bresenham circles and polygons in computer graphics(Computer graphics tutorials)

Course Website: http://www.comp.dit.ie/bmacnamee
Computer Graphics 4:
Bresenham Line
Drawing Algorithm,
Circle Drawing &
Polygon Filling

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Contents
In today’s lecture we’ll have a look at:
– Bresenham’s line drawing algorithm
– Line drawing algorithm comparisons
– Circle drawing algorithms
• A simple technique
• The mid-point circle algorithm
– Polygon fill algorithms
– Summary of raster drawing algorithms

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The Bresenham Line Algorithm
The Bresenham algorithm is
another incremental scan
conversion algorithm
The big advantage of this
algorithm is that it uses only
integer calculations
Jack Bresenham
worked for 27 years at
IBM before entering
academia. Bresenham
developed his famous
algorithms at IBM in
the early 1960s

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The Big Idea
Move across the x axis in unit intervals and
at each step choose between two different y
coordinates
2 3 4 5
2
4
3
5
For example, from
position (2, 3) we
have to choose
between (3, 3) and
(3, 4)
We would like the
point that is closer to
the original line
(xk, yk)
(xk+1, yk)
(xk+1, yk+1)

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The y coordinate on the mathematical line at
xk+1 is:
Deriving The Bresenham Line Algorithm
At sample position xk+1
the vertical separations
from the mathematical
line are labelled dupper
and dlower
bxmy k ++= )1(
y
yk
yk+1
xk+1
dlower
dupper

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So, dupper and dlower are given as follows:
and:
We can use these to make a simple decision
about which pixel is closer to the mathematical
line
(cont…)
klower yyd −=
kk ybxm −++= )1(
yyd kupper −+= )1(
bxmy kk −+−+= )1(1

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This simple decision is based on the difference
between the two pixel positions:
Let’s substitute m with ∆y/∆x where ∆x and
∆y are the differences between the end-points:
(cont…)
122)1(2 −+−+=− byxmdd kkupperlower
)122)1(2()( −+−+
∆
∆
∆=−∆ byx
x
y
xddx kkupperlower
)12(222 −∆+∆+⋅∆−⋅∆= bxyyxxy kk
cyxxy kk +⋅∆−⋅∆= 22

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So, a decision parameter pk for the kth step
along a line is given by:
The sign of the decision parameter pk is the
same as that of dlower – dupper
If pk is negative, then we choose the lower
pixel, otherwise we choose the upper pixel
(cont…)
cyxxy
ddxp
kk
upperlowerk
+⋅∆−⋅∆=
−∆=
22
)(

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Remember coordinate changes occur along
the x axis in unit steps so we can do
everything with integer calculations
At step k+1 the decision parameter is given
as:
Subtracting pk from this we get:
(cont…)
cyxxyp kkk +⋅∆−⋅∆= +++ 111 22
)(2)(2 111 kkkkkk yyxxxypp −∆−−∆=− +++

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But, xk+1 is the same as xk+1 so:
where yk+1- yk is either 0 or 1 depending on
the sign of pk
The first decision parameter p0 is evaluated
at (x0, y0) is given as:
(cont…)
)(22 11 kkkk yyxypp −∆−∆+= ++
xyp ∆−∆= 20

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The Bresenham Line Algorithm
BRESENHAM’S LINE DRAWING ALGORITHM
(for |m| < 1.0)
1. Input the two line end-points, storing the left end-point
in (x0, y0)
2. Plot the point (x0, y0)
3. Calculate the constants Δx, Δy, 2Δy, and (2Δy - 2Δx)
and get the first value for the decision parameter as:
4. At each xk along the line, starting at k = 0, perform the
following test. If pk < 0, the next point to plot is
(xk+1, yk) and:
xyp ∆−∆= 20
ypp kk ∆+=+ 21

