define the set for "non-parallel to screen axes" lines?

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What has to be considered for HDM regarding stereo and motion parallax? [2]

1. Real and virtual left right screens

2. Larger virtual screens at longer distance for porper focus

What has to be considered for ego-centric perspective regarding stereo and motion parallax? [2]

1. Projection coupled to user’s viewpoint

2. Requires real-time head tracking without latency // motion sickness

What are the equations for shearing to symmetric view volume regarding stereo and motion parallax?

P = SH_xy(-Ex/Ez,-Ey/Ez) = 4x4 diag with P_31 -Ex/Ez and P_32 -Ey/Ez

How does S-mapping works?

T(u,v) → T’(x_i,y_i,z_i)

Explain the vectors L, N, R and V of the Phong model?

[phong.rendering, 4]

1. L is the incoming light vector

2. N is the vector orthogonal to the screen

3. R is the reflected vector caused by L

4. V is the vector to viewer from reflection point

What can happen calculating L*N or R*V?

[phong.rendering, 2]

1. Dot product might be negative, assuming all light gets reflected away

2. Clamp result to [0,1] first

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IVR Chapter 5 Rendering

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