COMPUTER GRAPHICS
Prepared by
S.MAHALAKSHMI
Asst. Prof(Sr) / SCSE
VIT University
Programming in SRGP
INTRO
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Advantages of Raster Displays
Fill areas with uniform color or
repeated pattern with 2 or more
colors.
Stores images which allows
manipulation @ fine level : individual
pixels can be read or written and
arbitrary portions can be copied or
moved.
SRGP(Simple Raster Graphics
Package)
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Device independent Graphics package
Similar to Macintosh QuickDraw
Raster package and Xlib package of
Windows system
SRGP (Continued..)
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Drawing with SRGP
Basic Interaction Handling
Raster Graphics Features
Limitations of SRGP
1.Drawing with SRGP
 1.1
Specification of Graphics
Primitives
 1.2 Attributes
 1.3 Filled primitives and attributes
 1.4 Saving and Restoring Attributes
 1.5Text
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1.1 Specification of Graphics
Primitives
Drawing in integer graphics package such as SRGP
is like plotting graphs on graph paper with very fine
grid
Grid varies from 80-120 points per inch to 300 or
more on high resolution displays
(0,0)--> bottom left of the screen . +ve x increases
towards right and +ve Y increases towards top
1.1 continued...
SRGP supports basic collections of
primitives
a)Lines
b)Polygons
c) Circles
d)Ellipses
e)Text
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Lines and Poly lines
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Void SRGP_lineCoord(int x1, int y1, int
x2, int y2);
eg:
SRGP_lineCoord(0,0,100,300)
(or)
void SRGP_line(point pt1,point pt2);
Point is a defined type, a record holding
2 integer x,y values
typedef struct
{int x,y;
}point;
Lines and Poly lines
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A sequence of lines connecting
successive vertices is called polyline.
There are 2 polyline procedures.
These take array as parameter.
Void SRGP_polyLineCoord (int vertexcount,
int*xArray, int*yArray);
Void SRGP_polyLine(int vertexcount, point
*vertices);
continued..
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/* plot the axes*/
SRGP_lineCoord(175,200,320,200);
SRGP_lineCoord(200,140,200,280);
/*plot the data*/
SRGP_polyLineCoord (12 ,months,
balanceOfTrade);
SRGP_polyLine(7, bowtieArray);
contiued..
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Markers and polymarkers
It is convenient to place
markers(dots,asterisks or circles) at the data
points on graphs.
Void SRGP_markerCoord(intx,inty);
Void SRGP_marker(point pt)
Void SRGP_polyMarkerCoord (int vertexcount,
int*xArray, int*yArray);
Void SRGP_polyMarker(int vertexcount, point
*vertices);
continued...
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Polygons and Rectangles
--> To draw an outline polygon, we
can either specify a polyline on itself
by making the first and last vertices
identical (or) we can use
void SRGP_polygon(int vertexCount, point
*vertices)
Now bowtieArray is of only 6 points it
automatically closes the figure.
Continued
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Any rectangle can be specified as a
polygon having 4 vertices.
Uses only 2 primitives(lower left &
upper right)
Void SRGP_rectangleCoord(int leftx,
intbottomY,int rightX, int topY);
Void SRGP_rectanglePt(point bottomLeft,
point topRight);
void SRGP_rectangle(rectangle rect);
typedef struct{ point bottomLeft,topRight;
}rectangle;
Rectangles continued..
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Point SRGP_defPoint(intx,int y);
Rectangle SRGP_defRectangle(int
leftX,int bottomY,int rightX, int topY);
Circles and Ellipses
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Circles are Special types of ellipses.
The term ellipse arc used for
circular,elliptical,closed or partial arcs.
It is easy to specify arcs via upright
rectangles (bounding boxes or extents)
Void SRGP_ellipseArc(rectangle
extentRect, float startAngle, float
endAngle);
Circles and ellipses
 Relationship
between rectangular and
circular angle is
 Rectangular angle=arctan(tan(circularangle).width/height)+
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adjust
where the angles are in radians and
adjust = 0, for 0<= circular angle<∏/2
adjust = ∏/2 , for ∏/2 <=circular angle < 3∏/2
adjust = 2∏, for 3∏/2<= circular angle < 2∏
1.2 Attributes
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The specification of a primitive can be
controlled by specification of its
attributes
Line style and Line width
Color
Penstyle
Attributes are global state variables
that retain their values until they are
changed explicitly
Void SRGP_setLineStyle(lineStyle
CONTINUOUS / DASHED /DOTTED/….);
Void SRGP_setLineWidth(int widthValue);
Attributes..
