it is a Visible surface detection method is also known as depth buffer method. In this method detect the visible surface by using the distance of the object from the projections plane.

1. A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 1
COMPUTER GRAPHICSCOMPUTER GRAPHICS
Visible Surface DetectionVisible Surface Detection
(Z-Buffer/Depth Buffer)(Z-Buffer/Depth Buffer)

2. A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 2
DEPTH-BUFFER METHODDEPTH-BUFFER METHOD
 Compares surface depth values throughout a scene
for each pixel position on the projection plane
 Usually applied to scenes only containing polygons
 Fast approach due to easy depth values computation
 Also often called the z-buffer method
(x2, y2) & z2
(x3, y3) & z3
(x1, y1) & z1
 (x1, y1), (x2, y2) &
(x3, y3) are the pixel
positions of surfaces
S1, S2, and S3
respectively.
 z1, z2 and z3 defines
the depth values
(distance) of surfaces
S1, S2, and S3
respectively from the
View Plane

3. A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 3
DEPTH-BUFFER METHOD (Cont…)DEPTH-BUFFER METHOD (Cont…)
1. Initialise the depth buffer and frame buffer so that for
all buffer positions (x, y)
depthBuff(x, y) = 1.0
frameBuff(x, y) = bgColour
2. Process each polygon in a scene, one at a time
– For each projected (x, y) pixel position of a
polygon, calculate the depth z (if not already
known)
– If z < depthBuff(x, y), compute the surface colour
at that position and set
depthBuff(x, y) = z
frameBuff(x, y) = surfColour(x, y)
 After all surfaces are processed depthBuff and frameBuff
will store correct values

4. A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 4
DEPTH CALCULATIONDEPTH CALCULATION

5. ExampleExample
A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 5
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
6 6 6 6 6 6 6
6 6 6 6 6 6
6 6 6 6 6
6 6 6 6
6 6 6
6 6
6
6 6 6 6 6 6 6 1
6 6 6 6 6 6 1 1
6 6 6 6 6 1 1 1
6 6 6 6 1 1 1 1
6 6 6 1 1 1 1 1
6 6 1 1 1 1 1 1
6 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
6 6 6 6 6 6 6 1
6 6 6 6 6 6 1 1
6 6 6 6 6 1 1 1
6 6 6 6 1 1 1 1
6 6 6 1 1 1 1 1
6 6 1 1 1 1 1 1
6 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
Surface-1
Depth Buffer
Depth Buffer Frame Buffer

6. ExampleExample
A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 6
6 6 6 6 6 6 6 1
6 6 6 6 6 6 1 1
6 6 6 6 6 1 1 1
6 6 6 6 1 1 1 1
6 6 6 7 1 1 1 1
6 6 7 7 7 1 1 1
6 7 7 7 7 7 1 1
1 1 1 1 1 1 1 1
6 6 6 6 6 6 6 1
6 6 6 6 6 6 1 1
6 6 6 6 6 1 1 1
6 6 6 6 1 1 1 1
6 6 6 1 1 1 1 1
6 6 1 1 1 1 1 1
6 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
7 1 1 1 1 1 1 1
7 1 1 1 1 1 1
7 1 1 1 1 1
7 1 1 1 1
7 1 1 1
7 1 1
1 1 1 1 1 1 1 1
7 7 7 7 7
7 7 7 7
7 7 7
7 7
7
6 6 6 6 6 6 6 1
6 6 6 6 6 6 1 1
6 6 6 6 6 1 1 1
6 6 6 6 1 1 1 1
6 6 6 1 1 1 1 1
6 6 1 1 1 1 1 1
6 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
Surface-2Depth Buffer
Depth Buffer Frame Buffer

7. A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 7
Iterative Calculations (cont…)Iterative Calculations (cont…)
top scan line
bottom scan line
y scan line
y - 1 scan line
x x’

8. A. K. Biswas, Dept. Of Computer Apllication, B.I.T., Durg 8
DISADVANTAGES OF DEPTH BUFFERDISADVANTAGES OF DEPTH BUFFER
This method only find out one visible surface
at each pixel position that means it deals
with only Opaque surface.
1
2
3
4
5
6
1 Red
2 Red
3 Green
4 Blue
5 Green
6 Red