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The Bresenham Line Algorithm (cont…)
ACHTUNG! The algorithm and derivation
above assumes slopes are less than 1. for
other slopes we need to adjust the algorithm
slightly
Otherwise, the next point to plot is (xk+1, yk+1) and:
5. Repeat step 4 (Δx – 1) times
xypp kk ∆−∆+=+ 221

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Bresenham Example
Let’s have a go at this
Let’s plot the line from (20, 10) to (30, 18)
First off calculate all of the constants:
– Δx: 10
– Δy: 8
– 2Δy: 16
– 2Δy - 2Δx: -4
Calculate the initial decision parameter p0:
– p0 = 2Δy – Δx = 6

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Bresenham Example (cont…)
17
16
15
14
13
12
11
10
18
292726252423222120 28 30
k pk
(xk+1,yk+1)
0
1
2
3
4
5
6
7
8
9

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Bresenham Exercise
Go through the steps of the Bresenham line
drawing algorithm for a line going from
(21,12) to (29,16)

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Bresenham Exercise (cont…)
17
16
15
14
13
12
11
10
18
292726252423222120 28 30
k pk
(xk+1,yk+1)
0
1
2
3
4
5
6
7
8

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Bresenham Line Algorithm Summary
The Bresenham line algorithm has the
following advantages:
– An fast incremental algorithm
– Uses only integer calculations
Comparing this to the DDA algorithm, DDA
has the following problems:
– Accumulation of round-off errors can make
the pixelated line drift away from what was
intended
– The rounding operations and floating point
arithmetic involved are time consuming

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A Simple Circle Drawing Algorithm
The equation for a circle is:
where r is the radius of the circle
So, we can write a simple circle drawing
algorithm by solving the equation for y at
unit x intervals using:
222
ryx =+
22
xry −±=

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(cont…)
20020 22
0 ≈−=y
20120 22
1 ≈−=y
20220 22
2 ≈−=y
61920 22
19 ≈−=y
02020 22
20 ≈−=y

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(cont…)
However, unsurprisingly this is not a brilliant
solution!
Firstly, the resulting circle has large gaps
where the slope approaches the vertical
Secondly, the calculations are not very
efficient
– The square (multiply) operations
– The square root operation – try really hard to
avoid these!
We need a more efficient, more accurate
solution

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Eight-Way Symmetry
The first thing we can notice to make our circle
drawing algorithm more efficient is that circles
centred at (0, 0) have eight-way symmetry
(x, y)
(y, x)
(y, -x)
(x, -y)(-x, -y)
(-y, -x)
(-y, x)
(-x, y)
2
R

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Mid-Point Circle Algorithm
Similarly to the case with lines,
there is an incremental
algorithm for drawing circles –
the mid-point circle algorithm
In the mid-point circle algorithm
we use eight-way symmetry so
only ever calculate the points
for the top right eighth of a
circle, and then use symmetry
to get the rest of the points
The mid-point circle
algorithm was
developed by Jack
Bresenham, who we
heard about earlier.
Bresenham’s patent
for the algorithm can
be viewed here.

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Mid-Point Circle Algorithm (cont…)
(xk+1, yk)
(xk+1, yk-1)
(xk, yk)
Assume that we have
just plotted point (xk, yk)
The next point is a
choice between (xk+1, yk)
and (xk+1, yk-1)
We would like to choose
the point that is nearest to
the actual circle
So how do we make this choice?

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Let’s re-jig the equation of the circle slightly
to give us:
The equation evaluates as follows:
By evaluating this function at the midpoint
between the candidate pixels we can make
our decision
222
),( ryxyxfcirc −+=





>
=
<
,0
,0
,0
),( yxfcirc
boundarycircletheinsideis),(if yx
boundarycircleon theis),(if yx
boundarycircletheoutsideis),(if yx

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Assuming we have just plotted the pixel at
(xk,yk) so we need to choose between (xk+1,yk)
and (xk+1,yk-1)
Our decision variable can be defined as:
If pk < 0 the midpoint is inside the circle and
and the pixel at yk is closer to the circle
Otherwise the midpoint is outside and yk-1 is
222
)
2
1()1(
)
2
1,1(
ryx
yxfp
kk
kkcirck
−−++=
−+=

