Graphics Pipeline Clipping Techniques

 
Graphics Pipeline
Clipping
 
CMSC 435/634
 
Graphics Pipeline
 
Object-order approach to rendering
Vertex Processing
Clipping
Rasterization
Fragment Processing
Visibility & Blending
 
Transformations
Vertex components of shading
 
Find the visible parts of primitives
 
Break primitives into fragments/pixels
 
Fragment components of shading
 
Which do we see, how do they combine?
 
Why Clip?
 
Window sides
Draw less
Some rasterization algorithms need everything on
screen
Near
Draw less
Don’t divide by 0
Don’t divide by negative z
Far
Draw less
Constrain Z range
 
Clipping & Culling
 
Cull: decide not to draw an object at all
Clip: slice to keep 
just
 the visible parts
Trivial Reject: Entirely off-screen
Trivial Accept: Entirely on screen
 
4
Clipping Lines
Lines intersecting a rectangular clip region are
always clipped into a single line segment
Clip against one window edge at a time
5
Clip
Rectangle
 
Clipping Endpoints
 
For a point at (
x
,
y
) to be inside the clipping
rectangle
 
6
 
x
min 
 x 
 x
max
, y
min 
 y 
 y
max
 
Clipping Conditions
 
Both endpoints are inside (AB)
One endpoint in, another end outside (CD)
Both outside (EF, GH, IJ)
May or may not be in, further calculations needed
 
7
 
Cohen-Sutherland Line Clipping
 
 
First, endpoint pairs are checked for trivial
acceptance
If not, region checks are performed in order to
trivially reject certain lines
If both x pairs are <0 or >1,
then it lies outside (EF)
If both y pairs are <0 or >1,
then it too lies outside
 
8
 
Cohen-Sutherland Line Clipping
 
Create bit code for each endopint
Each region is assigned a 4-bit code (outcode)
 1
st
 bit – above top edge
y
 > 
y
max
2
nd
 bit – below bottom edge
y < y
min
3
rd
 bit – right of right edge
x > x
max
4
th
  bit – left of left edge
x < x
min
 
9
 
Efficient Computation of Bit-Code
 
Compute each bit
First bit is the sign bit of 
y
max
y
Second bit is 
y
y
min
Third bit is the sign bit of x
max
 – x
Forth bit is x – x
min
 
10
 
Bit-Code Trivial Rejects and Accepts
 
If both bit codes are zero – trivial accept
If endpoints are both outside of same edge,
they will share that bit
This can easily be computed as a logical 
and
operation – trivial reject if non-zero result
If not, then need to split line at clip edge,
discard portion outside, continue testing
 
11
 
Cohen-Sutherland
Line Clipping Algorithm
 
12
 
code1
 = outcode from endpoint1
code2
 = outcode from endpoint2
i
f
 
(
c
o
d
e
1
 
=
=
 
0
 
&
&
 
c
o
d
e
2
 
=
=
 
0
)
 
t
h
e
n
 
trivial_accept
e
l
s
e
 
i
f
 
(
c
o
d
e
1
 
&
 
c
o
d
e
2
 
!
=
 
0
)
 
t
h
e
n
 
trivial_reject
e
l
s
e
 
clip against left
 
clip against right
 
clip against bottom
 
clip against top
i
f
 
(
a
n
y
t
h
i
n
g
 
i
s
 
l
e
f
t
)
 
t
h
e
n
  
accept clipped segment
Homogeneous Clipping
 
Works for 3D planes
If point is inside clipping plane:
 
Point on line:
 
Intersection:
 
Polygon Clipping
 
Many cases (new edges, discarded edges)
Multiple polygons may result after clipping a
single polygon
 
14
 
Sutherland-Hodgman Polygon Clipping
 
Divide and conquer
Simple problem is to clip polygon against a
single infinite clip edge
Sequence of 4 clips against clipping rectangle
 
15
 
Sutherland-Hodgman Polygon Clipping
 
Algorithm moves around the polygon from 
v
n
to 
v
1
 and then on back to 
v
n
At each step
Check (v
i
 to v
i+1
) line against the clip edge
Add zero, one, or two vertices to the output
 
16
 
Sutherland-Hodgman Polygon Clipping
 
At each step, 1 of 4 possible cases arises
1) Edge is completely inside clip boundary, so add vertex 
p
 to
the output list
2) Intersection 
i
 is output as vertex because it intersects with
boundary
3) Both vertices are outside boundary, so neither is output
4) Intersection 
i
 and vertex 
p
 both added to output list
 
17
 
Sutherland-Hodgman Algorithm
 
18
 
Sutherland-Hodgman(
array
)‏
 
vertex 
S
 = 
array
[ length(
array
) - 1 ]
f
o
r
 
(
 
j
 
=
 
0
 
;
 
j
 
<
 
l
e
n
g
t
h
(
a
r
r
a
y
)
 
