Lect2 scan convertinglines

Computer Graphics:
Line Drawing Algorithms

Sudipta Mondal

Sudipta.hit@gmail.com

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Contents
Graphics hardware
The problem of scan conversion
Considerations
Line equations
Scan converting algorithms
– A very simple solution
– The DDA algorithm
Conclusion

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Graphics Hardware
It’s worth taking a little look at how graphics
hardware works before we go any further
Images taken from Hearn & Baker, “Computer Graphics C version

How do things end up on the screen?

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Architecture Of A Graphics System

Display
Frame Video
Processor Monitor
Memory Buffer Controller

Display System
CPU
Processor Memory

System Bus

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Output Devices
There are a range of output devices
currently available:
– Printers/plotters
– Cathode ray tube displays
– Plasma displays
– LCD displays
– 3 dimensional viewers
– Virtual/augmented reality headsets
We will look briefly at some of the more
common display devices

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Basic Cathode Ray Tube (CRT)
Fire an electron beam at a phosphor coated
screen

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Draw one line at a time
Raster Scan Systems

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Colour CRT
An electron gun for each colour – red, green
and blue

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Plasma-Panel Displays
Applying voltages to
crossing pairs of

conductors causes
the gas (usually a
mixture including
neon) to break
down into a glowing
plasma of electrons
and ions

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Liquid Crystal Displays
Light passing
through the liquid

crystal is twisted
so it gets through
the polarizer
A voltage is
applied using the
crisscrossing
conductors to stop
the twisting and
turn pixels off

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The Problem Of Scan Conversion
A line segment in a scene is defined by the
coordinate positions of the line end-points
y

(7, 5)

(2, 2)

x

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The Problem (cont…)
But what happens when we try to draw this
on a pixel based display?

How do we choose which pixels to turn on?

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Considerations
Considerations to keep in mind:
– The line has to look good
• Avoid jaggies
– It has to be lightening fast!
• How many lines need to be drawn in a typical
scene?
• This is going to come back to bite us again and
again

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Line Equations
Let’s quickly review the equations involved
in drawing lines
y Slope-intercept line
equation:
yend y = m× x +b
where:
yend - y0
y0 m=
xend - x0
b = y 0 - m × x0
x
x0 xend

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Lines & Slopes
The slope of a line (m) is defined by its start
and end coordinates
The diagram below shows some examples
of lines and their slopes
m=∞
m = -4 m=4
m = -2 m=2

m = -1 m=1

m = - 1 /2 m = 1 /2
m = - 1 /3 m = 1 /3

m=0 m=0

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A Very Simple Solution
We could simply work out the corresponding
y coordinate for each unit x coordinate
Let’s consider the following example:
y

(7, 5)
5

2
(2, 2)

x
2 7

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A Very Simple Solution (cont…)

5

4

3

2

1

0

0 1 2 3 4 5 6 7

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y
(7, 5)
5 First work out m and b:
5-2 3
m= =
2
7-2 5
(2, 2)
3 4
b = 2- *2 =
2 3 4 5 6 7
x 5 5
Now for each x value work out the y value:
3 4 3 3 4 1
y (3) = × 3 + = 2 y ( 4) = × 4 + = 3
5 5 5 5 5 5
3 4 4 3 4 2
y (5) = × 5 + = 3 y (6) = × 6 + = 4
5 5 5 5 5 5

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Now just round off the results and turn on
these pixels to draw our line
3
7 y (3) = 2 » 3
6 5
5 1
4
y (4) = 3 » 3
5
3
4
2 y (5) = 3 » 4
1 5
0 2
y (6) = 4 » 4
0 1 2 3 4 5 6 7 8 5

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However, this approach is just way too slow
In particular look out for:
– The equation y = mx + b requires the
multiplication of m by x
– Rounding off the resulting y coordinates
We need a faster solution

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A Quick Note About Slopes
In the previous example we chose to solve
the parametric line equation to give us the y
coordinate for each unit x coordinate
What if we had done it the other way
around?
y -b
So this gives us: x =
m
yend - y0
where: m = and b = y0 - m × x0
xend - x0

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A Quick Note About Slopes (cont…)
Leaving out the details this gives us:
2 1
x(3) = 3 » 4 x ( 4) = 5 » 5
3 3
We can see easily that 7
this line doesn’t look 6
very good! 5
4
We choose which way 3
to work out the line 2

pixels based on the 1
0
slope of the line
0 1 2 3 4 5 6 7 8

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If the slope of a line is between -1 and 1 then
we work out the y coordinates for a line based
on it’s unit x coordinates
Otherwise we do the opposite – x coordinates
are computed based on unit y coordinates
m=∞
m = -4 m=4
m = -2 m=2
m = -1 m=1

m = - 1 /2 m = 1 /2
m = - 1 /3 m = 1 /3

m=0 m=0

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5

4

3

2

1

0

0 1 2 3 4 5 6 7

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The DDA Algorithm
The digital differential
analyzer (DDA) algorithm
takes an incremental
approach in order to
speed up scan conversion
The original differential
Simply calculate yk+1 a n a l yze r w as a p h ys i c a l
based on yk machine developed by
Vannevar Bush at MIT in the
1930’s in order to sol ve
ordinary differential equations.

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The DDA Algorithm (cont…)
Consider the list of points that we
determined for the line in our previous
example:
(2, 2), (3, 23/5), (4, 31/5), (5, 34/5), (6, 42/5), (7, 5)
Notice that as the x coordinates go up by
one, the y coordinates simply go up by the
slope of the line
This is the key insight in the DDA algorithm

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When the slope of the line is between -1 and 1
begin at the first point in the line and, by
incrementing the x coordinate by 1, calculate
the corresponding y coordinates as follows:
yk +1 = yk + m
When the slope is outside these limits,
increment the y coordinate by 1 and calculate
the corresponding x coordinates as follows:
1
xk +1 = xk +
m

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Again the values calculated by the equations
used by the DDA algorithm must be rounded
to match pixel values

(xk+1, round(yk+m)) (round(xk+ 1/m), yk+1)

(xk, yk) (xk+ 1/m, yk+1)
(xk+1, yk+m) (xk, yk)

(xk, round(yk)) (round(xk), yk)

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DDA Algorithm Example
Let’s try out the following examples:
y y (2, 7)
7

(7, 5)
5

2 2
(2, 2) (3, 2)

x x
2 7 2 3

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DDA Algorithm Example (cont…)

7

6

5

4

3

2

0 1 2 3 4 5 6 7

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The DDA Algorithm Summary
The DDA algorithm is much faster than our
previous attempt
– In particular, there are no longer any
multiplications involved
However, there are still two big issues:
– 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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Conclusion
In this lecture we took a very brief look at
how graphics hardware works
Drawing lines to pixel based displays is time
consuming so we need good ways to do it
The DDA algorithm is pretty good – but we
can do better
Next time we’ll like at the Bresenham line
algorithm and how to draw circles, fill
polygons and anti-aliasing

Lect2 scan convertinglines

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