1. Solid Area Scan Conversion or Visible Surface Detection

2. P (viewing point)
P (viewing point)

3. visible-surface detection
Given a set of 3D object and viewing point
This process find which lines or parts of the object are visible from the viewing point ,
So that it can draw only those part
Find which surfaces are visible
 visible-surface detection
Remove those surfaces that should not be visible
 hidden-surface removal.

4. Visible Surface Detection algorithm
Visible surface detection algorithms are broadly classified as:
Object Space Methods: Compares objects and parts of objects to each other within the scene definition to determine which surfaces are visible
Image Space Methods: Visibility is decided point-by-point at each pixel position on the projection plane
Image space methods are by far the more common

5. Visible Surface Detection algorithm contd…
Two main types of algorithms:
Object space: Determine which part of the object are visible
Image space: Determine per pixel which point of an object is visible
Object space Image space

6. Visible surface detection algorithms are based on following 2 concepts:
Sorting : It facilitate the depth comparison by ordering
the object surface or lines in the scene
according to their distance from the view plane
Coherence : This method are used to take advantage of
regularities in a scene
( It contain intervals of constant pixel intensity &
scan pattern change (a little), from one line to other)

7. Visible Surface Detection Algorithms
Depth Comparison
Z-buffer or Depth Buffer
Depth Sorting or Painter’s
Area Subdivision or Warnock’s
Back Face detection
A – Buffer
Scan line
BSP tree
OCTree

8. Z-Buffer(Extension of frame buffer) (Image space)
Frame buffer  stores the intensity value of each
pixel in image space.
Z-buffer(depth buffer)  stores the z-coordinate
value of each pixel in image space.
When an object in 3D is projected on a view plane as
projection pt(x,y) 
it’s z-value is compared with the previously stored
z-value. If it is smaller then new pixel is written to the
frame buffer and z-buffer is updated with new z-value

9. S3 S2
y axis S1 Z3 Z2 Z1
x axis
Order of scanning is
S2s3s1
z axis

10. For (each position on each polygon surface)
Two buffers are needed: depth and color.
Initialize the buffers:
depth(x,y) = Zbackground
color(x,y) = Ibackground
For (each position on each polygon surface)
calculate z for each (x,y)position
if (z(x,y) < depth(x,y)) {
depth(x,y) = Z
color(x,y) = Isurf }

11. Painter’s algorithm (image space & object space)
This algorithm process the polygon as if they
were being painted into the screen in order of their distance from the viewer.
It has following basic steps 
Surface are sorted in order of decreasing depth
Surface are scan converted in order, starting with surface of greatest depth.

12. By Painter’s
Algorithm

13. STEP 1 6 5
Store the polygon in array by uniquely numbering them 4 2 3 1 1 2 3 4 5 6
STEP 2
For each polygon maintain a linked list named as front list which will contain the polygon no. in front of it in a sequence
STEP 3
For each polygon maintain a counter named as behind counter which will contain the no. of polygon behind to it

14. Initialization 1 2 3 4 5 6 6 5 4 2 3 1 Behind Counter Polygon Array
Front List 2 5 1 2 4 3 1 2 1 5 3 1 3 4 2 5 6

15. STEP 4 (loop)
Repeat step 5 & step 6 till all polygon behind counter is -1
STEP 5
Draw all polygon’s whose behind counter is 0
STEP 6
After drawing
Step through the polygon in the front list, decrease their behind counter by 1.
For drawn polygon make it’s behind counter as -1

16. 1 2 3 4 5 6 6 ITERATION -- 1 2 New Behind Counter Polygon Array
Front List
Behind Counter 2 1 5 1 -1 2 4 3 1 1 2 1 5 3 3 1 4 2 2 5 -1 6

17. 1 2 3 4 5 6 6 ITERATION -- 2 4 2 New Behind Counter Polygon Array
Front List
Behind Counter 1 1 5 1 -1 -1 2 4 3 1 1 1 5 3 -1 3 4 2 2 5 -1 -1 6

18. 1 2 3 4 5 6 6 ITERATION -- 3 4 2 3 New Behind Counter Polygon Array
Front List
Behind Counter 1 5 1 -1 -1 2 4 3 1 -1 1 5 3 -1 3 -1 4 1 2 5 -1 -1 6

19. 1 2 3 4 5 6 6 ITERATION -- 4 4 2 3 1 New Behind Counter Polygon Array
Front List
Behind Counter -1 5 1 -1 -1 2 4 3 1 -1 -1 1 5 3 -1 3 -1 4 1 5 -1 -1 6

20. 1 2 3 4 5 6 6 5 ITERATION -- 5 4 2 3 1 New Behind Counter
Polygon Array
Front List
Behind Counter -1 -1 5 1 -1 -1 2 4 3 1 -1 -1 1 5 3 -1 3 -1 4 -1 5 -1 -1 6

21. Depth-Sorting Algorithm 1
Summary
Image and Object space
Sort surfaces for depth
Draw them back to front
H&B 16-6:

22. For following type of polygons this will not work
Disadvantage
For following type of polygons this will not work
Solution
Cyclically overlapping polygons require cutting
Piercing polygons require clipping

23. Back-face Detection or Elimination
A point (x, y, z) is “inside” a surface with plane parameters A, B, C, and D if
A point (x, y, z) is “outside” a surface with plane parameters A, B, C, and D if
We cannot see the back-face of solid objects:
Hence, these can be ignored

24. Back-face elimination
The polygon is a front face if
Where,
V is a vector in the viewing direction from the eye(camera)
N is the normal vector to a polygon surface V N

25. Back-face elimination
The polygon is a back face if V N
Where,
V is a vector in the viewing direction from the eye(camera)
N is the normal vector to a polygon surface

26. Back-face elimination
Object-space method
Works fine for convex polyhedra: ±50% removed
Concave or overlapping polyhedra: require additional processing
Interior of objects can not be viewed
Partially visible front faces