Clipping, Rasterization & Hidden Surface Removal

1. Chapter 5
Clipping & Rasterization
by
Ahmed Daoud

2. Presentation
Outline
• Introduction
• Clipping
- Line segment clipping: parametric form
- Cohen-Sutherland algorithm
- Liang-Barsky algorithm
- Polygon clipping
- Clipping in 3-D
• Rasterization
- Points
- Lines
- Polygons
- Inside-Outside Testing
- Crossing (odd - even) test
- Winding test
- Fill and Sort
- Flood fill
- Scanline fill
- Odd-even fill

3. Introduction

4. Introduction
• At end of the geometric pipeline, vertices have been
assembled into primitives (lines, planes, etc.)
• Must clip out primitives that are outside the view
volume
• Algorithms based on representing primitives by lists of
vertices
• Must find which pixels can be affected by each
primitive
Fragment generation
Rasterization or scan conversion

5. High-Level view of the graphics process
Application
program
Graphics
system
Frame
buffer
Vertices Pixels
• Two main steps:
- We must pass every geometric object through the system.
- we must assign a color to every pixel in the color buffer
that is displayed

6. Clipping

7. What is clipping?
• Clipping is the process of eliminating objects that lie
outside the viewing volume and thus cannot be
visible in the image.
• Clipping is the process of determining which
primitives, or part of primitives, fit within the clipping
or view volume defined by the application program

8.  2D against clipping window
 3D against clipping volume
 Easy for line segments polygons
 Hard for curves and text
- Convert to lines and polygons first

9. Line
segment
clipping
▸ A clipper decide which primitives, or part of
primitives, can possibly appear on the display and
be passed on to the rasterizer
▸ Primitives that fit within the specified view volume
pass through the clipper or are accepted
▸ Primitives that cannot appear on the display are
eliminated or rejected

10. Line
segment
clipping
Cohen-Sutherland algorithm
2D clipping

11. • A point (x, y) is coded according to its position with respect to the viewport boundaries with a
4-bit code called outcode

13. Cases of
outcodes
in Cohen-
Sutherland
algorithm

14. • Let the two endpoints of a line segment be (x1, y1) and (x2, y2)
and o1 = outcode(x1, y1) , o2 = outcode (x2, y2)
• There are 4 cases:
Case 1: o1 = o2 = 0
Both endpoints inside window  no clipping
Case 2: o1 != 0, o2 = 0 or o1 = 0, o2 != 0
One endpoint inside window  clip one end (given by outcode != 0)
Test against each edge in a fixed order and clip
Code intersection point, repeat process to see if 2nd clipping required
Case 3: o1 & o2 != 0 (Bitwise AND)
If endpoints are both on same side of a window edge discard.
Case 4: o1 & o2 = 0 (Bitwise AND)
The endpoints are both outside but across adjacent edges:
a) may intersect the extensions  line segment outside the window
b) may intersect the edges  line segment must be clipped at both
adjacent edges

15. Line segments and the parametric form
Line segments can be specified in many ways:
• 1) Slope and y intercept
y = m x + c m = slope Will not work for vertical line (m = ∞)
• 2) One point (x1, y1) and slope
y = y1 + m (x – x1) Will not work for vertical line (m = ∞)
• 3) Two points
(y – y2) / (x – x2) = (y – y1) / (x – x1) Will not work if x = x1 or x2
• 4) Parametric form: 2 points and a parameter
x = x1 u + x2 (1-u) y = y1 u + y2 (1-u) Will work for any line.
Furthermore, says that
• 0 <= u <= 1 (x, y) lies between (x1, y1) and (x2, y2)
• u < 0 (x, y) lies outside on the side of (x1, y1)
• u > 1 (x, y) lies outside on the side of (x2, y2)

16. Line
segment
clipping
Liang-Barsky algorithm
▸Consider the parametric form of a line
segment
p(a) = (1-a)p1+ ap2 1 ≥ a ≥ 0

17. Liang-
Barsky
algorithm
▸ We can distinguish between the cases by looking at
the ordering of the values of a where the line
determined by the line segment crosses the lines that
determine the window

