Term
Explain the visibility problem. 

Definition
Not everything in our world is visible. The camera has a finite length of view. Objects get in the way, sometimes of themselves. So, we need techniques to eliminate unnecessary objects or parts of objects. 


Term
Explain the naive clipping algorithm. 

Definition
Clip each pixel before accessing the frame buffer. The algorithm proceeds as follows: def SetPixel(x,y, colour): if x >=0 and x <= x_max and y >=0 and y <=y_max: frame[x,y]= colour 


Term
Explain the advantages and disadvantages of naive clipping. 

Definition
It always works but it is slow because it adds unnecessary tests to the innner most loop of the output producing code. 


Term
Explain CohenSutherland lineclipping. 

Definition
We split the world into 9 regions. The window placed in the middle one. Three regions above window, three regions below, 1 to the left, 1 to the right. This gives 4bit outcodes. The first bit: above top edge, 2nd bit: below bottom edge, 3rd: right of right edge, 4th:left of left edge. If the condition is true bit is set.Trivial acceptance, trivial rejection. Explain the algorithm in steps. 


Term
Explain the LiangBarsky algorithm for polygon clipping. 

Definition
Consider the polygon as list of vertices. Consider each line segment in turn. Only four possible cases: totally in, totally out, going in, going out. 


Term
Explain backface culling. 

Definition
Only draw a polygon if it's outside faces the camera. N: normal vector, n: direction towards the viewpoint from the centre of the polygon and th is the angle between them N.n = Nncos(th) if cos(th) is positive then polygon points towards the camera. 


Term
Explain painters algorithm. 

Definition
Sort the polygons according to furthest z. Draw the polygons in reverse order, starting with the furthest away. Polygons furthest away will be drawn first and then overwritten by those nearer. 


Term
Problem with Painters algorithm. 

Definition
Real polygons can overlap in z as well as xy. If those conditions are met: Overlap in x or y Is one either fully or fully behind of a plane defined by one polygon. Then split the polygons accordingly. This is real painter's algorithm. 


Term
How does the zbuffer work? Explain the algorithm. 

Definition
We have a zbuffer with the same size as the frame buffer. It is used to keep the distance from the camera of the nearest object at that pixel. Algorithm: if z_current < z(buffer): update the colour in the frame buffer, update the value in the zbuffer. else do nothing. 


Term
Discuss the advantages and disadvantages of the zbuffer algorithm being used for visibility. 

Definition
Advantages: Easy to do in hardware Can use smart memory of comparisons Easy to do in software No spacial cases, just pass z to set pixel routine Works well with things other than planes, more general Disadvantages Must draw everything, can use backface culling to eliminate some 


Term
Explain the technique used so we don't have to calculate distance at each pixel. 

Definition
Instead of calculating distance(z) at each pixel. Calculate distance at vertices and used bilinear interpolation to find the corresponding value at the required pixel. 

