# Shared Flashcard Set

## Details

Clipping and visibility in computer graphics
Not all objects are visible in our world, so there's a need to eliminate objects or parts of objects that are not visible. Test algorithms for doing that.
11
Computer Science
01/18/2013

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
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 Cohen-Sutherland line-clipping.
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 4-bit out-codes. 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 Liang-Barsky 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 themN.n = |N||n|cos(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 yIs 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 z-buffer work? Explain the algorithm.
Definition
 We have a z-buffer 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 z-buffer.elsedo nothing.
Term
 Discuss the advantages and disadvantages of the z-buffer algorithm being used for visibility.
Definition
 Advantages:Easy to do in hardwareCan use smart memory of comparisonsEasy to do in softwareNo spacial cases, just pass z to set pixel routineWorks well with things other than planes, more generalDisadvantagesMust draw everything, can use back-face 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 bi-linear interpolation to find the corresponding value at the required pixel.
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