IVR Chapter 5 Rendering
Questions about the lecture 'Introduction to Virtual Reality' of the RWTH Aachen Chapter 5 Rendering
Questions about the lecture 'Introduction to Virtual Reality' of the RWTH Aachen Chapter 5 Rendering
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Cartes-fiches
What are the steps and transitions of the rendering pipeline? [4]
1. Transformation →[Culling] 2. Projection →[Clipping] 3. Scan Conversion →[Hidden surface removal] 4. Shading
Name three types of reflection on surfaces regarding rendering? [3]
1. Direct
2. Multi-surface
3. Subsurface
Name three types of direct reflection on surfaces regarding rendering? [3]
1. Mirror // Perfect specular
2. Directed diffuse
3. Diffuse
Which reflection is considered in real time CG and VR?
Local reflection
Which reflection is considered in photo realistic CG?
Global reflection
What has to be considered for global reflection? [3]
1. Interaction of different object surfaces
2. Ray tracing
3. Radiosity
Name a model for reflection?
Phong model
What is the principle of the phong model regarding reflection? [2]
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]
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]
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. Ix = 1/y-y’ *(I*(ys-y’) + I’*(y-ys)) with point between point p and p' and x of {a,b}
3. Is = 1/xb-xa *(Ia*(xb-xs) + Ib*(xs-xa)) with Ia left and Ib 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?
Name two algorithms for detecting lines in scan conversion? [2]
1. DDA and 2. Bresenham (1962)
What is the input for both line detecting algorithms in scan conversion?
Input (xS,yS) and (xE,yE) with yE>yS
Define the DDA algorithm for detecting lines in scan conversion? [2]
1. Compute x=xS and m=(xE-xS)/(yE-yS)
2. For every y between yS and yE label (round(x),y) and x+=m
What is the idea of the Bresenham algorithm for detecting lines in scan conversion? [2]
1. From pixel (x,y) two possibilities for next step
2. Choose next step due to decision variable d
Which two possibilities exist for the next steps in the Bresenham algorithm for detecting lines in scan conversion? [2]
1. East to (x+1,y) and 2. north east to (x+1,y+1)
How does the decision regarding d in the Bresenham algorithm for detecting lines in scan conversion works? [3]
1. d=2*dy – dx
2. Go east if d>0
3. Go north east if d<=0
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 (xi,xi+1) of A[y]
2.2 If z-value(x,y) > z-buffer(x,y) then update z-buffer
2.3 Compute diffuse intensity Id and set framebuffer (x,y,Id)
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 [xw,yw,zw] → Projection → 3. Screen [xs,ys]
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
