Lect3 bresenham circlesandpolygons
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Transcript of Lect3 bresenham circlesandpolygons
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Sudipta Mondal
Computer Graphics :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 isanother incremental scanconversion algorithmThe big advantage of thisalgorithm is that it uses onlyinteger calculations
J a c k B r e s e n h a mworked for 27 years atIBM before enteringacademia. Bresenhamdeveloped his famousalgorithms at IBM int h e e a r l y 1 9 6 0 s
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The Big Idea
Move across the x axis in unit intervals andat each step choose between two different ycoordinates
2 3 4 5
2
4
3
5For example, fromposition (2, 3) wehave to choosebetween (3, 3) and(3, 4)We would like thepoint that is closer tothe original line
(xk, yk)
(xk+1, yk)
(xk+1, yk+1)
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The y coordinate on the mathematical line atxk+1 is:
Deriving The Bresenham Line Algorithm
At sample positionxk+1 the verticalseparations from themathematical line arelabelled dupper and dlower
bxmy k ++= )1(
yyk
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 decisionabout which pixel is closer to the mathematicalline
Deriving The Bresenham Line Algorithm(cont…)
klower yyd -=
kk ybxm -++= )1(
yyd kupper -+= )1(bxmy kk -+-+= )1(1
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This simple decision is based on the differencebetween the two pixel positions:
Let’s substitute m with ∆y/∆x where ∆x and∆y are the differences between the end-points:
Deriving The Bresenham Line Algorithm(cont…)
122)1(2 -+-+=- byxmdd kkupperlower
)122)1(2()( -+-+DD
D=-D byxxyxddx kkupperlower
)12(222 -D+D+×D-×D= bxyyxxy kk
cyxxy kk +×D-×D= 22
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So, a decision parameter pk for the kth stepalong a line is given by:
The sign of the decision parameter pk is thesame as that of dlower – dupper
If pk is negative, then we choose the lowerpixel, otherwise we choose the upper pixel
Deriving The Bresenham Line Algorithm(cont…)
cyxxyddxp
kk
upperlowerk
+×D-×D=
-D=
22
)(
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Remember coordinate changes occur alongthe x axis in unit steps so we can doeverything with integer calculationsAt step k+1 the decision parameter is givenas:
Subtracting pk from this we get:
Deriving The Bresenham Line Algorithm(cont…)
cyxxyp kkk +×D-×D= +++ 111 22
)(2)(2 111 kkkkkk yyxxxypp -D--D=- +++
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But, xk+1 is the same as xk+1 so:
where yk+1 - yk is either 0 or 1 depending onthe sign of pkThe first decision parameter p0 is evaluatedat (x0, y0) is given as:
Deriving The Bresenham Line Algorithm(cont…)
)(22 11 kkkk yyxypp -D-D+= ++
xyp D-D= 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-pointin (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 thefollowing test. If pk < 0, the next point to plot is(xk+1, yk) and:
xyp D-D= 20
ypp kk D+=+ 21
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The Bresenham Line Algorithm (cont…)
Remember: The algorithm and derivationabove assumes absolute value of slopes areless than 1. For other slopes we need toadjust the algorithm slightly.
Otherwise, the next point to plot is (xk+1, yk+1) and:
5. Repeat step 4 (Δx – 1) times
xypp kk D-D+=+ 221
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Bresenham Example
Let’s have a go at thisLet’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…)
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Bresenham Exercise
Go through the steps of the Bresenham linedrawing algorithm for a line going from(21,12) to (29,16)
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Bresenham Exercise (cont…)
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Bresenham Line Algorithm Summary
The Bresenham line algorithm has thefollowing advantages:
– An fast incremental algorithm– Uses only integer calculations
Comparing this to the DDA algorithm, DDAhas the following problems:
– Accumulation of round-off errors can makethe pixelated line drift away from what wasintended
– The rounding operations and floating pointarithmetic 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 circleSo, we can write a simple circle drawingalgorithm by solving the equation for y atunit x intervals using:
222 ryx =+
22 xry -±=
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A Simple Circle Drawing Algorithm(cont…)
20020 220 »-=y
20120 221 »-=y
20220 222 »-=y
61920 2219 »-=y
02020 2220 »-=y
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A Simple Circle Drawing Algorithm(cont…)
However, unsurprisingly this is not a brilliantsolution!Firstly, the resulting circle has large gapswhere the slope approaches the verticalSecondly, the calculations are not veryefficient
– The square (multiply) operations– The square root operation – try really hard to
avoid these!We need a more efficient, more accuratesolution
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Eight-Way Symmetry
The first thing we can notice to make our circledrawing algorithm more efficient is that circlescentred at (0, 0) have eight-way symmetry
(x, y)
(y, x)
(y, -x)
(x, -y)(-x, -y)
(-y, -x)
(-y, x)
(-x, y)
2R
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Mid-Point Circle Algorithm
Similarly to the case with lines,there is an incrementalalgorithm for drawing circles –the mid-point circle algorithmIn the mid-point circle algorithmwe use eight-way symmetry soonly ever calculate the pointsfor the top right eighth of acircle, and then use symmetryto get the rest of the points
The mid-point circlea l g o r i t h m w a sdeveloped by JackBresenham, who weheard about earlier.