18. Polygon
clipping
▸Polygons need to be clipped against a rectangular (usually)
window.
▸Not as simple as line segment clipping
Clipping a line segment yields at most one line segment
Clipping a polygon can yield multiple polygons
▸ Sometimes clipped against non-rectangular windows
▸Consider rectangular windows:
Convex polygon  Clipped convex polygon (only one)
Concave polygon  May result in more than one
clipped piece

19. Strategies
1. Brute force
2. Triangulation = Replace nonconvex with convex pieces
(Tessellation)
3. Clipping as a Black Box  Pipeline approach
1. Brute Force Straightforward algorithm: Apply Cohen-Surtherland or Liang-Barsky edge by edge

20. 2. Tessellation and
Convexity
▸Replace nonconvex (concave) polygons with a set of triangular
polygons (a tessellation)
▸Also makes fill easier

21. 3. Black box
approach: clipping
pipeline
▸Can consider line segment clipping as a process that takes in two
vertices and produces either no vertices or the vertices of a clipped line
segment  Clipping against each side of window is independent of
other sides
▸Can use four independent clippers in a pipeline

22. Exmaple: Black Box:
Line clipping

23. ▸Polygon can be clipped successively against the boundaries of the window edge by
edge

24. Clipping
in 3-D
▸Cohen Sutherland in 3D
▸Use 6-bit outcodes
▸When needed, clip
line segment
against planes

25. Clipping
in 3-D
▸Cohen Sutherland in 3D

26. Rasterization

27. What is
Rasterization?
• This is (almost) the last stage in the viewing
pipeline
• Produces fragments from the remaining objects
and them into discrete representation, in the
frame buffer, for display
• The resulting objects from the previous
transformations are essentially polygons which
must now be rasterized, that is, they must be
converted to addresses in the buffer and the
contents of the address loaded with
color/intensity information.
• Sometimes called scan conversion

28. Rasterization
▸This scan conversion can be considered from simple
to complicated
▸Scan conversion of :
▸Points
▸Lines
▸Polygons

29. Points
• The color buffer is n x m array of pixels
• The address is now the address of a pixel address
(ix, iy)
• A color or intensity is now stored in the buffer
location corresponding to (ix, iy)  write_pixel (ix, iy,
value);

30. Lines
• Line L is given by the points in the segment (ix1, iy1)
- (ix2, iy2)
• These points must be scan converted 
rounded/truncated to integer addresses on the
screen.
• Line scan conversion algorithms
• DDA algorithm
• Bresenham algorithm

31. Lines
• Points on line are obtained from the differential
equation dy/dx = m
• where m = (y2 – y1) / (x2 – x1) = Δy / Δx 0 ≤ m ≤ 1
▸Coordinates are integers  pixels generated
successively with increments of 1 in the x variable.
▸Change in y = Δy = m Δx
▸Δx always 1  Δy = m
for (ix=x; ix <= ix2; ix++)
{
y += m;
write_pixel(x, round (y),line_color);
}

32. Ploygons
 Inside-Outside Testing
- Crossing test = Odd-even test
- Winding test (for non-simple polygons)
 Fill and Sort
- Flood fill
- Scanline fill
- Odd-even fill

33. Inside-Outside
Testing
1) Crossing test = Odd-even test
- Let p be a point inside a simple polygon any ray from
p must cross an odd number of edges of the polygon
- Let p be a point outside a simple polygon any ray from
p must cross an even number of edges of the polygon
2) Winding test (for non-simple polygons)
- we must decide how to determine whether a given point
is inside or outside of the polygon.
- Winding Number Count clockwise encirclements of
point:
odd  inside even  outside

34. Winding test
Example

35. Flood fill
▸Fill can be done recursively from an initial point, seed
point, located inside the polygon (WHITE)
▸Scan convert edges into buffer in edge/inside color
(BLACK)
void flood_fill(int x, int y)
{
if(read_pixel(x,y)= = WHITE) {
write_pixel(x,y,BLACK);
flood_fill(x-1, y);
flood_fill(x+1, y);
flood_fill(x, y+1);
flood_fill(x, y-1);
}
}

36. Hidden-Surface Removal

37. What is hidden-
surface removal?
• Determines which fragments correspond to objects
that are visible.
• Those that are in the view volume and are not
blocked from view by other objects closer to the
camera.