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To ensure things are as efficient as possible
we can do all of our calculations
incrementally
First consider:
or:
where yk+1 is either yk or yk-1 depending on
the sign of p
( )
( ) 2
2
1
2
111
2
1]1)1[(
2
1,1
ryx
yxfp
kk
kkcirck
−−+++=
−+=
+
+++
1)()()1(2 1
22
11 +−−−+++= +++ kkkkkkk yyyyxpp

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The first decision variable is given as:
Then if pk < 0 then the next decision variable
is given as:
If pk > 0 then the decision variable is:
r
rr
rfp circ
−=
−−+=
−=
4
5
)
2
1(1
)
2
1,1(
22
0
12 11 ++= ++ kkk xpp
1212 11 +−++= ++ kkkk yxpp

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The Mid-Point Circle Algorithm
MID-POINT CIRCLE ALGORITHM
• Input radius r and circle centre (xc, yc), then set the
coordinates for the first point on the circumference of a
circle centred on the origin as:
• Calculate the initial value of the decision parameter as:
• Starting with k = 0 at each position xk, perform the
following test. If pk< 0, the next point along the circle
centred on (0, 0) is (xk+1, yk) and:
),0(),( 00 ryx =
rp −=
4
5
0
12 11 ++= ++ kkk xpp

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The Mid-Point Circle Algorithm (cont…)
Otherwise the next point along the circle is (xk+1, yk-1)
and:
4. Determine symmetry points in the other seven octants
5. Move each calculated pixel position (x, y) onto the
circular path centred at (xc, yc) to plot the coordinate
values:
6. Repeat steps 3 to 5 until x >= y
111 212 +++ −++= kkkk yxpp
cxxx += cyyy +=

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Mid-Point Circle Algorithm Example
To see the mid-point circle algorithm in
action lets use it to draw a circle centred at
(0,0) with radius 10

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(cont…)
9
7
6
5
4
3
2
1
0
8
976543210 8 10
10 k pk
(xk+1,yk+1) 2xk+1 2yk+1
0
1
2
3
4
5
6

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Mid-Point Circle Algorithm Exercise
Use the mid-point circle algorithm to draw
the circle centred at (0,0) with radius 15

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(cont…)
k pk (xk+1,yk+1) 2xk+1
2yk+1
0
1
2
3
4
5
6
7
8
9
10
11
12
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6
5
4
3
2
1
0
8
976543210 8 10
10
131211 14
15
13
12
14
11
16
1516

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Mid-Point Circle Algorithm Summary
The key insights in the mid-point circle
algorithm are:
– Eight-way symmetry can hugely reduce the
work in drawing a circle
– Moving in unit steps along the x axis at each
point along the circle’s edge we need to
choose between two possible y coordinates

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Filling Polygons
So we can figure out how to draw lines and
circles
How do we go about drawing polygons?
We use an incremental algorithm known as
the scan-line algorithm

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Scan-Line Polygon Fill Algorithm
2
4
6
8
10 Scan Line
0
2 4 6 8 10 12 14 16

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The basic scan-line algorithm is as follows:
– Find the intersections of the scan line with all
edges of the polygon
– Sort the intersections by increasing x
coordinate
– Fill in all pixels between pairs of intersections
that lie interior to the polygon

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(cont…)

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Line Drawing Summary
Over the last couple of lectures we have
looked at the idea of scan converting lines
The key thing to remember is this has to be
FAST
For lines we have either DDA or Bresenham
For circles the mid-point algorithm

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Summary Of Drawing Algorithms

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6
2 3 41
5
4
3

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M
6
2 3 41
5
4
3

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M
6
2 3 41
5
4
3

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Blank Grid
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0
8
976543210 8 10
10

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