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Mid-Point Circle Algorithm (cont…)
(xk+1, yk)
(xk+1, yk-1)
(xk, yk)
Assume that we havejust plotted point (xk, yk)The next point is achoice between (xk+1, yk)and (xk+1, yk-1)We would like to choosethe point that is nearest tothe actual circleSo how do we make this choice?
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Mid-Point Circle Algorithm (cont…)
Let’s re-jig the equation of the circle slightlyto give us:
The equation evaluates as follows:
By evaluating this function at the midpointbetween the candidate pixels we can makeour decision
222),( ryxyxfcirc -+=
ïî
ïí
ì
>=<
,0,0,0
),( yxfcirc
boundarycircle theinsideis),(if yxboundarycircleon theis),(if yx
boundarycircle theoutsideis),(if yx
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Mid-Point Circle Algorithm (cont…)
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 andand the pixel at yk is closer to the circleOtherwise the midpoint is outside and yk-1 iscloser
222 )21()1(
)21,1(
ryx
yxfp
kk
kkcirck
--++=
-+=
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Mid-Point Circle Algorithm (cont…)
To ensure things are as efficient as possiblewe can do all of our calculationsincrementallyFirst consider:
or:
where yk+1 is either yk or yk-1 depending onthe sign of pk
( )( ) 22
12
111
21]1)1[(
21,1
ryx
yxfp
kk
kkcirck
--+++=
-+=
+
+++
1)()()1(2 122
11 +---+++= +++ kkkkkkk yyyyxpp
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Mid-Point Circle Algorithm (cont…)
The first decision variable is given as:
Then if pk < 0 then the next decision variableis given as:
If pk > 0 then the decision variable is:
r
rr
rfp circ
-=
--+=
-=
45
)21(1
)21,1(
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 acircle centred on the origin as:
• Calculate the initial value of the decision parameter as:
• Starting with k = 0 at each position xk, perform thefollowing test. If pk < 0, the next point along the circlecentred on (0, 0) is (xk+1, yk) and:
),0(),( 00 ryx =
rp -= 45
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 octants5. Move each calculated pixel position (x, y) onto the
circular path centred at (xc, yc) to plot the coordinatevalues:
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 inaction lets use it to draw a circle centred at(0,0) with radius 10
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Mid-Point Circle Algorithm Example(cont…)
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Mid-Point Circle Algorithm Exercise
Use the mid-point circle algorithm to drawthe circle centred at (0,0) with radius 15
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Mid-Point Circle Algorithm Example(cont…)
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Mid-Point Circle Algorithm Summary
The key insights in the mid-point circlealgorithm are:
– Eight-way symmetry can hugely reduce thework in drawing a circle
– Moving in unit steps along the x axis at eachpoint along the circle’s edge we need tochoose between two possible y coordinates
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Filling Polygons
So we can figure out how to draw lines andcirclesHow do we go about drawing polygons?We use an incremental algorithm known asthe scan-line algorithm
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Scan-Line Polygon Fill Algorithm
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0 2 4 6 8 10 12 14 16
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Scan-Line Polygon Fill Algorithm
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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Scan-Line Polygon Fill Algorithm(cont…)
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Line Drawing Summary
Over the last couple of lectures we havelooked at the idea of scan converting linesThe key thing to remember is this has to beFASTFor lines we have either DDA or BresenhamFor circles the mid-point algorithm
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Mid-Point Circle Algorithm (cont…)
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Mid-Point Circle Algorithm (cont…)
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Mid-Point Circle Algorithm (cont…)
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Blank Grid
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