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OUTPUT PRIMITIVES
Output Primitives: Basic geometric structures used to describe scenes.
(points, straight line segment, circles and other conic sections, quadric surfaces, spline curve and surfaces, polygon color areas, and character strings)
These picture components are often defined in a continuous space.
OUTPUT PRIMITIVES
In order to draw the primitive objects, one has to first scan convert the object.
Scan convert: Refers to the operation of finding out the location of pixels to be intensified and then setting the values of corresponding bits, in the graphic memory, to the desired intensity code.
SCAN CONVERTING A POINT
A random-scan (vector) system
stores point-plotting instructions in the display list, and coordinate values in
these instructions are converted to deflection voltages that position the electron beam at the screen locations to be plotted during each refresh cycle.
SCAN CONVERTING A POINT
A mathematical point P(x,y) needs to be scan converted to a pixel. If the coordinate values of point P are
(3.3, 7.3) then the floor function of x’=3 and y’=7, is plotted.
SCAN CONVERTING A POINT (IN VECTOR DISPLAY)
x´=Floor(x) and y´=Floor(y)
All points that satisfy:
are mapped to pixel (x´,y´)
1
1
yyy
xxx
SCAN CONVERTING A LINE
The Cartesian slope- intercept equation for a straight line is:
12
12
xxyy
m
11xmyb
bxmy
xmy my
x
SCAN CONVERTING A LINE These equation form the basic for
determining deflection voltage in analog devices.
xmy
|m|<1
my
x
|m|>1
SCAN CONVERTING A LINE On raster system, lines are plotted
with pixels, and step size (horizontal & vertical direction) are constrained by pixel separation.
SCAN CONVERTING A LINE
We must sample a line at discrete positions and determine the nearest pixel to the line at each sampled position.
DDA ALGORITHM Algorithm is an incremental scan
conversion method. Based on calculating either or If |m|<1,
x y
1
)1(
Y
x
myy
xmy
kK
1
CHARACTER GENERATION
Letters, numbers, and other character can be displayed in a variety of size and styles.
CHARACTER GENERATION
There are three basic methods to generate characters,
1. Hardware based: Logic is built into the graphics terminal and drawn quickly.
2. Bitmap based: utilizes pixels and copies appropriate portion from frame buffer to display
3. Vector based: provides with unlimited possibilities with character style, size, shape