;
 
j
+
+
 
)
 
d
o
 
vertex 
P
 = 
array
[
 j 
]
i
f
 
(
 
P
 
i
s
 
i
n
s
i
d
e
 
c
l
i
p
 
p
l
a
n
e
 
)
 
t
h
e
n
i
f
 
(
 
S
 
i
s
 
i
n
s
i
d
e
 
c
l
i
p
 
p
l
a
n
e
 
)
 
t
h
e
n
 
 
/
*
 
c
a
s
e
 
1
 
*
/
    
Output( 
P
 )
e
l
s
e
/
*
 
c
a
s
e
 
2
 
*
/
    
Output( ComputeIntersection( 
S
, 
P
, clip plane ) )‏
    
Output( 
P
 )‏
e
l
s
e
 
i
f
 
(
 
S
 
i
s
 
i
n
s
i
d
e
 
c
l
i
p
 
p
l
a
n
e
 
)
 
t
h
e
n
/
*
 
c
a
s
e
 
2
 
*
/
   
Output( ComputeIntersection( P, S, clip plane ) )‏
e
l
s
e
/
*
 
c
a
s
e
 
3
 
*
/
   
no op
  
S
 = 
P
Slide Note
Embed
Share

Explore the concepts of graphics pipeline clipping, including object-order rendering, vertex and fragment processing, visibility, blending, reasons for clipping, culling, and various methods like line and endpoint clipping. Dive into algorithms like Cohen-Sutherland for efficient line clipping.


Uploaded on Sep 13, 2024 | 0 Views


Download Presentation

Please find below an Image/Link to download the presentation.

The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. Download presentation by click this link. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.

E N D

Presentation Transcript


  1. Graphics Pipeline Clipping CMSC 435/634

  2. Graphics Pipeline Object-order approach to rendering Transformations Vertex components of shading Vertex Processing Find the visible parts of primitives Clipping Break primitives into fragments/pixels Rasterization Fragment components of shading Fragment Processing Which do we see, how do they combine? Visibility & Blending

  3. Why Clip? Window sides Draw less Some rasterization algorithms need everything on screen Near Draw less Don t divide by 0 Don t divide by negative z Far Draw less Constrain Z range

  4. Clipping & Culling Cull: decide not to draw an object at all Clip: slice to keep just the visible parts Trivial Reject: Entirely off-screen Trivial Accept: Entirely on screen 4

  5. Clipping Lines Lines intersecting a rectangular clip region are always clipped into a single line segment Clip against one window edge at a time F D D D H B B C C H H J E A A G G J G I Clip Rectangle I 5

  6. Clipping Endpoints For a point at (x,y) to be inside the clipping rectangle xmin x xmax, ymin y ymax 6

  7. Clipping Conditions Both endpoints are inside (AB) One endpoint in, another end outside (CD) Both outside (EF, GH, IJ) May or may not be in, further calculations needed 7

  8. Cohen-Sutherland Line Clipping First, endpoint pairs are checked for trivial acceptance If not, region checks are performed in order to trivially reject certain lines If both x pairs are <0 or >1, then it lies outside (EF) If both y pairs are <0 or >1, then it too lies outside 8

  9. Cohen-Sutherland Line Clipping Create bit code for each endopint Each region is assigned a 4-bit code (outcode) 1st bit above top edge y > ymax 2nd bit below bottom edge y < ymin 3rd bit right of right edge x > xmax 4th bit left of left edge x < xmin 9

  10. Efficient Computation of Bit-Code Compute each bit First bit is the sign bit of ymax y Second bit is y ymin Third bit is the sign bit of xmax x Forth bit is x xmin 10

  11. Bit-Code Trivial Rejects and Accepts If both bit codes are zero trivial accept If endpoints are both outside of same edge, they will share that bit This can easily be computed as a logical and operation trivial reject if non-zero result If not, then need to split line at clip edge, discard portion outside, continue testing 11

  12. Cohen-Sutherland Line Clipping Algorithm code1 = outcode from endpoint1 code2 = outcode from endpoint2 if (code1 == 0 && code2 == 0) then trivial_accept else if (code1 & code2 != 0) then trivial_reject else clip against left clip against right clip against bottom clip against top if (anything is left) then accept clipped segment 12

  13. Homogeneous Clipping Works for 3D planes If point is inside clipping plane: Point on line: Intersection:

  14. Polygon Clipping Many cases (new edges, discarded edges) Multiple polygons may result after clipping a single polygon 14

  15. Sutherland-Hodgman Polygon Clipping Divide and conquer Simple problem is to clip polygon against a single infinite clip edge Sequence of 4 clips against clipping rectangle 15

  16. Sutherland-Hodgman Polygon Clipping Algorithm moves around the polygon from vn to v1 and then on back to vn At each step Check (vi to vi+1) line against the clip edge Add zero, one, or two vertices to the output 16

  17. Sutherland-Hodgman Polygon Clipping At each step, 1 of 4 possible cases arises 1) Edge is completely inside clip boundary, so add vertex p to the output list 2) Intersection i is output as vertex because it intersects with boundary 3) Both vertices are outside boundary, so neither is output 4) Intersection i and vertex p both added to output list 17

  18. Sutherland-Hodgman Algorithm Sutherland-Hodgman(array) vertex S = array[ length(array) - 1 ] for ( j = 0 ; j < length(array) ; j++ ) do vertex P = array[ j ] if ( P is inside clip plane ) then if ( S is inside clip plane ) then else elseif ( S is inside clip plane ) then Output( ComputeIntersection( P, S, clip plane ) ) else no op S = P /* case 1 */ Output( P ) Output( ComputeIntersection( S, P, clip plane ) ) Output( P ) /* case 2 */ /* case 2 */ /* case 3 */ 18

More Related Content

giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#