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Each attribute has a default.. Eg., default for
linestyle is CONTINUOUS that for line width is
1.
Line style can be represented as bit mask and
it is scan converted by SRGP.
For eg.. Contimuous all 1’s
DASHED 1111001111001111…..
Attributes for marker primitive
Void SRGP_setMarkerSize(int sizeValue);
 Void SRGP_setMarkerStyle(markerStyle
MARKER_CIRCLE / MARKER_SQUARE/…);
 The default is circle style
 COLOR
 bilevel system the color value is 0 and 1.
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-1 black, 0 white for black on white
devices
1 green, 0 black for green on black devices
Attribute-color
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The integer color attribute is an index to
SRGP;s color table. Each entry defines
color or gray scale value.
There are 2d entries and d is the depth of
frame buffer
2 methods to specify colors
1. use integers 0 and 1 directly
2. use color names supported by SRGP
We can also select color by calling
Void SRGP_setColor(int colorIndex)
1.3 Filled Primitives and Their
Attributes
Primitives can be drawn in 2 ways
 Outline
 Filled
 SRGP’s filled versions of area
defining primitives draw the interior
pixels with no outline
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Code for creating filled Primitives
Output for the previous code
Odd Parity rule
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We need a rule for specifying which
regions are interior, and thus should be
filled and which is exterior
This can done in SRGP through odd parity
rule.
To check whether the region lies inside/
outside
Choose a test point
Choose a ray that starts at the test point
extends indefinitely in any direction and
does not pass any vertices.
If this ray intersect the polygon outline an
odd no of times ->interior
else exterior
Fill style and fill pattern for areas
4
ways
 Void SRGP_setFillStyle(drawStyle
SOLID/ BITMAP_PATTERN_OPAQUE /
BITMAP_PATTERN_TRANSPARENT/
PIXMAP_PATTERN);
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There is no distinction in opaque &
transparent in pix map patterns
Application screen background
Background color  0 bits in bitmap
patterns used in opaque mode.
 Suppose user expects the uniform screen
background pattern then the application
should set up the background in desired
pattern. Default solid color 0
 Erase primitive  is done by redraw them
in the application background
pattern.(Quick & dirty technique). This
yield damaged image if primitives
overlaps.
 Damage repair is done by respecifying
primitives in application data
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1.4 Saving and restoring Attributes
Attributes can be saved for later
restoration.
 It does not affect the global attribute
state.
 SRGP does this attribute group via
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void SRGP_inquireAttributes(attributeGroup * group);
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void SRGP_setAttributes(attributeGroup * group);
1.5 Text
Specifying and implementing text drawing is
always complex in graphics package.
(style,font,bold,italic,width,spacing)
 In SRGP text is horizontally aligned and
character widths vary, but space b/w
characters is constant.
 High quality documents need specialized
packages that offer more ctrl.
 Eg. Post script
 Text in SRGP is generated by
 void SRGP_text(point origin, char *text);
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1.5 Text continued..
The location of text is ctrlled by
specification of its origin/Anchor point
 Text primitive’s appearance is determined
by only 2 attributes (color and font)
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void SRGP_setFont(int valueIndex)
Each character in the font is defined as
rectangular bitmap.
 SRGP draws a character by filling a
rectangle using character bitmap as
pattern in transparent mode.
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Formatting text
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Void SRGP_inquireTextExtent(Char*text, int
*width, int * height, int *descent);
Code for formatting text
2.Basic Interaction Handling
Topics to be discussed
 Human Factors
 Logical Input Devices
 Sampling versus Event driven Processing
 Sample mode
 Event mode
 Pick correlation for Interaction Handling
 Setting Device Measure and Attributes
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2.1 Human Factors
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Interaction style(look and feel)
Ease of learning and use
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Guidelines for user computer interaction
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Provide simple and Consistent interaction
sequence
Do not overload the user with too many
different options
Show the available options clearly
Give appropriate feedback
Allow the user ti recover gracefully from
mistakes.
2.2 Logical Input Devices
Device types in SRGP
 2 logical input devices supported by
SRGP
 a) Locator
 b) Keyboard
• SRGP maps the logical devices to physical
devices(eg locator to mouse, joystick,
tablet….)
• SRGP provides only one logical locator and
one keyboard device
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2.3 Sampling versus Event driven
Processing